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Root Systems Research for
Bioinspired Resilient Design: A
Concept Framework for Foundation
and Coastal Engineering
Elena Stachew
1
, Thibaut Houette
1
and Petra Gruber
2
*
1
Biomimicry Research and Innovation Center BRIC, Department of Biology, The University of Akron, Akron, OH, United States,
2
Biomimicry Research and Innovation Center BRIC, Myers School of Art and Department of Biology, The University of Akron,
Akron, OH, United States
The continuous increase in population and human migration to urban and coastal areas
leads to the expansion of built environments over natural habitats. Current infrastructure
suffers from environmental changes and their impact on ecosystem services. Foundations
are static anchoring structures dependent on soil compaction, which reduces water
infiltration and increases flooding. Coastal infrastructure reduces wave action and
landward erosion but alters natural habitat and sediment transport. On the other hand,
root systems are multifunctional, resilient, biological structures that offer promising
strategies for the design of civil and coastal infrastructure, such as adaptivity,
multifunctionality, self-healing, mechanical and chemical soil attachment. Therefore, the
biomimetic methodology is employed to abstract root strategies of interest for the design
of building foundations and coastal infrastructures that prevent soil erosion, anchor
structures, penetrate soils, and provide natural habitat. The strategies are described in
a literature review on root biology, then these principles are abstracted from their biological
context to show their potential for engineering transfer. After a review of current and
developing technologies in both application fields, the abstracted strategies are translated
into conceptual designs for foundation and coastal engineering. In addition to presenting
the potential of root-inspired designs for both fields, this paper also showcases the main
steps of the biomimetic methodology from the study of a biological system to the
development of conceptual technical designs. In this way the paper also contributes to
the development of a more strategic intersection between biology and engineering and
provides a framework for further research and development projects.
Keywords: root architecture, root research, biomimicry, bioinspired design, building foundations, coastal
engineering
INTRODUCTION
Currently, 40% of the global population lives in cities and by 2050, this number will increase to 66%
(Li, 2018). 40% of the global population and 75% of the world’s megacities are within 100 km of a
coastline and this percentage is also expected to increase (Mayer-Pinto et al., 2019). These population
migration trends highlight the need for built infrastructure, competing for space with natural habitats
that provide essential protective and regulating ecosystem services (Duraiappah et al., 2005;Lotze
Edited by:
Barbara Mazzolai,
Italian Institute of Technology (IIT), Italy
Reviewed by:
Matthew Aaron Robertson,
Queen’s University, Canada
Olusegun Oguntona,
University of Johannesburg,
South Africa
*Correspondence:
Petra Gruber
pgruber@uakron.edu
Specialty section:
This article was submitted to
Soft Robotics,
a section of the journal
Frontiers in Robotics and AI
Received: 09 April 2020
Accepted: 08 April 2021
Published: 26 April 2021
Citation:
Stachew E, Houette T and Gruber P
(2021) Root Systems Research for
Bioinspired Resilient Design: A
Concept Framework for Foundation
and Coastal Engineering.
Front. Robot. AI 8:548444.
doi: 10.3389/frobt.2021.548444
Frontiers in Robotics and AI | www.frontiersin.org April 2021 | Volume 8 | Article 5484441
REVIEW
published: 26 April 2021
doi: 10.3389/frobt.2021.548444
et al., 2006;Grimm et al., 2008). Compounded by climate change,
damage to the built environment from natural disasters incurs
massive economic losses (Tamura and Cao, 2010;Dinan, 2017).
Continued urban migration results in growth of infrastructure
and of impermeable surface cover. The overuse of material with
respect to foundation construction specifically, also increases soil
compaction. Soil compaction and impermeability compromise
water storage and infiltration and so contribute to increasing
risks of flooding and erosion (Yang and Zhang, 2011;Alaoui
et al., 2018). Soil erosion becomes a problem for foundations as
their anchorage depends on soil stability. Increasing frequency and
intensity of storm events will also impose more severe loading
scenarios (Dinan, 2017). Reducing soil compaction, preventing
erosion, and adapting to extreme loading scenarios are crucial
needs, questioning the current design of building foundations. A
multifunctional adaptive approach to foundation engineering
should aim at alleviating flooding and erosion potential, while
also lowering material usage in construction required to support a
structure under various loading scenarios. As seen in the evolution
of biological systems, multifunctionality typically increases design
complexity. The difficulty of inserting complex structures in the
soil without significant excavation in current civil engineering
methods limits foundation design to simple morphologies.
Coastal infrastructure protects populations and the built
environment against wave action and landward erosion (Bulleri
and Chapman, 2010;McLachlan and Defeo, 2018). Continued
coastal migration and the effects of climate change require more
protective infrastructure that is also substantially larger in size and
scale (Ferrario et al., 2014). This trend eliminates, displaces, or
fragments natural coastal habitats which provide multiple
significant ecosystem functions (Barbier et al., 2008;Strain et al.,
2018), not to mention substantially decreasing biodiversity for
some of the most diverse global ecosystems (Duarte, 2009).
Additionally, traditional coastal engineering practices often
cause downstream erosion, wave reflection, bottom scour and
subsequent increased nearshore wave heights, and disruption of
natural nearshore littoral transport (Silvester, 1972;McLachlan and
Defeo, 2018). A multifunctional adaptive approach to coastal
engineering should aim at wave attenuation, dissipation, and
dispersion to reduce wave action and erosion potential, while
also creating physical conditions, such as quiescent flow regimes
and habitat refuge spaces, to increase and maintain biodiversity
across multiple taxa (e.g., plants, macroinvertebrates, and fish).
We propose that the overarching design framework of
biologically inspired design (BID), hereinafter referred to as
bioinspired design, can inform the development of sustainable,
multifunctional, and adaptive innovations to built infrastructure.
Bioinspired design utilizes inspiration from nature to develop
technical outcomes (Lenau et al., 2018). In our case,
understanding how living organisms embed and stabilize
themselves with minimal disruption and degradation to their
surroundings, dynamic environment is crucial to our application
areas of building foundations and coastal infrastructure. Natural
ecosystems contain herbaceous vegetation, woody plants, and
trees, in which roots contribute significantly to anchorage of an
aboveground structure and subsequent substrate stability. In the
case of mangroves and other coastal forests, their root systems
must significantly contribute to wave attenuation and substrate
stability along coasts for survival (Koch et al., 2009). Roots also
perform multiple functions other than anchorage and substrate
stability and adapt to changes detected in the surrounding soil
environment through a variety of mechanisms (Malamy, 2005).
Therefore, we hypothesize that the study of root systems informs
multiple engineering design applications in the areas of
foundation and coastal engineering.
Within the framework of bioinspired design lies both
biomimetics and biomimicry (Lenau et al., 2018). For the scope
of this work we utilize the terms synonymously and employ
primarily the problem-driven process of biologically inspired
design as our research methodology to present design proposals
for our specific application areas. Problem driven biologically
inspired design takes on a technical question that is answered
by a strategic search for analogous solution in biology. The first step
in the problem-driven bioinspired design process and in our
research investigation is an assessment of common practices,
uses, and applications to identify the technical shortcomings of
current building foundation and coastal infrastructure designs.
Next, these shortcomings are abstracted, so that the problem, its
context, constraints, and necessary functions can be transposed to
biology and connected to biological analogs. Principles are
extracted from biological models (in our case, root systems) out
of their natural context, so that they may be emulated in
technological solutions (Vincent et al., 2006;Fayemi et al.,
2017). While biomimicry primarily follows the same design
steps as biomimetics, its unique attribute is on an ecological
philosophy and ethos to meet the challenges of sustainable
development (Benyus, 2011;Lenau et al., 2018).
To demonstrate the hypothesis that the study of root systems
informs multiple engineering design applications through the
overarching design lens of bioinspired design, we present an
overview of relevant root biology in “Roots as Biological Model”
section, with a special focus on adaptation and biomechanics.
Through the biomimetics process, specific biological information
is then related to infrastructure problems and vulnerabilities
through a functional translation in a comprehensive analogy
table in “Abstraction and Analogy”section (Table 1).
“Application of Root Biology to Technical Designs”section
presents a range of current and future innovative bioinspired
design concepts for the fields of building foundation and coastal
engineering, followed by Discussion and Conclusion in sections
“Discussion”and “Conclusion”.
ROOTS AS BIOLOGICAL MODEL
Rather than a comprehensive encyclopedia this section provides a
general overview of root biology and an understanding of
strategies and mechanisms found in root systems for
mechanical anchorage, soil stability, and other dynamic
external loading conditions relevant for biomimetic
translation to the two application spaces of building foundation
and coastal infrastructure design. Additionally, there is a general
introduction to the use of root systems (and other woody
components) in natural constructions by humans, whose
Frontiers in Robotics and AI | www.frontiersin.org April 2021 | Volume 8 | Article 5484442
Stachew et al. Root Research for Bioinspired Design
strategies and mechanisms of construction are also relevant to
biomimetic translation.
Root Basics
Root Structure Components
Root structure is generally described by four regions or zones
(Bidlack et al., 2011). These regions, starting from the end of the
root, are the root cap, region of cell division, region of elongation,
and region of maturation. The root cap and apical meristem
located in the region of cell division are the only regions that
push through the soil. The other regions remain stationary. Root
diameter gradually increases through addition of secondary tissues
in the region of elongation (i.e., radial growth). Lastly, the region of
maturation is where root hairs are produced. These are short-lived
extensions that adhere tightly to soil particles and increase the total
water and mineral nutrient absorptive surface of the root.
Root Classification: Different Types of Roots
There are three main types of roots: primary (i.e., seminal),
adventitious (i.e., nodal), and lateral roots (Malamy, 2005).
Primary roots stem from seed, while nodal roots initiate from
non-root tissue and are coordinated with aboveground shoot
development. Many mature plants have a combination of taproot
(a thick, vertical, centrally located primary root) and diffuse
fibrous (i.e., nodal) root systems (Malamy, 2005;Bidlack et al.,
2011). Lateral roots develop by branching, which is coordinated
with root elongation (Lecompte and Pagès, 2007), with an
equilibrium maintained between root number and length
(Malamy, 2005). From a spatial perspective, structural “coarse”
roots (sometimes referred to as basal roots) are often near the base
of the stem. Their primary function is anchorage, and they may
develop considerable secondary thickening. Fine “thin”roots are
often much further away from the stem (sometimes referred to as
distal roots). Their primary functions are soil exploration to
source water and nutrients.
Root Growth Processes
Axial growth and radial growth are the two main types of root
growth processes (Hodge et al., 2009). Axial growth is defined as
the root extending in length and the tip pushing forward into the
soil, with the root parts behind the elongation zone anchored in
the soil. The direction of root elongation is triggered by different
tropisms, such as gravitropism and hydrotropism (Lynch and
Brown, 2001). Axial growth is significantly limited when zones
with high soil mechanical resistance is present (Hoad et al., 2001).
Radial growth is defined as additional layers of growth on
individual roots, root thickening, or secondary thickening (Hodge
et al., 2009). This growth process is important in expanding the
range of root functions, including axial transport properties,
mechanical strength and anchorage, storage capacity, and
protection against predation, drought, or pathogens.
Root System Architecture and Morphology
Root System Architecture, or spatial configuration of the root
system, varies greatly depending on plant species, soil
composition, water, nutrient, and mineral availability
(Malamy, 2005;Hodge et al., 2009). The shape of a root
system is characterized by how the roots occupy the soil and
is defined specifically by the traits of root depth, lateral root
expansion, and root length densities. The shape of the root system
can also be described by abstract synthetic descriptors like fractal
dimensions (Tatsumi et al., 1989). The structure is characterized
by root system components and their relationships, defined by
the traits of root gradients, cross section, topology, and
connection between roots (i.e., branching angle) (Malamy,
2005). Root topology describes the abstracted pattern of root
branching. Topological order is an important parameter of root
trait analysis as it can be a stronger predictor of mechanical
properties than root diameter (Mao et al., 2018).
