GEOCELL REINFORCED FOUNDATIONS

Abstract

Geocell-reinforcement in ground improvement is being used very extensively in present days. It is a three dimensional honeycombed confinement system, made of geosynthetics, which significantly improves the bearing capacity of soft soils, specially, in foundations, and pavements applications. Apart from improving the soil strength, it has also been extensively used in various slope stabilization, embankment construction and railway track applications. Various parameters are needed to be considered and designed for the application of geocell systems, like: geometrical parameters of geocell, its location and infill soil characteristics This paper briefly discussed about geocell reinforced foundation systems and various parameters influencing the performance of the same. The developments and parametric studies on geocell foundation performances are discussed giving the optimum values of various parameters.

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Training Course on ‘Introduction to Geosynthetics and Their Applications’
December 14, 2015, NIT Surat
GEOCELL REINFORCED FOUNDATIONS
A. Murali Krishna1 and Arghadeep Biswas2
1Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati, India.
Email: amurali@iitg.ernet.in
2Department of Civil Engineering, Jalapaiguri Govt. Engg. College, Jalapaiguri, India.
Email: arghadeep.biswas@gmail.com
Abstract
Geocell-reinforcement in ground improvement is being used very extensively in present days.
It is a three dimensional honeycombed confinement system, made of geosynthetics, which
significantly improves the bearing capacity of soft soils, specially, in foundations, and
pavements applications. Apart from improving the soil strength, it has also been extensively
used in various slope stabilization, embankment construction and railway track applications.
Various parameters are needed to be considered and designed for the application of geocell
systems, like: geometrical parameters of geocell, its location and infill soil characteristics
This paper briefly discussed about geocell reinforced foundation systems and various
parameters influencing the performance of the same. The developments and parametric
studies on geocell foundation performances are discussed giving the optimum values of
various parameters.
Keywords: Ground improvement, geocell reinforcement, geosynthetics, foundations, bearing
capacity
INTRODUCTION
The credit of introducing the systematic concept of reinforcing the soil has been with
Vidal (1969). Over the time, soil-reinforcement has been modified according to new
inventions and requirements. The metallic strip-reinforcements in the beginning (Binquet and
Lee, 1975) were replaced by polymeric sheet-type-reinforcement and afterwards, the versatile
geosynthetics in different forms such as geotextile, geogrid, and geocells have superseded all.
Geocell, the three-dimensional honeycombing confining system, was devised by Webster and
Watkins (1977). In last few decades, the benefits of geocell-reinforcements, considering
several aspects such as material-strength, geometry, placement depths etc., have been
demonstrated by several investigators. Several field applications (Johnson, 1982; Bush et al.,
1990; Cowland and Wong, 1993; Emersleben and Meyer, 2008) and laboratory studies
(Shimizu and Inui, 1990; Dash et al., 2001, 2003; Sitharam et al., 2005, 2007; Zhang et al.,
2010; Mehrjardi et al. 2013; Tafreshi et al., 2010; Biswas et al., 2013, 2015) have highlighted
the benefits of three dimensional geocell reinforcement. This paper briefly discusses about
geocell reinforced foundation systems and various parameters influencing the performance of
the same.
MECHANISM OF GEOCELL REINFORCEMENT
Geocells, made of geosynthetics such as geotextiles or geogrids, are thermally welded
or mechanically bonded interconnected pocket-structures, in the form of mattress, used with
in-filled soil. General reinforcing mechanisms of geocell is confining the in-fill soil from
shearing away and derive anchorage resistance through the surrounding soil against the
applied load. Geocell walls cut the potential failure planes (which would be the case in
unreinforced condition) and force it to go deeper in to the soil as shown in Fig. 1. The
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Training Course on ‘Introduction to Geosynthetics and Their Applications’
December 14, 2015, NIT Surat
interconnected pockets provide all round confinement to the in-filled soil and resists from
squeezing out under shear. In addition, the geocell-walls derive interfacial resistances with
surrounding soil through its apertures and develop anchorage with the soil to improve the
load bearing capacity of the reinforced-system (Fig. 2). As a whole, the in-filled geocell-soil