There are three main categories of root system morphology
(Ennos, 2000). The plate morphology, often found in mature
trees, is characterized by thick lateral roots radiating horizontally
or slightly obliquely from the main stem, followed by tapering and
branching, in addition to sinker roots originating from lateral roots
close to the stem. The taproot morphology, characterized by the
single, centrally located taproot, is often found in dicotyledon species
(Ennos and Fitter, 1992) and some rainforest pioneer species (Crook
et al., 1997). Coronal and prop root morphology is often found in
monocotyledon species, as they cannot undergo radial growth and
therefore cannot produce a taproot. This type is characterized by
thick lignified nodal roots growing obliquely from the stem (Ennos,
1991;Ennos et al., 1993). Many species possess intermediate
morphologies (Crook and Ennos, 1998). Intraspecific root
grafting seen in forests is believed to contribute to mechanical
support and nutrient exchange (Graham and Bormann, 1966;
Kumar et al., 1985;Keeley, 1988). Additionally, root system
morphology can be affected by symbiotic root—microorganism
relationships in the rhizosphere, such as mycorrhizal fungi and
actinomycete bacteria (Steeves and Sussex, 1989;Hodge et al., 2009).
Root Function, Development, and
Adaptation
Root Adaptation to Soil Patches
To effectively deploy in transient soil patches rich in moisture or
nutrients, roots exhibit significant morphological plasticity through
modular root structure and tissue differentiation along the root axis
(Hodge et al., 2009). Drew and Saker (1975) reported an increase in
lateral root initiation in soil patches, while Linkohr et al. (2002)
found a repression of lateral root elongation outside the patches.
Root systems also shed roots when resource uptake becomes
insufficient (Hodge et al., 2009).
Root Adaptation to Soil Density, Compaction,
Resistance, and Moisture
Roots must overcome soil resistance to displace soil particles as
the root grows. As a result, root diameter increases and root
elongation decreases with increasing soil strength (Correa et al.,
2019). Soil zones of variable resistance impact root growth rate,
morphology, orientation, and the local soil-root environment
(Hodge et al., 2009 and associated references therein). Roots
generally follow the path of least resistance, leading to distinct
environments compared to the bulk soil (Pierret et al., 1999;
Hodge et al., 2009).
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Stachew et al. Root Research for Bioinspired Design
To grow through soils, root tips need to generate enough force
to expand a hole in the soil, exceed frictional resistance of the root
tip with the soil particles, and exceed the internal tension in the
root cell walls (Bengough et al., 2011). It is suggested that up to
80% of total penetration resistance results from friction (Greacen
et al., 1968;Bengough et al., 1997). The friction between soil
particles and roots, presence of root hairs, and potential root
trajectory also assist in anchoring the root, so that tissues in the
elongation zone can push the root tip forward (Bengough et al.,
2011).
Circumnutations (i.e., revolving nutation), present in all plant
organs (Hart, 1990;Kiss, 2006;Mugnai et al., 2007), are the result
of differential growth, resulting in active growth movement
following an elliptical path in a left-handed or right-handed
rotation (Johnsson, 1997). The role of root circumnutations is
still debated, but Dottore et al. (2018) found that this movement
reduces the pressure and energy required to penetrate soil.
Roots passively secrete low molecular weight organic
compounds in the rhizosphere, called root exudates. These
exudates promote microbial activity and soil stabilization
through mucus like adhesion, known as mucilage (Tisdall
et al., 1978;Cheshire, 1979;Amellal et al., 1998). Rapid
wetting/drying cycles induce shrinkage and cracks in the soil,
which reduces hydraulic conductivity due to the presence of large
pores in the soil matrix (Grant and Dexter, 1989). Czarnes et al.
(2000) found that a root mucilage analog (e.g., polygalacturonic
acid) stabilized the soil structure against the disruptive effects of
wetting/drying cycles.
Root Adaptation to Continual Water Inundation and
High Salinity
Flooding induces ethylene productionintheroot,which
signals increased nodal and lateral root formation posited
for increased stability (Hodge et al., 2009). Mangroves,
exhibiting a complex stilted root network, only exist in
tropical climates in cyclically submerged environments, with
muddy, waterlogged anoxic soils and high salinity. Mangrove
roots can generally be classified into four types: stilt root, knee
root, snorkel root, and buttress root (Tomlinson, 2016). To
obtain adequate oxygen supply from the air to belowground
roots, mangroves increase adventitious root production
specifically with spongy, erenchymous tissue near the
sediment surface (Hodge et al., 2009). Pneumatophores,
vertical erect roots that emerge from shallow adventitious
roots (Bidlack et al., 2011), are known to slow water
currents, attenuate waves, and increase sedimentation
(Mazda et al., 1997;Hogarth, 2015).
Contractile Roots Adapted to Environments With Low
Water Availability
Contractile roots are found across multiple plant groups, which
mostly inhabit environments with harsh seasons such as drought
or cold temperatures (Pütz, 2002). This behavior, which protects
plant organs and young shoots from harsh conditions by pulling
them down into the soil, is also known to improve plant
anchorage and water uptake (Jernstedt, 1984;Pütz, 2002;
North et al., 2008;Bidlack et al., 2011).
Root Biomechanics
The Root-Soil Plate: Effects of Behavior as One
Mechanical Entity
In Coutts (1983) and associated references therein, various studies
on the behavior of rooted soil under stress found that tree roots
increased soil shear strength by 1–17 kPa. When resistance of the
root-soil interface is higher than the surrounding soil strength, the
root-soil mass behaves as a single unit under applied load, known
as the root-soil plate. This plate is especially visible in uprooted
trees. Root-soil resistance is affected by branching and distribution
in number and size of roots. Roots stiffen the soil similarly to how
rebar rods stiffen a beam, as they mostly resist tensile loads.
Depending on soil conditions, root breakage and slippage
through soil are the main failure mechanisms. In clay soils
where the soil resistance is greater, roots slip instead of break,
meaning that root-soil resistance is more a function of soil
resistance than root morphology and strength (Waldron, 1977).
Root morphology and strength play a greater role when the soil
moisture content is a little below its saturation point.
Effect of Root Hairs on Anchorage and Growth
Ennos (1989) suggests in a study on sunflowers that root hairs
play a major role for the anchorage of young plants against
uprooting by increasing the effective root surface area in contact
with the soil. Additionally, Stolzy and Barley (1968) saw an
increase in tension resistance of individual roots of Pisum
sativum seedlings with root hairs compared to ones without
root hairs. According to Ennos (2000), it is far less likely that
root hairs are useful in the anchorage of mature plants, since root
hairs are only produced near the tip of elongating roots in the
maturation zone where mechanical stresses are relatively low for
large mature plants. In this case, the major mechanical role of root
hairs is in root tip growth, as root hairs anchor the root while the
tip is pushed forward through the soil (Stolzy and Barley, 1968;
Ennos, 2000;Bengough et al., 2011).
Effect of Roots on Slope Stability
Trees reduce soil erosion and prevent shallow landslides
through a network of coarse and fine roots just below the
surface that increase the shear strength of the soil medium,
and sinker roots that anchor the surface layers to a deeper, more
stable soil mass (Nicoll et al., 2005). Structural root mass has
been found to be greater on the upslope side of exposed trees on
slopes, explaining the increase in resistance to upslope
overturning (Nicoll and Ray, 1996). Liang et al. (2017)
demonstrated in a slope stability simulation using a 3D
printed root structure that root strengthening pushes the soil
shear plane deeper in the soil. Root strengthening depends on
species-specific root mechanical properties, surrounding
confining stress, depth of the initial soil slip plane, and root
morphology. The maximum reinforcing effect from root
strengthening may require increased root depth of sinker
roots and lateral extension to enhance soil shear strength. In
a Fiber Bundle Model (FBM) framework to estimate root
cohesion, Arnone et al. (2016) found that the effects of root
water uptake may be more significant than mechanical
reinforcement for slope stability, especially in fine soils.
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Stachew et al. Root Research for Bioinspired Design
Effect of Roots on Wave Attenuation
In a mature mangrove forest, Mazda et al. (1997) observed that
wave attenuation does not decrease with increasing water depth,
which is very beneficial in cases of storm surge and sea level rise.
Mangrove swamps of Rhizophora spp. or Bruguiera spp. have
intricate and large pneumatophores and therefore provide
resistance to flow due to increased projection area. Wave
energy loss is caused by both bottom friction and flow
resistance (i.e., drag force) by mangrove vegetation (trees,
trunks, and roots) through the entire water column. The
submerged tree volume and projection area of aboveground
mangrove root morphology play a significant role in
attenuating tsunami inundation flow (Ohira et al., 2013).
Effect of Roots to Lateral Aboveground Stresses
The components and relevant parameters of anchorage under
lateral forces (e.g., wind) include root-soil plate dimension, root
and soil tensile strength beneath the plate, root-soil resistance
specifically on the windward side, and stiffness at the pivot point
at the base of the tree (Coutts, 1983). A root system of a tree
subjected to wind loads responds through increased growth of the
roots aligned with the plane of stimulation (Nicoll and Dunn,
2000). On the leeward side, bending and compressive forces push
the root-soil interface against the soil below. On the windward
side, tensile and/or shear forces are present due to uplifting.
A study conducted by Tamasi et al. (2005) showed that wind
loading on young Quercus robur L. trees resulted in increased
total lateral root number and length in wind stressed trees
compared to control trees. Wind loading appears to result in
increased growth of more lateral roots and higher structural
root mass on the leeward side. Root systems of adult Picea
sitchensis trees exposed to a natural prevailing wind had higher
structural root mass on the leeward side instead of the
windward side (Nicoll and Ray, 1996). A study conducted
by Stokes et al. (1995,1997) on young Picea sitchensis,showed
greater numbers of both windward and leeward roots, more
elongated and branched morphology, and increased root
diameter.
Although tap roots play a role in initial tree anchorage
(Crook et al., 1997), evidence suggests that lateral roots are
the major component of anchorage in response to dynamic
loading conditions (Ennos et al., 1993;Stokes et al., 1995;Ennos,
2000;Stokes, 2002;Dupuy et al., 2003;Cucchi et al., 2004). If
there are too many roots in the soil however, the soil will likely
fail in shear and tension at the edge of the soil-root plate (Ennos
et al., 1993). Ennos (2000) also notes that plants minimize the
total energy cost of anchorage when exposed to uprooting
potential by only strengthening (i.e., thickening) the basal
parts of the root system.
The location of roots defines their cross-sectional shape.
Bending resistance seems to occur through changes in
structural roots cross-sections, producing I-beam, T-beam, and
oval cross-sections (Rigg and Harrar, 1931). Ennos (2000)
describes the components of root system morphology that
resist lateral stresses. The plate morphology has three
components of anchorage: resistance of leeward hinge to
bending, resistance of the windward roots to uprooting, and
weight of the root-soil plate. The taproot morphology has two
components of anchorage: soil compressive resistance and
taproot bending resistance. The morphology of coronal and
prop roots also have two components: soil compressive
resistance and buckling resistance of the windward roots.
Effect of Buttress Roots to Lateral Aboveground
Stresses
Uneven secondary thickening between root and stem results in
the development of supporting buttresses (Bidlack et al., 2011).
Crook et al. (1997) studied the anchorage of taproot systems:
buttressed trees of Aglaia and Nephelium possessing sinker roots,
and non-buttressed Mallotus wrayi trees with thin lateral roots.
Buttresses provided six times more anchorage than the thin
lateral roots of non-buttressed trees and approximately 60% of
the anchorage acting in tension and compression. Buttresses of
tropical trees are also more often found on the less dense side of
an asymmetric crown, suggesting that buttresses partly serve as
tension elements to equalize mechanical stresses (Young and
Perkocha, 1994;Crook and Ennos, 1998). In addition, buttresses
are believed to reduce the risk of buckling failure (Young and
Perkocha, 1994), and reduce bending and concentration of stress
at the base of the tree (Mattheck et al., 1993).
Root Utilization in Human Constructions
Tree root systems have been directly utilized in several natural
constructions by humans. These constructions are an example of
bio-utilization or biotechnology, and in combination with
traditional engineered or technical components, can take on
the form of bio-hybrid approaches. In the case of streambank
stabilization and restoration ecology practices using large woody
debris (LWD—e.g., fallen trees, stumps, rootwads, and branches)
(Svoboda and Russell, 2011), their biological analogs are in beaver
dams and complexes (Wright et al., 2002), natural woody debris
(WD), and natural log jams (Larson et al., 2001). Naturally
occurring LWD jams were removed from many rivers for
flood control and navigation during the 20th century
(Montgomery et al., 2003), but these structures are currently
being re-introduced due to benefits such as habitat complexity
and restoration, debris retention, in addition to erosion
protection, stabilization, and grading control (USBR and
ERDC, 2016).
Abbe et al. (1993) studied the distinctive patterns exhibited by
natural LWD jams, identifying categories and types of
accumulations and jams by size, position, orientation,
frequency, and type of WD. Continued in the study by Abbe
and Montgomery (2003), this categorization provides a
framework and typological basis for which to describe the
ways these jams influence stream geomorphology, floodplain
formation, and riparian habitat. Different LWD configurations
and jams produce erosional and depositional zones at varying
lengths downstream of the structure and/or within the structure
depending on hydraulic and geomorphic project objectives.
These jams can also be designed to freely move during higher
velocity flows or persist for centuries as stable structures (Svoboda
and Russell, 2011). The position of logs within a stream channel,
wood density, and decay rates as a function of tree species and
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Stachew et al. Root Research for Bioinspired Design
TABLE 1 | Analogy table (“Abstraction and Analogy”section).
Biological role models Functions/Working principles Problems/vulnerabilities
Soil erosion 1 Root/soil plate network behaving as one
entity due to adhesion between soil particles
and presence of root hairs Coutts (1983);
Bailey et al. (2002)
Network of thread-like elements in contact
with granular media to distribute load
prevents movement of this media in
response to tensile and shear forces
Soil erosion around building foundations; for
example, during heavy precipitation events, or
exposed location on a steep slope/cliff (with or
without precipitation)
2 Single root fan facing upstream deflects
flow, additionally disrupts, partitions and
slows the flow that passes through fan via
drag, resulting in less scour within the
structure Svoboda and Russell (2011)
Single flow deflection structure oriented in
direction of predominant flow, composed of
cylindrical elements with variable length,
cross section, diameter/width, orientation
and curvature arranged in a non-uniform
porous branching pattern that disrupts flow
through structure
High water velocity leading to erosion and poor
habitat conditions
3 Position and orientation of several tightly
placed rootwads in naturally occurring,
stable log jams, including those constructed
by beaver for habitat Abbe et al. (1997);
Abbe and Montgomery (2003);Svoboda
and Russell (2011)
Large cylindrical elements with complex
fractal-like endings facing the flow act as
key anchoring and stabilizing elements of a
single assembled porous yet stable
structure of multiple elements
Coastal erosion and scour, specifically caused by
wave action and reflection
4 Irregular distribution, configuration and
porosity of roots and tree trunks in mangrove
swamps resulting in flow obstruction/wave
attenuation Mazda et al. (1997);Kazemi et al.
(2017)
Semi-rigid elements in a varied distribution
of spacing and orientation in a continuous
and connected system causing wave
attenuation with reduced reflection; also
increasing drag, which reduces
downstream flow velocity and shear stress
High velocities and wave action in nearshore area
leading to coastal erosion, turbidity, poor habitat
conditions due to high water flow and poor water
quality, and inland flooding risk
Structural support 5 Root system architecture recruiting large
volume of soil and surface area to support
tree and respond to variable loading
conditions
Structural support through a wider
distributed network of elements
Low resilience of foundation piles to changing
loading conditions due to limited volume of soil
used for support due to simple shape
6 Interweaving of roots and root grafting
between trees of same species contributing
to mechanical support Graham and
Bormann (1966);Kumar et al. (1985);Keeley
(1988)
Continuous weaving of thread and stem like
elements into a connected network in
granular media
New engineering structures not connected to or
benefiting from existing artificial structures
already in place
7 Asymmetric root morphology resisting
asymmetric loading conditions due to wind
and weight of tree canopy Young and
Perkocha (1994);Nicoll and Ray (1996);
Nicoll and Dunn (2000);Tamasi et al. (2005)
Structural adaptation under asymmetrical
load by increasing number of rigid elements
on the compression side and thread-like
elements on the tension side
Engineering structures not designed to support
and adapt to specific directional loading
conditions
8 Differentiated root morphology for sloped
terrain Reubens et al. (2007);Danjon et al.
(2008);Stokes et al. (2009);Liang et al.
(2017)
Main deep sinker element providing
anchorage with shallow thread-like
elements retaining soil particles in a sloped
terrain to stabilize structure and media
Engineering structures—such as foundations
and coastal infrastructure—lacking specialized
adaptation or design for sloped terrain
9 Adapted root distribution to chemical and
mechanical soil conditions Ennos (2000)
Adaptation of structural morphology to
changing environment
Fixed engineering structures unable to change/
adapt to changing environment
10 Mangrove root morphology supporting and
erating the tree in both low-tide (roots
surrounded by air) and high tide (roots
surrounded by water) environments Ohira
et al. (2013);Hogarth (2015)
Flexible branching/network able to transfer
varying loads to granular media when
surrounded by fluid of different densities
Structures built for one water level not effective
outside of their designed range (e.g., seawall
height unable to counter sea level rise)
11 Buttresses transferring loads from the trunk
to the soil/root plate Young and Perkocha
(1994);Crook et al. (1997)
Element connection shape optimized for
stress reduction based on the tension
triangles rule Mattheck et al. (2006)
Stress concentrations in connections
12 Development of a "T" or "I" cross section in
structural roots Nicoll and Ray (1996);Nicoll
(2006)
Adaptation of the element’s cross-sectional
profile in response to specific loading
conditions
Fixed cross section of elements, overdesigned to
resist diverse loading conditions
13 Design of lateral roots and root hairs that
physically attach to soil particles at the micro
scale Bailey et al. (2002)
Increase loading capacity of macro
structures through skin frictional contact
between granular media and network of
thread-like elements by integrating highly
textured micro surfaces
Foundations designed at macro scale not utilizing
micro interactions between foundation and soil
particles to increase loading capacity
14 Root mucilage enhancing bond strength
between soil particles and roots to
counteract soil shrinkage/expansion caused
by rapid wetting/drying cycles Czarnes et al.
(2000);Galloway et al. (2020)
Increase loading capacity of macro
structures by attaching thread-like elements
to granular media with chemical adhesion
Foundations not chemically connected to the soil
particles at the micro scale for increased loading
capacity
(Continued on following page)
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Stachew et al. Root Research for Bioinspired Design
moisture content, all affect the structure’s stability and life
expectancy, but will commonly exceed the design life of most
engineering projects (Abbe et al., 1997).
LWD drag depends on the cross-sectional area of a flow
obstruction, incident flow velocity, an “obstruction form
descriptor”coefficient, and a blockage coefficient equal to the
ratio of the structure’s total cross-sectional area to the channel
cross-sectional area perpendicular to flow (Gippel et al., 1992;
Abbe and Montgomery, 1996;Abbe et al., 1997). Similar
geometrical parameters governing drag are also seen in a
study conducted to examine the flow-structure interactions of
modeled mangrove circular patches (Kazemi et al., 2017).
Porosity, defined in the study as the ratio of submerged root
volume to total defined volume, spacing ratio between cylindrical
models of mangrove roots, and flexibility are influencing
parameters for drag and mean downstream velocity.
ABSTRACTION AND ANALOGY
Based on the review of root biology and current problem areas, an
analogy table (Table 1) was created to link relevant biological
principles with technical problems or vulnerabilities in the civil
and coastal engineering fields via an identified abstracted
function and working principle. The table can be read from
both sides: starting with root biology, it allows for linking the
working principle to an engineering field and starting with the
technical problem area, it allows for linking to a working principle
also present in biology. The main themes (i.e., soil erosion,
structural support, soil penetration, conditions for living
organisms, and multifunctionality), point to broader problem
areas. This table provides an overview of the translation
opportunities that were found from investigating both biology
and engineering through the lens of biomimetics.
TABLE 1 | (Continued) Analogy table (“Abstraction and Analogy”section).
Biological role models Functions/Working principles Problems/vulnerabilities
Soil penetration 15 Cone shaped root morphology due to
growth resulting in diameter gradient along
root axis from thin root tip (earliest growth) to
thick root flair (mature growth)
Tapered element to facilitate penetration of
granular media
Mechanical resistance of soil overcome with
higher forces to penetrate soil
16 Contractile root behavior pulling the plant
into the soil to protect plant organs from
extreme temperature, low moisture and
increase anchorage North et al. (2008)
Creating a shortening of the attachment to
lower attached element to reduce exposure
to extreme conditions, also increases
tensile force and improves anchoring
Engineering structures degrading over time
under weathering and tensile structures yielding
under constant loading
17 Root turning in the soil by differential growth
response, triggered by auxin distribution in
the elongation zone Chen et al. (1999);
Blancaflor and Masson (2003)
Turning in a granular media by differential
expansion of a thread-like element
Inability to change direction of soil penetration in
granular media when driving foundation piles into
soil, mostly vertical or near-vertical orientation
18 Root hairs and root curvature anchoring the
root allowing the root tip to move forward in
the soil due to cell elongation Bengough
et al. (2011)
Combination of functions: Anchorage and
size expansion from anchoring point,
therefore resulting in forward movement
Construction equipment limited to pushing and
expanding a structure just from the surface
through the soil
19 Circumnutations of root tip to find path of
least resistance in the soil to facilitate growth
Minorsky (2003);Migliaccio et al. (2013)
Moving the tip of the digging element in a
circular or spiral path to find least resistance
regions in granular media
Difficulty of finding path of least resistance when
digging or pushing through granular media
Conditions for
living organisms
20 Space between mangrove roots differing
with respect to height Twilley and Day (2013)
Distribution and geometry of voids with
respect to organism body size supporting
habitats for organisms, diverse predator-
prey interactions and prey refuge
Lack of habitat complexity along hardened
shorelines reducing diverse food web
interactions
21 Snag/root roughness preferred substrate for
invertebrate colonization, increasing
foraging habitat for prey fish Angermeier and
Karr (1984);Wallace and Benke (1984);
Benke et al. (1985)
Heterogeneous surface textures and
structures
Hard, flat and smooth surfaces of coastal
infrastructure reducing habitat availability for
sessile or habitat-forming organisms
Multifunctionality 22 Thermal energy absorption from the soil by
roots and distribution to the tree Ballard et al.
(2009)
Utilizing stable temperature of soil to heat/
cool a system
Current building foundations not designed to
actively contribute to geothermal exchange in
buildings
23 Mangrove root adaptation in anaerobic, high
salinity, waterlogged soils Robertson and
Alongi (1992);Hogarth (2015)
System able to develop in harsh
environment due to adaptive survival
strategies that creates favorable
environment for other systems to function
and exchange resources
Static, heavy and bulky structures required to
provide stability of waterlogged muddy soils
eliminating space for natural habitat
24 Root system and soil exchange of nutrients,
carbon and water, also between mycorrhizal
fungi when present
Constant exchange of resources with the
environment to enhance growth and
adaptation to stimuli
Engineering structures unable to facilitate
exchange of water and resources with the soil
(e.g., water uptake, water discharge, carbon
sequestration)
25 Self-healing properties of trees by
accretional growth around wounds Bloch
(1952);Cremaldi and Bhushan (2018)
Adaptive gap closure through material
accretion
Engineering structures—such as foundations -
often inaccessible for active repair
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APPLICATION OF ROOT BIOLOGY TO
TECHNICAL DESIGNS
In this section, we present the two main application areas for the
problem-driven biomimetic approach, building foundations and
coastal infrastructure. We discuss current practices, limitations
and shortcomings, followed by a broad listing of current
experimental and innovative solutions. We end by exploring a
range of speculative, bioinspired design concepts informed by
root biology to illustrate the biomimetic approach.
Building Foundations
Current Foundation Design and Problem Review
Building foundations transfer aboveground structural stresses to
the underlying soil by transmitting gravitational loads, stabilizing
the structure against overturning and lateral movement, and
providing resistance to uplift. Current foundations function by
creating a contact surface area with the soil bearing the loads, pre-
consolidating the underlying soil, utilizing the foundation weight
under gravity, and/or anchoring it to a rock layer (Hobst and
Zajíc, 1983).
Depending on soil conditions and loading scenario,
foundation design follows two main types: shallow and deep
foundations. Shallow foundations, such as strip footing, spread
footing, or raft, transfer loads to the soil close to the soil surface
and are used for low loading capacities. Deep foundations, such as
piles, drilled shafts, and caissons, are used for high intensity heavy
building types and resist lateral and uplifting forces. They are also
used when the upper layers of soil are weak, collapsible,
expansive, or subject to soil erosion (Das, 2007). They can
reach depths of hundreds of meters into the ground (Frost
et al., 2017).
The structural capacity of foundation piles depends on the
bearing capacity of the pile tip and lateral friction of the pile
(Das, 2007). Foundation pile design is determined by loading
type, subsoil conditions, and location of the water table. In weak
soils, point bearing piles are built up to the rock surface or into a
strong soil layer if within reasonable depth. Otherwise, piles
relying on friction with the soil particles or increased soil
compaction are placed. In clayey soils, adhesion also helps to
holdthepileinplace(Das, 2007). Vibro-compaction and vibro-
replacement methods are economical and well-established
techniques to improve weak or loose soils through
compaction (Baumann and Bauer, 1974). Depending on pile
design and material, different techniques are used to insert them
in the ground. Piles are driven in the soil with various types of
hammers or vibratory drivers, but other techniques may be
employed for specific scenarios such as jetting and partial
augering (Das, 2007).
Typical foundation piles are made of wood (e.g., timber
piles), concrete (e.g., precast or cast-in-situ piles), and steel
(e.g., pipes or rolled H-section piles) (Das, 2007). Steel piles are
easily managed, supporting high driving stresses, penetrating
hard soil layers, and carrying relatively high loads. They are
expensive, subject to corrosion, and may be damaged during soil
insertion. Precast concrete piles also support high driving
stresses while resisting corrosion, but they are more difficult
to maneuver and properly cut. Cast-in-situ piles are cheaper,
and the steel cast can be inspected before pouring the concrete,
but the casing may be damaged during soil insertion and the
resulting pile can be difficult to connect after pouring. Timber
piles are limited in terms of driving force and loading conditions
(i.e., capacity and direction). Composite piles are composed of
different materials which are difficult to join, so they are not
widely used (Das, 2007).
There are several limitations and shortcomings to current
foundation design, engineering, and construction practices. First,
deep foundations are limited to simple vertical or near vertical
(i.e., 0°with respect to the pile axis) cylindrical piles, due to the
inability of current drill and dig construction techniques to
actively change direction in the soil (Frost et al., 2017). It has
been demonstrated that increasing the angle of foundation piles
from 0°to 15°and 30°increases the loading capacity of the
foundation due to a larger bearing area (i.e., surface area of the
soil in contact with the pile and supporting pile weight) (Frost
et al., 2017). Compared to a traditional smooth vertical pile, the
introduction of a branching angle of 15°doubled the downward
bearing capacity, and a branching angle of 30°tripled this capacity
(Frost et al., 2017). Additionally, orchard tree root systems
showed an increase of vertical pullout resistance by
8–13 times compared to traditional micropile foundations of
comparable volume and mass (Burrall et al., 2020). Second,
foundations are monofunctional as they are only designed to
support a structure, while we use other artificial subsurface
technical structures for other functions (e.g., energy
conversion). Third, the capacity of foundations to resist loads
and forces is not dynamic and adaptable (some exceptional
technologies exist for earthquake prone applications).
Foundations are usually built as static structures and are
expected to maintain morphology and materiality over time.
They cannot adapt to changing environmental conditions, such
as varying loads applied to the structure and soil movement,
therefore operating on a single timescale. Lastly, foundations are
located underground, therefore inaccessible for maintenance.
The use of materials that lack self-healing properties requires an
over-design to counter this potential drift in performance
over time.
Root systems can serve as inspiration as they share similar
functionality and design requirements with foundations such as
anchorage and soil penetration, but also provide adaptability and
multifunctionality. Root systems possess a large bearing area
compared to their volume, due to their branched morphology
and the presence of microstructures. Complex root morphology
is also a result of the multiple functions provided for the tree such
as soil exploration, nutrient/water exchange and transport,
anchorage, and thermal regulation, which in turn provides
additional ecosystem services such as erosion prevention.
Additionally, root systems adapt and respond to stimuli over
multiple timescales (e.g., daily fluctuations and constant long-
term loads) through transient (e.g., damping) and permanent
responses (e.g., reaction wood growth, self-healing). Since root
systems are part of a living multicellular organism, they can heal
and regenerate tissues of their anatomy. The foundation designs
of the future could mimic these root system strategies.
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With this biomimetic approach to foundation design, multiple
research questions arise: Which design parameters informed by
root systems would increase the loading capacity of foundations
without adding weight? How can complex branched structures be
inserted in soil with minimal disturbance? How can a foundation
actively or passively adapt to changing external internal loads as
informed by root adaptation mechanisms? Which additional
functions could be fulfilled by foundations other than
anchoring a building in place? How could biological
organisms be integrated into bio-hybrid foundation designs
and to what benefit?
Current Innovative Solutions for Foundation Design
In the following list, we summarize current innovative strategies
for future foundation designs, from morphological variation to
integration of biological organisms. They are organized under
four main topics of interest referring to the analogy table in
“Abstraction and Analogy”section (Table 1): soil erosion,
structural support, soil penetration, and self-healing, as an
aspect of multifunctionality.
•Preventing soil erosion—Various geosynthetic products are
available on the market. The stabilizing effect of a thread-
like element in granular media has been investigated by the
placement of a textile filament layer by layer around loose
rocks and exposed at the Chicago Architecture Biennial
2015.
1
Additionally, bacteria that bind to soil particles, have
been used to strengthen the mechanical properties of soil
through Microbial Induced Calcium Carbonate
Precipitation (MICP) (DeJong et al., 2006;Whiffin et al.,
2007;Van Wijngaarden et al., 2011). The use of genetically
modified bacteria to precipitate calcium carbonate when soil
pressure is detected to react to changing loading conditions
is tested with computational models (Dade-Robertson et al.,
2018).
•Geometric alternatives to support structures—Foundation
geometry is a defining factor for total loading capacity and
pile displacement (Frost et al., 2017). Conical piles provide
an increased bearing capacity compared to straight-sided
cylindrical piles (Manandhar and Yasufuku, 2012). The
lateral surface texture of foundation piles is another
parameter to increase loading capacity by increasing
shear strength of its interface with soil (Martinez and
Frost, 2017). Research in this field stressed the need to
design foundation surface roughness, in opposition to
current smooth or only randomly structured construction
materials, such as randomly textured high-density
polyethylene geomembranes and roughly finished
concrete (Frost et al., 2002). Biological textures, such as
snakeskin, were studied to produce bioinspired surfaces
designed for foundation piles and yielded promising
results for increasing directional friction (Martinez et al.,
2018;Martinez et al., 2019).
•Robots for soil penetration—Due to the difficulty of
inserting non-linear structures in soils, burrowing robots
inspired by animal (Calderón et al., 2016;Khosravi et al.,
2018;Calderón et al., 2019) and plant strategies (Sadeghi
et al., 2014;Hawkes et al., 2017;Sadeghi et al., 2017;Dottore
et al., 2018;Greer et al., 2019;Ozkan-Aydin et al., 2019) have
been explored in the past decade. For example, an
earthworm inspired robot mimicking peristaltic waves by
activating axial and radial contraction, was built with three
silicone body segments and able to crawl through straight
and curved pipes (Calderón et al., 2016;2019). While
current soil monitoring techniques use probes that are
pushed in the ground, self-burrowing probes, based on
radial expansion of sections of the probe, have been
studied and simulated in sandy soils (Khosravi et al.,
2018). Animals use their musculature to move and dig in
soils whereas the root tip of plants grows through the
substrate (Sadeghi et al., 2014). Root systems, and
especially root tip growth, have served as inspiration for
growing robots (Del Dottore et al., 2018). For example, root
tip growth has been translated into a robot that can sense its
environment and grow in diverse directions through
additive manufacturing (Sadeghi et al., 2014;Sadeghi
et al., 2017). Directional growth by extension of the body
tip has also been translated in soft robotics to conform to
constrained environments (Hawkes et al., 2017). In
addition, the influence of circumnutation to facilitate soil
penetration has been tested with artificial probe tips
(Dottore et al., 2018). These movements have also been
implemented in a soft robot growing in a 2D environment
(Ozkan-Aydin et al., 2019).
•Self-healing–Self-healing in biology has been explored and
is being translated into bioinspired healing materials with
the following mechanisms: protective coating, autogenous
healing, shape memory, chemical activity, vascular systems,
and bio-healing (Cremaldi and Bhushan, 2018). Bio-healing
refers to the use of biological organisms to perform self-
healing, such as spore-forming bacteria in self-healing
concrete (Jonkers, 2007). Concrete also can self-heal cracks
with water and carbon dioxide through chemical activity (Li
and Yang, 2007). The crack closure of two different systemsof
self-healing concretes, based on polyurethane and
superabsorbent polymers, has been successfully tested on
large-scale prototypes (e.g., concrete beams of 150 mm ×
250 mm ×3000 mm) (Van Tittelboom et al., 2016). Self-
healing concrete has yet to be tested at the scale and
environmental conditions of building foundations.
Root-Inspired Design Proposals for Building
Foundations
Studying, abstracting, and transferring biological root system
strategies to the field of foundation engineering can yield
innovative designs, addressing the shortcomings of current
foundation designs. In the following section, various
bioinspired design strategies are presented at an abstract
conceptual level, disregarding scaling and materiality at this
point, which should be explored in further research projects.
1
https://selfassemblylab.mit.edu/rock-printing/
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Root-Inspired Erosion Prevention
The following soil retention concept for a foundation is inspired
by the erosion prevention of root systems in sloped terrains. Root
systems have the capacity to prevent soil erosion by soil particle
retention through entanglement and chemical bondage, but
foundations are only designed to structurally support a
structure (row 1 of Table 1). Foundations located in
environments subject to soil erosion such as riverbanks, cliffs,
or steep slopes would benefit from erosion prevention measures.
The root system of a tree growing in sloped terrain develops
vertical roots along with horizontal thin lateral roots retaining soil
particles downslope of the tree (row 8 of Table 1). A technical
building foundation could mimic this strategy and combine a
main vertical structure to anchor the building with a secondary
structure to retain soil particles (Figure 1). A mesh, similar to
existing geosynthetic fabrics or a network of laterally branched
elements near the surface, can be integrated in the design of
foundations to reduce erosion. Soil particle retention could be
achieved through chemical or mechanical attachment.
Root-Inspired Structural Support
The first concept of root-inspired structural support is inspired by
the root grafting strategy found in forests to achieve cooperative
building foundations (row 6 of Table 1). A newly constructed
foundation can be connected to existing infrastructures to
increase the load bearing area and the volume of soil recruited
to support the structure, while making the foundation more
resilient under extreme loading scenarios. Additionally, the
connection to existing structures can provide an interface to
exchange resources, such as water and thermal energy. The
multifunctional aspect of this network of foundations is
further described in section “Multifunctional Root-Inspired
Foundations”.
The second concept is on structural optimization based on
both root adaptation to specific loading conditions as well as
machine learning (row 7 and 12 of Table 1). Studying the
adaptation of root systems to changing loads and
environments can inform the design of root-inspired structural
support systems subjected to similar loads. Computer simulations
and machine learning can be used to process root adaptation data
and apply the algorithms to foundation design. The following
steps are required. First, root trait data about adaptation to
various loading scenarios needs to be collected. A database
will be populated with relevant traits in relation to the type of
loads applied to the tree. A machine learning algorithm can then
simulate how a root system would react to specific loading
conditions. Finally, the morphology of this simulated root
system could be used to inform the design of a new
foundation (Figure 2).
The third concept of root-inspired structural support aims at
translating the hierarchical structure of root systems for the
transfer of structural loads to soil particles down to the
microscale (row 13 of Table 1) through highly textured
foundation surfaces. As this is interconnected with the soil
insertion techniques, those concepts are explored in section
“Root-Inspired Soil Penetration Devices”. Biological
adhesives could also be secreted by the foundation to
strengthen the bond between the foundation and soil particles
as an analogy for mucilage (row 14 of Table 1).
Root-Inspired Soil Penetration Devices
In biology, multiple mechanisms allow organisms from animals
to plants to move through granular media. The main question
addressed in the following concepts is how to transfer biological
strategies of root systems to an artificial soil penetrating system.
The first concept of root-inspired soil penetration is on
foundation pile tips inspired by the tapered root tip
geometry that facilitates soil penetration (row 15 of Table 1).
The tip geometry of a semi-flexible linear element affects its
interaction with soil particles and the resulting path through
soils during soil insertion. Therefore, controlling tip geometry
could serve to guide a semi-flexible pile to follow a specificpath
(Figure 3).
The second concept of root-inspired soil penetration is on
branched foundations, emerging from the previous concept on
tip geometry. First, the cross section of a pile made of semi-
flexible material is extended into multiple thinner elements.
Driving this dissected pile in the soil will produce a branched
geometry that increases the load bearing area (row 5 of Table 1).
The branched geometry is expected to be a result of the pile
material properties, geometry of the dissected elements and their
FIGURE 1 | Soil retaining concept of a foundation on sloped
terrain—The primary deep foundation piles support the structure beyond the
potential shear surface and the secondary root-inspired network holds soil
particles in place.
FIGURE 2 | Structural optimization concept based on root adaptation
and machine learning—The design of a root-inspired foundation follows the
root traits adapted to a specific loading condition.
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Stachew et al. Root Research for Bioinspired Design
tips, and soil properties (Figure 4A). The tip geometry can also be
actively controlled to distribute the branched structure
throughout the soil in a specific arrangement. This concept of
branched foundations can be applied to an entire pile tip
(Figure 4A) or tip parts (Figure 4B).
The third concept of root-inspired soil penetration is on
hierarchical foundations, based on the ability of roots to
produce a complex branched structure in the soil through
initial insertion of linear elements only. This strategy
facilitates soil penetration while providing structural support
at a later stage. Following this analogy, foundations can be
designed for multi-phase implementation. A smooth linear
vertical foundation pile can first be inserted in the ground.
Thinner linear elements can then be pushed from this vertical
pile into the soil laterally to improve anchorage (Figure 5).
These lateral elements can also serve as anchors to push against
while the foundation tip is driven deeper into the soil through
axial expansion (row 18 of Table 1).
FIGURE 3 | Foundation pile tips concept—When driven in the soil, a semi-flexible foundation pile with a symmetric tip remains straight (A). In practice, soil particle
arrangement will cause minor deflections depending on pile and soil properties. With an asymmetric tip, the same pile is expected to turn toward the acute side (B,C).
FIGURE 4 | Branched foundation concept–This figure shows the application to the entire pile (A) or to parts of the tip (B). When the pile is driven into the soil, the
dissected elements follow different paths based on their geometry, flexibility and soil properties. The dissected elements can be controlled to reach a desired depth.
FIGURE 5 | Hierarchical foundation concept—First, the smooth vertical pile is driven in the soil (left). Once the pile is in place, individual semi-flexible elements are
laterally pushed into the soil (right).
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Stachew et al. Root Research for Bioinspired Design
The fourth concept of root-inspired soil penetration devices is
on texture alteration through material decay. Biodegradable
material is placed around a highly textured foundation pile to
create a smooth surface which facilitates soil penetration. Once
inserted in the soil, this material will biodegrade and expose the
highly textured surface, from the third concept of root-inspired
structural support (“Root-Inspired Structural Support”section,
row 13 of Table 1 and Figure 6). For this concept, additional
bioinspiration of directional friction is interesting, especially if the
directionality of the surface structure could change over time and
by this, control the movement of the element through the soil.
Bacteria known to precipitate calcium carbonate can be
introduced under the biodegradable layer to further strengthen
the bond between the foundation and soil particles.
Multiphase design was further conceptualized through the
investigation of shape-change materials and structures for
increased friction of foundation piles with weak soils, for
example in wetlands (refer to the third concept of root-
inspired structural support in “Root-Inspired Structural
Support”section and row 13 of Table 1). Shape-change
behaviors are used in three different concepts to counter the
trade-off between the ease of pile insertion in soils and the surface
friction of the pile.
The first concept of shape-change foundations is based on the
swelling properties of hygroscopic materials when they absorb
water. A hygroscopic material is located behind a biodegradable
layer along the surface of a pile (Figure 7B). After placement in
the soil and decomposition of the biodegradable layer, the
hygroscopic material becomes exposed to water in wetland
soil. The water triggers material expansion, creating a three-
dimensional structure to increase the surface contact with soil
(Figure 7).
The second concept of shape-change foundations is based on
bi-layer materials, which change curvature under humidity
gradients. Bi-layer plywood materials, inspired by pinecones,
have been researched for their ability to bend under humidity
gradients and applied to architectural prototypes (Menges and
Reichert, 2015). Such composite material is located at the surface
of the foundation pile. Once inserted in the soil, water absorption
induces curvature change of the bilayer elements (Figure 8). The
success of such shape-change concepts also depends on the
resistance of the soil particles.
FIGURE 6 | Texture alteration concept—First, the smooth foundation pile is driven into the soil (left). Over time, the biodegradable material will decay (middle),
leaving the highly textured surface in contact with the soil particles (right).
FIGURE 7 | Shape-change foundation based on hygroscopic materials—The smooth foundation pile is driven into the so il (A-left), then the biodegradable material
decays (A-middle). The material decomposition exposes the hygroscopic material to the saturated soil, resulting in a three-dimensional structure and increased
anchorage through friction (A-right), (B-left and right) show the disposition of the materials before and after the decomposition of the biodegradable material.
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The third concept of shape-change foundations is based on
the behavior of auxetic structures with a negative Poisson’s
ratio. When stretched or compressed in one direction, they
also respectively expand or compress in the perpendicular
direction. By assembling auxetic and non-auxetic structures
together in a plane, stretching of the assembly in one direction
(see yellow arrows on Figure 9) induces a geometrical change
of the structure (see red and blue arrows on Figure 9A)
(Mirzaali et al., 2018). The assembly needs to be made of a
semi-flexible material to allow material deformation. The flat
assembly can be rolled to produce a cylindrical structure for a
foundation pile (Figure 9B). During soil insertion, the
structure can be locked and, once in place, released.
Compressive or tensile loads on the auxetic foundation pile
will create wrinkles leading to a higher bearing surface area
(Figure 9C).
Multifunctional Root-Inspired Foundations
The multifunctional foundation concept is inspired by the
added functionality in biological root systems and targets
preventing erosion and exchanging energy and resources
with the soil and other artificial structures (row 1 and 22 of
Table 1). With the development of self-burrowing
technologies and smart materials, multifunctional
foundations can be envisioned. The benefits of erosion
prevention have already been stated in “Root-Inspired
Erosion Prevention”section. Foundations and geothermal
systems abide by the same constraints of soil penetration and
anchorage. Their combination into a multifunctional system
could economize resources. Another further strategy to
exchange thermal energy is to connect buildings through
their foundations (refer to the cooperative concept in
“Root-Inspired Structural Support”section). Appliances
producing massive amounts of heat, such as data centers,
can serve as a heat source for buildings (Woodruff et al., 2014).
In addition to thermal energy, other resources such as water
can be exchanged between buildings (row 6 of Table 1). By
increasing load transfer throughfrictionwiththesoilmedium,
the surface area of the foundations needs to be increased, but
their weight can be decreased. As a result, hollow foundations
canbearouteforadditionalfunctionality, such as geothermal
energy, water storage and transport.
FIGURE 8 | Shape-change foundation based on bi-layer materials—The smooth foundation pile is driven into the soil (A-left). Over time, the bi-layer composite
material, exposed to humidity, curves outwards resulting in increased anchorage through friction (A-midd le), (A-right) presents a top view of this deployed pile system.
(B-left and right) show the disposition of the bi-layer composite material and the belt holding it in place, before and after the curvature change.
FIGURE 9 | Shape-change foundation based on auxetic behavior—The combination of auxetic and non-auxetic structures in a plane produces edge curvature
when compressed or stretched as simplified in (A). When this combined structure is longitudinally stretched (i.e., yellow arrows), the auxetic section (i.e., in red) stretches
while the non-auxetic one (i.e., in blue) shortens (A-left). The reverse behavior happens when the combined structure is compressed longitudinall y (A-right). When rolled
into a cylinder (B), the longitudinal compression or stretching produces horizontal wrinkles (C). This cylinder can serve as a vertical foundation pile to resist
compression and tensile loads.
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Stachew et al. Root Research for Bioinspired Design
The concepts presented in this section provide examples of
how strategies found in root systems can inform the design of
future foundations. These concepts do not take materiality,
scaling, and rigorous technical feasibility into consideration
however, but they should be the basis for future research and
development projects.
Coastal Infrastructure
Current Coastal Infrastructure Design and Problem
Review
Typical built coastal infrastructure serves two main objectives:
protection from wave action and landward erosion (USACE
and Army Engineer Waterways Experiment Station Coastal
Engineering Research Center, 1984). While generally effective
at these objectives, coastal structures are static, often
anchored, and therefore cannot adapt to rapid, dynamic
conditions. In light of climate change, current static
structures do not hold up to raising water levels, storm
surges, and flooding. For example, a post Hurricane Katrina
rebuild of the New Orleans, LA, USA seawall was almost
overtopped by waves from storm surge in 2018.
2
Additionally, hard infrastructure alters and displaces the
structure and function of natural habitats that existed
before, eliminating both significant biodiversity, and habitat
complexity that supports trophic structure development for a
rich, interconnected food web, refuge for mobile organisms
and fish, and attachment surfaces for sessile and habitat-
forming organisms (Strain et al., 2018).
Common typologies of hardened infrastructure include:
shore-parallel attached smooth vertical or concave surfaces
(e.g., seawalls, such as bulkheads), shore-parallel attached
sloped variable surfaces (e.g., revetments, such as riprap),
shore-perpendicular attachments (e.g., groins and jetties),
detached shore-parallel sloped above-water structures (e.g.,
breakwaters), and detached shore-parallel submerged
structures (e.g., breakwaters and artificial reefs). Shore-parallel
attached structures prevent erosion of land from wave action but
fragment the land-water interface and contribute to the loss of
natural habitats (Goodsell et al., 2007). Seawalls and some
revetments reflect waves, which increases nearshore turbulence
(Silvester, 1972). Often this turbulence is too rough for native
plants to establish and maintain, attracting invasive species
establishment. Increased turbulence also increases sediment
resuspension and reduces water clarity.
3
Depending on wave
action and nearshore particle size, sediment may be carried
through wave reflection out into the open shore, reducing the
available sediment budget for natural littoral deposition
processes. Riprap revetments can fail due to toe scour,
outflanking, wave overtopping and subsequent erosion of
material behind the revetment, and settlement. Wave reflection
also causes scour, deepening the water level adjacent to a seawall,
allowing for larger wave heights to approach the shore (Griggs
and Fulton-Bennett, 1988). Shore-perpendicular attached
structures redirect littoral transport to prevent erosion or
allow river mouths to remain deep enough for navigation in
the case of harbor infrastructure, but often cause downdrift
erosion due to a reduced available sediment budget for
continued nearshore transport. This disruption of natural
littoral processes induces a negative feedback loop, requiring
more downstream infrastructure to protect against this erosion
(Hanson and Lindh, 1993). Both detached above-water and
submerged structures attenuate waves (through surface wave
breaking and bottom roughness, respectively) and provide fish
habitat, but above-water structures restrict coastlines from
migrating landward or seaward in response to varying water
levels (McLachlan and Defeo, 2018;Scape/Landscape
Architecture PLLC, 2014).
Mangrove forests show a pathway to remediate these
shortcomings. Mangrove roots stabilize soils, while their
ecosystem provides habitat and a gradual land-water
transition. On a long time scale, mangrove forests migrate
landward or seaward in response to varying water levels
(Robertson and Alongi, 1992). Wave dissipation through these
complex flow obstruction configurations significantly reduces
wave reflection and subsequent turbulence in the nearshore
environment. Even if mangroves are overtopped by waves
during storm surge, the roots and trees still provide adequate
bottom roughness and flow obstruction to effectively attenuate
wave energies (Mazda et al., 1997). Manmade constructions using
wood, such as rootwad revetments, engineered log jams, crib-
walls, deflectors, weirs and pile dikes, also stabilize soils, reduce
flows, while also providing habitat and maintaining a more
gradual land-water transition. Interestingly, while these
structures are cheaper, exceed project design life, and often
match or exceed performance objectives compared to rock
structures, these LWD human constructions are rarely used
(Abbe et al., 1997). Additionally, mangrove roots, naturally
occurring log jams, and woody overhang along riverbanks or
shorelines, provide habitat for a variety of organisms. Complex
morphologies, such as root systems, protect from wave action and
stabilize sediment to primarily provide anchorage for an
aboveground structure, as well as provide habitat. Complex
morphologies would similarly allow for multifunctional coastal
infrastructure design.
To undertake a redesign of coastal infrastructure that expands
beyond its primary objectives of protection from wave action and
landward erosion, a biomimetic approach via the study and
abstraction of root systems can be employed. Investigating
specific themes of erosion prevention, multifunctionality,
spatial variability, and adaptation to dynamic external loads
involves answering the following research questions: What is
the minimum level of complexity required from a root-inspired
structure (e.g., topology, orientation of elements, density,
distribution of individual cross-sections across topological
orders, distribution of orientation across topological orders,
texture) and at what scale(s) is it most effective for the
following functions of (a) wave dissipation and dispersion (vs.
wave reflection), (b) downstream development of reduced flow
velocities through the depth of the water column that match
2
https://www.nytimes.com/interactive/2018/02/24/us/new-orleans-flood-walls-
hurricanes.html
3
https://www.mishorelinepartnership.org/erosion-at-the-shoreline.html
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Stachew et al. Root Research for Bioinspired Design
preferential velocity ranges of native taxa, (c) flow speeds for
sediment deposition, and (d) refuge/pore space habitat creation?
What minimum combination of effective length scales as
informed by root systems are needed to discourage localized
erosion and scour development by dissipating the formation of
vortices? Furthermore, can topographically complex
infrastructure be produced that more closely resembles the
structure and function of the natural habitat (such as woody
overhang, exposed root systems) that has been displaced? With
regards to adaptation and multifunctionality, coastal
infrastructure of the future should adapt to changing external
loads such as wave height, storm surge, sediment movement, or
landslides in sloped banks or shorelines. Coastal infrastructure
should also participate in additional ecosystem services such as
provision of habitats, nutrient cycling, and carbon
sequestration.
Current Innovative Solutions for Coastal Infrastructure
Design
In the following list, we summarize current alternative
approaches to traditional engineered coastal infrastructure
design, spanning complex forms, coastal ecosystem restoration,
to living infrastructure. They are organized under four main
topics of interest referring to the analogy table in “Abstraction
and Analogy”section (Table 1): soil erosion, structural support,
conditions for living organisms, and multifunctionality.
•Geo- and bio-textile fabrics to prevent soil erosion–As
mentioned in “Current Innovative Solutions for
Foundation Design”section, geosynthetic products are
currently used to stabilize soils through placement of a
polymeric textile filament layer by layer around loose
rocks, gravel, or sediment. This practice is also seen in
coastal engineering. Geotextile tubes or bags, a synthetic
fabric filled with sediment, are used to line riverbanks,
shorelines, or protect young plant seedlings as part of a
nearshore ecological restoration initiative. Biodegradable
coconut coir pith logs packed in tubular netting, known
as coir logs, are an example of soil bioengineering that
reduce water velocities at the edge of slopes, shorelines, and
riverbanks (Rella and Miller, 2012).
•Complex concrete forms for increased structural
support—Concrete forms for revetments, breakwaters, or
additional reinforcement of seawalls have become more
complex since the 1950s with inventions such as
Tetrapods, Akmons, Seabees, Accropodes, Xblocs, dolos,
and KOLOS. Their complex shapes, pack density, and
porosity allow for wave dissipation that reduces wave
run-up, overtopping and reflection, but also facilitates
interlocking of individual units and increased stability of
the overall structure (Dupray and Roberts, 2009).
•Establishing conditions for living organisms through
ecosystem conservation and restoration—Wetlands,
mangroves, coral reefs, oyster reefs, and salt marshes are
proving cheaper and more effective in reducing wave energy
than building hard artificial structures. Meta-analysis of the
literature indicates that coral reefs reduce wave heights by
70%, salt marshes by 72%, mangroves by 31%, and seagrass/
kelp beds by 36% (Ferrario et al., 2014;Narayan et al., 2016).
•Establishing conditions for living organisms through eco-
engineering—Locations such as harbors, nearshore
navigation routes, and dense urban areas are not suitable
for restoration. In this case, ecological engineering or “eco-
engineering”is an approach that considers recovery of prior
ecosystem services in the design of hard infrastructure
(Mayer-Pinto et al., 2017). Habitat features to increase
fish productivity or biodiversity of key functional groups
of organisms can be integrated via textures, crevices, pits,
intertidal water retaining features, raises, ledges, ridges, and
soft, flexible protruding elements such as rope, ribbon, or
twine (Strain et al., 2018). Grooves, dimples, and grooved
shelf features were incorporated into the submerged toe
blocks of offshore breakwaters in Lake Erie, part of the Great
Lakes freshwater system, to increase habitat for fish and
invertebrates with limited success (Suedel et al., 2016).
•Multifunctional, living infrastructure—ECOncrete uses a
special concrete mix to lower the pH closer to that of
seawater, a common criticism of traditional marine grade
concrete, to facilitate organism attachment and growth
(Finkel and Ido, 2017). The concrete blocks are formed
with molds to create the surface texture and roughness to
promote attachment by oysters, bryozoans, coralline algae,
and several other habitat-forming species (Perkol-Finkel
and Sella, 2015). Uses include offshore breakwaters,
revetments, seawall panels, or attachments to existing
seawall panels.
4
Reef Design Lab 3D prints unique
surface features on seawall panels using marine grade
concrete to improve recreational fishing opportunities
and increase biodiversity, specifically to maximize
colonization of native species.
5
Mangrove Reef Wall was
first studied to understand flow-structure development
behind modeled mangrove roots, as well as wave
attenuation and sediment deposition characteristics to
create bioinspired infrastructure (Kazemi et al., 2017).
The current application of this research is a living seawall
application for wave attenuation, colonization, and
increased biodiversity.
•Multifunctionality of hard infrastructure to assist with
coastal restoration and rehabilitation—Awiderangeof
coastal restoration and rehabilitation projects use hard
modular structures from concrete mixtures. TetraPOT,
by designer Sheng-Hung Lee at National Cheng Kung
University, creates an interlocking system of concrete
pods that use mangrove trees and roots to keep the
pods in place as a line of coastal defense along
shorelines.
6
Reef Design Lab takes a similar approach
with a reusable planter to promote mass planting of a
native mangrove species for coastal defense.
7
CEMEX
4
https://econcretetech.com/
5
https://www.reefdesignlab.com/
6
https://www.jamesdysonaward.org/en-GB/2016/project/tetrapot/
7
https://www.reefdesignlab.com/mangroveplanters
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Stachew et al. Root Research for Bioinspired Design
created the Rhizolith Island (“Isla Rhizolith”) prototype,
consisting of a mosaic of floating concrete structures with
a“head”and a “fin”that functions as a seed carrier for
mangroves, to restore mangrove forests while also
providing coastal protection. The finisalsodesigned
to serve as marine habitat, offering shelter for fish and
surfaces for barnacles.
8
Reef Ball also uses a specialized
concrete mixture to lower the pH and a textured outer
surface to promote growth of transplanted corals. Reef
Ball uses similar principles to develop concrete domes to
serve as oyster beds for oyster reef restoration. Used in
more than 70 countries, on more than 4,000 projects,
there are more than 700,000 Reef Balls in the oceans
around the world.
9
•Complex scaffolding to establish conditions for living
organisms—Grow Oyster Reefs LLC has created complex
concrete scaffolds mimicking the oyster shape, in
addition to mimicking the oyster shell’smaterial
formulathroughacalcium-enriched, patent-pending
mixture that also aims to reduce nitrogen levels in
water.
10
Additionally, Reef Design Lab 3D prints
ceramic scaffolds using D-shape technology to assist
with coral reef rehabilitation.
5
These two examples
show the possibility of production of complex coastal
structures.
Root-Inspired Design Proposals for Coastal
Infrastructure
In this section, several bioinspired design strategies from the
biology of root systems are presented for coastal engineering.
Since these strategies do not depend on the availability of real
mangrove trees, riparian tree species, or rootwads, the properties
of these root-inspired structures can be fine-tuned according to
the learnings from biomechanics investigations. Parameters like
the distribution of cross-sections, lengths, spacing, branching
angles, and orientations, can be adjusted to a specific shoreline
reach with its predominant wave and storm surge conditions.
Additionally, the arrangement, stacking, and orientation of
several root-inspired structures can be adjusted for different
shoreline configurations and wave energy conditions, as well
as intended ecosystem service restoration goals and outcomes,
and/or maintenance strategies.
Root-Inspired Erosion Prevention
The first erosion prevention concept builds upon the concept of
engineered log jams and complexes discussed in “Root
Utilization in Human Constructions”section (row 3 in
Table 1), a windthrown tree overhang along a river or stream
still embedded in the bank by its root system, and mangrove roots
encouraging sediment deposition (row 4 in Table 1). If erosion is
of highest concern, a root-inspired structure (or several
structures) can be inserted perpendicular to a beach or
shoreline face with the root fan embedded in the shoreline
(Figure 10). The multi-scale elements of the root-inspired
structure, such as overall shape, topological orders, and
branching angle/orientation, will need to be tested to
determine their effects on vortex development, localized
erosion, and scour, so as not to be a further detriment to the
shoreline. These multi-scale elements could be engineered such
that vortices do not form (or are quickly dissipated) behind or
downstream from the structure, further enhancing sediment
deposition potential. Additionally, since groins and jetties
cause downstream erosion issues due to a perpendicular
element facing seaward into the nearshore, the seaward end of
a root-inspired structure can be truncated so as not to cause
similar issues. This truncation is shown in Figure 10 (left) as the
transparent ends of the trunk of a 3D modeled rootwad. This
seaward end could then be formed to provide heterogeneous
substrate for habitat. Sediment penetration of the complex root
fan like end of this structure into the shoreline face may be
difficult. Concepts to increase contact area with the sediment
previously described in “Root-Inspired Soil Penetration
Devices”section on building foundations could be employed.
This erosion prevention concept could also be utilized
particularly during high water years to protect shoreline
property. Depending on sediment type (silt, clay, sand, mud),
particle size distribution, and wave energy exposure, this concept
could unintentionally cause localized scour around the large
FIGURE 10 | Erosion prevention concept along a shoreline—The top view (left) shows modeled rootwads embedded along a shoreline with root fan facing
landward and truncated trunk (note transparency in open water) facing seaward. The side view (right) shows one rootwad with root fan embedded along a sloped
shoreline face.
8
https://www.cemex.com/-/cemex-develops-floating-concrete-island-to-revitalize-
mangrove-shorelines
9
https://www.eternalreefs.com/the-eternal-reefs-story/about-reef-balls/
10
http://www.growoysterreefs.com/
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Stachew et al. Root Research for Bioinspired Design
anchoring elements (Svoboda and Russell, 2011) and would need
to be tested to confirm its effects. Additionally, several engineered
log jams or complexes, such as bar apex jams (“Root Utilization
in Human Constructions”section), could potentially be
embedded in beach sediment at different distances from the
water line to provide erosion protection of the entire beach
front. The complexes can be designed more as a fixed
structure, but may still more closely mimic the process of both
large driftwood and windthrown trees near a historically forested
shoreline forming natural protective “structures”along a beach
(Abbe et al., 1997).
The second erosion prevention concept specifically addresses
additional engineered structures in or near waterways that can
cause significant erosion issues. This includes structures such as
bridge abutments and culverts, not primarily used for erosion
prevention of coasts, streams, or riverbanks, that cause localized
scour or erosion at the edge or slightly downstream of the
structure. These engineered structures could be redesigned
based on the geometry of root systems (Figure 11—right), in
order to reduce localized scour and erosion and additionally
deposit sediments further downstream that are a result of scour or
erosion. When implemented, rootwad-inspired structures will
also catch plastic waste (Figure 11—left) that could be collected at
regular intervals, to reduce overall transport of waste to lakes and
oceans. The same structure placed strategically at the bottom of a
river could reduce bedload movement and scour.
Root-Inspired Multifunctional Revetments
Multifunctional revetment design concepts are additional
iterations of the concept described in Figure 10, but with the
root fan facing seaward. This design concept (Figure 12)is
different from current wooden revetments in that there is a
complex flow obstructing end, but similar to riverbank
stabilization practices used in restoration ecology (row 2 in
FIGURE 11 | Erosion prevention concept for engineering structures—A typical exposed root system along a riverbank catching plastic debris (left) can be one of
several functions of an abstracted root structure (right) that could replace the ends of bridge abutments or the edges of culverts to reduce erosion and scour.
FIGURE 12 | Multifunctional revetment concept designs—(A)—A modeled rootwad illustrating wave attenuation with the root fan facing seaward, and the trunk
end embedded along the sloped shoreline face. (B)—Spacing between the root fan and sloped shoreline face shows possible passage for a fish corridor. (C)—A root fan
embedded more in the sediment bottom may provide greater toe protection of a steeper shoreline face. (D)—A mangrove like structure can attenuate waves in addition
to providing habitat (fish refuge) through spacing control.
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Stachew et al. Root Research for Bioinspired Design
Table 1 and “Root Utilization in Human Constructions”
section). One purpose of the root fan like end is wave
attenuation, breaking up and dissipating the waves due to the
density, orientation, and cross-section of the individual elements
in the structure (Figure 12A). Wave attenuation also in turn
reduces erosion potential. The spacing between the root fan and
shoreline face can be manipulated, creating space for a protected
fish corridor or passage behind the root fan, a slower moving
wake region for aquatic plants to establish and maintain, and/or
the ability for the shoreline to migrate landward or seaward
(Figure 12B). This sub-strategy tackles the larger theme of
creating conditions for living organisms (rows 20–21 in
Table 1.) Shipping and navigational activities, recreational
activities, and predominant wave conditions may restrict
available space. The root fan can also be angled down and
embedded into the sediment bottom, offering more traditional
toe protection for a sloped shoreline face in addition to habitat,
dependent on wave conditions (Figure 12C). Mangrove like
revetment structures could be adapted to provide habitats
(rows 20–21 in Table 1), by controlling spacing between
individual elements of a single structure (Figure 12D).
Root-Inspired Multifunctional Composite Structure
Two design concepts shown in Figure 13 illustrate the same
principle idea: use of multifunctional material composites. With
the advent of additive manufacturing, even in using traditional
coastal construction materials such as marine grade concrete and
ceramic (“Current Innovative Solutions for Coastal
Infrastructure Design”section), material composites reveal
new possibilities in multifunctional infrastructure. Building on
the fact that roots have different functions and respective
morphology, in addition to the morphological adaptation
principles illustrated in rows 7–9ofTable 1, material
properties could be varied. A composite structure may employ
more rigid, thicker material allocation in places exposed to wave
energy and erosion potential (i.e., higher stress), while softer,
more flexible material could be allocated in sheltered orientations
for habitat or refuge. Figure 13A shows this division in material
rigidity and flexibility in a root-inspired structure, while
Figure 13B shows a gradient in material rigidity in a standard
pile. As previously mentioned in Kazemi et al. (2017),flexibility of
a modeled mangrove root resulted in higher drag in shallow
waters; therefore, flexibility along the axis of a standard structural
pile may offer greater flow reduction in some lower flow scenarios
than a standard rigid pile. A structure modeled after an
engineered log jam could also have both rigid and flexible
elements assembled in one continuous, porous, yet stable
structure (row 3 and 4 in Table 1).
Root-Inspired Patterning for Multifunctional Seawall
This concept suggests a large-scale redesign of a seawall,
including micro and macro approaches. Building on the
existing living seawall innovations described in “Current
Innovative Solutions for Coastal Infrastructure Design”
section, a seawall could have large-scale undulations on the
entire face rather than just at the top and bottom like a recurved
seawall. The hypothesis is that this large-scale undulation
(Figure 14A)wouldsignificantly reduce wave reflection and
subsequent toe scour compared to a recurved seawall.
Figure 14B shows a seawall concept with a hierarchical
surface design. The designs of Reef Design Lab and
Mangrove Reef Wall (“Current Innovative Solutions for
Coastal Infrastructure Design”section) could also be
utilized at this scale. These existing designs offer spatial
variability, referring to horizontally heterogeneous and
topographically complex structures and surfaces typically
observed in natural habitats. Figures 14–Cmagnifies the
surface roughness and texture, building on the habitat utility
of snag/root roughness as described in row 21 of Table 1.While
a seawall does not mimic a root system in any tangible abstract
way, the concept of irregularity (row 4 of Table 1), root
curvature, spacing, and morphology can be integrated by the
application of two-dimensional patterns or three-dimensional
surface structures.
The concepts presented in this section provide an overview
of how strategies found in root systems can inform the design
of technical coastal infrastructure. These concepts do not take
into consideration however, materiality, scaling, and rigorous
FIGURE 13 | Multifunctional composites—(A)—A root-like structure with the black elements comprising a rigid material and the green elements comprising a
flexible material. A standard construction pile made of one material is shown in (B) while an innovative composite pile is shown in (C), composed of a material gradient
from more rigid (red) at the base of the pile to more flexible (grey) at the top of the pile.
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Stachew et al. Root Research for Bioinspired Design
technical feasibility, which could be further researched in
future projects.
DISCUSSION
This paper demonstrates utility of the bioinspired design
approach through the study of the biology of root systems to
inform multiple engineering design applications. Through design
of a comprehensive analogy table that relates specific biological
information about roots to engineering infrastructure problems
and vulnerabilities, functional principles were established to link
the two fields as outlined specifically through the biomimetic
process. These principles informed design proposals for
foundation and coastal engineering that can fulfill various
functions, such as erosion prevention, structural support, soil
penetration, and habitat creation. Many questions emerging from
this work are not addressed in this paper however, specifically in
the areas of materiality, technology, sustainability, and
implementation.
Considering materiality, typical foundation and coastal
engineering constructions use wood, concrete, rock, and steel.
The resources required to shape these traditional materials and
their desired material properties result in simple morphologies.
New techniques such as 3D/4D printing, dual-extrusion, D-shape
technology, and CNC machining, allow for customizable and
complex organic forms, such as root-inspired structures. Field
scanning techniques, parametric design, and advanced
manufacturing techniques could be combined into a unified
design process to customize structures to specific site
conditions and desired functions. Traditional engineering
materials can be shaped with these new technologies, but in
parallel, such technologies foster the exploration of a wide range
of material composites. Engineered material composites can be
highly tuned with specific properties and performance
characteristics to potentially respond and adapt to dynamic
loading conditions. A material’s engineered response at smaller
scales (i.e., micro- and nano-level) is akin to biomass
accumulation in locations of higher stress in trees. Added
functionality, complexity, and feedback loops through the
developing fields of biotechnology and synthetic biology can
also be considered in subsequent design iterations. The use of
living organisms (e.g., mycelium, coral polyps, oyster spat) and
modified living organisms can lead to emerging techniques like
MICP (Dade-Robertson et al., 2018). To abstract root systems
principles for foundation and coastal engineering, the transfer of
different timescales needs to be addressed in further research.
Damping systems, responsive MICP, and self-healing materials
could respond to everyday fluctuations. Digging/growing agents,
programmable structural growth, and design flexibility to
repurpose infrastructure meanwhile, could serve as an
adaptation to long-term loads.
Advanced technologies and materials could lead the way to
adaptable engineered systems. In foundation design, we can
envision adaptation through material or shape change
response to changing soil conditions, changing structural loads
throughout the lifetime or utility of the structure, or to strengthen
the foundation over time (similar to secondary thickening in root
systems). In coastal infrastructure design, we can envision
adaptation to the changing energetics of nearshore systems,
water levels, nutrient or pollutant concentrations (e.g., material
surface properties facilitating in removal or sequestration), and/
or dissolved oxygen provisioning for aquatic life. For achieving
sustainability, the design of a product should be evaluated for its
entire life cycle, which cannot be performed at this early design
stage. Therefore, concepts presented in this paper focus on the
primary functions required and opportunities for improving
existing practices toward greater sustainability, a key aspect of
biomimicry. Assessing the sustainability of these concepts would
need to question and include the longevity, adaptation, decay,
degradation, and/or reusability of such systems. Should elements
of foundation and coastal systems naturally decompose or
degrade in the ground or water, or should they be reusable or
recyclable? Does the design for adaptation to changing conditions
over time increase design complexity to the point where it may
lead to reduced sustainability? For material selection, biological-
FIGURE 14 | Multifunctional seawall patterning—(A)—Large-scale undulations on the entire face of the seawall so that it is no purely vertical, and no longer causing
wave reflection and scour as shown with the blue arrows. (B)—A simple fractal pattern on an individual seawall tile. (C)—Magnification of an individual seawall tile
illustrating surface roughness and texture.
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Stachew et al. Root Research for Bioinspired Design
based fillers originating from agricultural or construction waste
streams can be utilized in material composites. Especially in
coastal engineering, degradation of these inert biological-based
fillers in an engineered material composite is an additional
component to consider, whether by saltwater intrusion, ice, or
UV light. Degradation could be seen as beneficial, considering if
the by-products offer a food source for native organisms, do not
disrupt organismal primary productivity and reproductive cycles,
and if habitat-forming organisms may take the place of the
degrading structure over time. Must the coastal structure be
permanent, or once the desired physical conditions are
established (i.e., sediment deposition, coastal vegetation fully
established to reduce wave heights), the structure becomes
indistinguishable from its surroundings? Similarly, the life
cycle of a building foundation could be designed such that it
decomposes or “dies”(similar to lateral roots or root hairs no
longer needed for water and nutrient acquisition) when the
building is no longer occupied. The foundation could also
connect to the soil matrix at the individual soil particle scale
to continue preventing erosion, even though aboveground
structural support may no longer be needed.
Lastly, in the case of implementation, where do bioinspired
design concepts of built infrastructure fit in the existing array of
technical options? The possibility to reuse, retrofit, or recycle
existing foundations should be a priority to reduce waste
production and urban decay. Instead of following the ‘take-
make-dispose’linear process in building construction,
technological advancements allow for analysis and adaptation
of existing structures to current needs, instead of building new
structures to fit new needs. Future designs must follow a more
integrated approach, “managing engineered landscapes as
ecological systems,”that evolve, adapt, and respond through
time (DeJong et al., 2015). Additionally, root-inspired
structures should not replace necessary hard coastal
infrastructure in high energy nearshore systems where it is
required nor restoration of ecosystems in low energy systems
where it is possible. Their inclusion may offer additional
functionality or allow for conditions for successful ecosystem
restoration to take place in systems where these projects typically
cannot succeed.
CONCLUSION
The design of built infrastructure often regards soil properties as
stable through time. By default, building foundations to seawalls
are both bulky and heavy to respond to predominant loads and to
ensure stability and durability over a long lifetime. Dynamic
changes to soil properties and environmental conditions, in
addition to inefficient use of material and poorly optimized
construction by viewing soil as stable, compromises built
infrastructure performance.
While foundation designs are limited to simple vertical
geometries by current building techniques, diverse strategies
from root systems give insights to develop multifunctional
foundations able to anchor structures, prevent erosion, and
adapt to various stresses. Several conceptual designs were
made by abstracting and combining multiple root strategies
relating to adaptive soil penetration, surface texture, complex
topology, hierarchical morphology, self-healing materials, and
growth principles. Similarly, coastal infrastructure is often limited
to two technical objectives, simplifying form, material,
construction, and implementation, which displaces natural
habitat and exacerbates negative feedback loops in coastal
ecosystem functioning. Strategies adapted from root systems,
and in particular the ecosystems supported by mangrove and
other coastal forests, can serve to develop multifunctional coastal
infrastructure. In particular, principles relating to root system
architecture, surface texture, complex topology, material
gradients, and adaptive soil penetration were abstracted and
combined into several conceptual coastal infrastructure designs.
We conclude that bioinspired design concepts of built
infrastructure should be part of the mosaic of solutions offered
that provide protective, multifunctional, and livable spaces.
Therefore, this review of biological root systems and the
conceptualized biomimetic translations offers a new way of
thinking about technical problems and vulnerabilities in
engineering and broadly contributes to creating an improved
understanding and intersection of the fields of biology and
engineering.
AUTHOR CONTRIBUTIONS
ES wrote the largest share of this paper, including introduction,
root as a biological model, application to coastal infrastructure,
and discussion and conclusion. TH contributed to those parts
and wrote root biomechanics, and application to building
foundations. ES and TH together worked on the review of
root biology. TH contributed the figures. ES compiled all texts
and organized the writing. PG conceptualized the content,
supervised the research, and revised the article.
FUNDING
The work on this paper was partially funded by the faculty startup
fund of The University of Akron. Funding for Elena Stachew is
provided by the National Oceanic and Atmospheric
Administration (NOAA) Grant DNRFHCZ18B 309-02
DNRH0902, distributed by ODNR Office of Coastal
Management in Sandusky, Ohio and Cleveland Water Alliance.
ACKNOWLEDGMENTS
The shape change foundation concepts were created by in
collaboration with undergraduate students Joshua Davis and
Ashvi Shah. Contributions by Elena Stachew are a
collaborative result supported by a Biomimicry Fellowship
with sponsors Biohabitats and Cleveland Water Alliance.
The authors would like to thank Dr. Jake Miesbauer for
sharing his root biomechanics knowledge during inspiring
discussions.
Frontiers in Robotics and AI | www.frontiersin.org April 2021 | Volume 8 | Article 54844420
Stachew et al. Root Research for Bioinspired Design
REFERENCES
Abbe, T. B., Montgomery, D. R., Featherston, K., and McClure, E. (1993). A
Process-Based Classification of Woody Debris in a Fluvial Network; Preliminary
Analysis of the Queets River. Washington: EOS Transaction of the American
Geophysical Union, 296.
Abbe, T. B., Montgomery, D. R., and Petroff, C. (1997). “Design of Stable In-
Channel Wood Debris Structures for Bank Protection and Habitat Restoration:
An Example from the Cowlitz River, WA,”in Proceedings of the Conference on
Management Disturbed by Channel Incision. Oxford, MS: University of
Mississippi, 16, 8.
Abbe, T. B., and Montgomery, D. R. (1996). Large Woody Debris Jams, Channel
Hydraulics and Habitat Formation in Large Rivers. Regul. Rivers: Res. Mgmt. 12
(2–3), 201–221. doi:10.1002/(sici)1099-1646(199603)12:2/3<201::aid-
rrr390>3.0.co;2-a
Abbe, T. B., and Montgomery, D. R. (2003). Patterns and Processes of Wood
Debris Accumulation in the Queets River Basin, Washington. Geomorphology
51 (1), 81–107. doi:10.1016/s0169-555x(02)00326-4
Alaoui, A., Rogger, M., Peth, S., and Blöschl, G. (2018). Does Soil Compaction
Increase Floods? A Review. J. Hydrol. 557, 631–642. doi:10.1016/j.jhydrol.2017.
12.052
Amellal, N., Burtin, G., Bartoli, F., and Heulin, T. (1998). Colonization of Wheat
Roots by an Exopolysaccharide-ProducingPantoea Agglomerans Strain and its
Effect on Rhizosphere Soil Aggregation. Appl. Environ. Microbiol. 64 (10),
3740–3747. doi:10.1128/aem.64.10.3740-3747.1998
Angermeier, P. L., and Karr, J. R. (1984). Relationships between Woody Debris and
Fish Habitat in a Small Warmwater Stream. Trans. Am. Fish. Soc. 113 (6),
716–726. doi:10.1577/1548-8659(1984)113<716:RBWDAF>2.0.CO;2
Arnone, E., Caracciolo, E., Noto, L. V., Preti, F., and Bras, R. L. (2016). Modeling
the Hydrological and Mechanical Effect of Roots on Shallow Landslides. Water
Resour. Res. 52 (11), 8590–8612. doi:10.1002/2015WR018227
Bailey, P. H. J., Currey, J. D., and Fitter, A. H. (2002). The Role of Root System
Architecture and Root Hairs in Promoting Anchorage against Uprooting
Forces in Allium Cepa and Root Mutants of Arabidopsis Thaliana. J. Exp.
Bot. 53 (367), 333–340. doi:10.1093/jexbot/53.367.333
Ballard, J., Pasquale, C., and Howington, S. (2009). “A Heat and Fluid Transport
Simulation of a Soil-Root-Stem System,”in 39th AIAA Fluid Dynamics
Conference. Reston, Virginia: American Institute of Aeronautics and
Astronautics.
Barbier, E. B., Koch, E. W., Silliman, B. R., Hacker, S. D., Wolanski, E., Primavera,
J., et al. (2008). Coastal Ecosystem-Based Management with Nonlinear
Ecological Functions and Values. Science 319 (5861), 321–323. doi:10.1126/
science.1150349
Baumann, V., and Bauer, G. E. A. (1974). The Performance of Foundations on
Various Soils Stabilized by the Vibro-Compaction Method. Can. Geotech. J. 11
(4), 509–530. doi:10.1139/t74-056
Bengough, A. G., McKenzie, B. M., Hallett, P. D., and Valentine, T. A. (2011). Root
Elongation, Water Stress, and Mechanical Impedance: A Review of Limiting
Stresses and Beneficial Root Tip Traits. J. Exp. Bot. 62 (1), 59–68. doi:10.1093/
jxb/erq350
Bengough, A. G., Mullins, C. E., and Wilson, G. (1997). Estimating Soil Frictional
Resistance to Metal Probes and its Relevance to the Penetration of Soil by
Roots. Eur. J. Soil Sci. 48 (4), 603–612. doi:10.1111/j.1365-2389.1997.
tb00560.x
Benke, A. C., Henry, R. L., III, Gillespie, D. L., and Hunter, R. J. (1985). Importance
of Snag Habitat for Animal Production in Southeastern Streams. Fisheries 10
(5), 8–13. doi:10.1577/1548-8446(1985)010<0008:IOSHFA>2.0.CO;2
Benyus, J. M. (2011). A Biomimicry Primer. London: The Biomimicry Institute and
the Biomimicry Guild
Bidlack, J. E., Jansky, S. H., and Stern, K. R. (2011). “Chapter 5: Roots and Soils,”
in Stern’s Introductory Plant Biology. 12th ed. New York, NY: McGraw-
Hill, 622.
Blancaflor, E. B., and Masson, P. H. (2003). Plant Gravitropism. Unraveling the
Ups and Downs of a Complex Process. Plant Physiol. 133 (4), 1677–1690.
doi:10.1104/pp.103.032169
Bloch, R. (1952). Wound Healing in Higher Plants. II. Bot. Rev. 18 (10), 655–679.
doi:10.1007/bf02957556
Bulleri, F., and Chapman, M. G. (2010). The Introduction of Coastal Infrastructure
as a Driver of Change in Marine Environments. J. Appl. Ecol. 47 (1), 26–35.
doi:10.1111/j.1365-2664.2009.01751.x
Burrall, M., DeJong, J. T., Martinez, A., and Wilson, D. W. (2020). Vertical Pullout
Tests of Orchard Trees for Bio-Inspired Engineering of Anchorage and
Foundation Systems. Bioinspiration Biomimetics 16 (1), 016009. doi:10.1061/
9780784482834.026
Calderón, A. A., Ugalde, J. C., Chang, L., Zagal, J. C., and Pérez-Arancibia, N. O.
(2016). “Design, Fabrication and Control of a Multi-Material-Multi-Actuator
Soft Robot Inspired by Burrowing Worms,”in 2016 IEEE International
Conference on Robotics and Biomimetics (ROBIO). 31–38.
Calderón, A. A., Ugalde, J. C., Chang, L., Zagal, J. C., and Pérez-Arancibia, N. O.
(2019). An Earthworm-Inspired Soft Robot with Perceptive Artificial Skin.
Bioinspiration Biomimetics 14 (5), 056012. doi:10.1088/1748-3190/ab1440
Chen, R., Rosen, E., and Masson, P. H. (1999). Gravitropism in Higher Plants. Plant
Physiol. 120 (2), 343–350. doi:10.1104/pp.120.2.343
Cheshire, M. V. (1979). Nature and Origin of Carbohydrates in Soils. London; New
York: Academic Press.
Correa, J., Postma, J. A., Watt, M., and Wojciechowski, T. (2019). Soil Compaction
and the Architectural Plasticity of Root Systems. J. Exp. Bot. 70 (21), 6019–6034.
doi:10.1093/jxb/erz383
Coutts, M. P. (1983). Root Architecture and Tree Stability. Plant Soil 71, 171–188.
doi:10.1007/bf02182653
Cremaldi, J. C., and Bhushan, B. (2018). Bioinspired Self-Healing Materials:
Lessons from Nature. Beilstein J. Nanotechnol. 9 (1), 907–935. doi:10.3762/
bjnano.9.85