mattress behave like a semi-rigid slab which redistribute the incoming load over wider area
onto the underlying soil leading to improved performance of the system with reduced stress
intensity and the associated settlements.
Fig. 1 Foundation behavior in different configurations
Fig. 2 Various mechanisms with geocell reinforcement
GEOCELL-REINFORCED FOUNDATION SYSTEM
A typical geocell-reinforced foundation system is shown in Fig. 3 for a footing of
diameter D. Two types of soils can be noticed in the figure: the soil-1 is the native soil
underneath the reinforced-soil and the geocell-reinforced fill-soil is shown as soil-2. To
improve the bearing capacity of foundation soil, geocells are placed directly over the native
soft ground and then the pockets of geocell are filled with either using native soil or using
better granular materials like sand or gravel. If it is filled by the native soil, then soil-1 and
soil-2, shown in the Fig. 3, are one and the same (clay-clay or sand-sand). However, as per
general practice, the geocell-pockets are filled with granular materials such as sand or gravel
for its better interfacial properties. In that case, the two soil medium will be different
(sand/gravel-clay). The geocell mattress dimensions and placement details are referred as
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Training Course on ‘Introduction to Geosynthetics and Their Applications’
December 14, 2015, NIT Surat
different parameters shown in the Fig. 3. The ‘h’, ‘b’, and ‘d’ are representing the height,
width, and pocket size of the geocell mattress, respectively. The placement depth below the
footing or the thickness of soil-cushion provided in between the footing and geocell-mattress
is denoted by ‘u’.
Fig. 3 Typical geocell-reinforced foundation system
STUDIES ON GEOCELL-REINFORCED SYSTEMS
Successful field applications inspired researchers for rigorous parametric study to use
geocell more effectively. Rajagopal et al. (1999) investigated strength and stiffness behavior
of individual geocell-sand system through triaxial tests. Dash et al. (2001) reported a detail
parametric study on formation pattern, geometry, and placement depth, stiffness of the
geocell material and relative density (ID) of the in-filled sand of geocell-sand foundation
system. About 8 fold improvement in bearing capacity with geocell was observed. Pokharel
et al. (2010) investigated effect of shape, type, embedment depth of footing, height of geocell
and quality of in-filled material on geocell reinforced foundation bed and found that circular
shaped geocell pocket gave better result than elliptical shape.
According to general field conditions, several model studies have also been carried out with
geocell-sand mattress over soft clay subgrade. Mandal and Gupta (1994) investigated
responses of geocell-sand foundation mattress over soft marine clay. Krishnaswamy et al.
(2000) investigated behavior of a model footing rested on geocell-sand reinforced
embankment over soft clay. Emersleben and Meyer (2008) performed model and field test
with full-scale traffic loading on geocell-sand mattress over soft clay subgrade. Zhang et al.
(2010) proposed bearing capacity calculation method of geocell reinforced foundation system
considering “Lateral resistance effect”, “Vertical stress dispersion effect” and “Membrane
effect”.
Few laboratory model investigations have also been carried out with geocell-reinforced clay
over clay subgrade. Sitharam et al. (2005 & 2007) investigated clay-filled geocell mattress
over soft clay subgrade. About 5 fold increases in bearing capacity was achieved. It was
reported about 90% reduction in settlement due to geocell-clay mattress over soft clay
subgrade.
d
D
Soil -2
h
H
u
b
Geocell-Mattress
Soil -1
Footing
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Training Course on ‘Introduction to Geosynthetics and Their Applications’
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INFLUENCING PARAMPETRS
Performance of geocell-reinforced foundation system depends on various parameters.
Laboratory model studies revealed several influencing parameters having immense effect on
geocell-reinforced foundation system.
Formation Pattern
The chevron pattern (Fig. 4) of geocell formation was reported to be more beneficial over
diamond pattern as it have more joints per unit area so as the rigidity (Webster, 1979; Dash et
al., 2001; Krishnaswamy et al., 2000; Pokharel et al., 2010).
Fig. 4 Pattern of Geocell Formation (after Dash et al. 2001)
Pocket Size
The pocket size (d) of a geocell-mattress is generally expressed as the diameter of an
equivalent circular area of the geocell pocket opening (in plan). It has been recommended
that the pocket opening should be less than the loading diameter and found that the smaller
geocell-pockets give higher performance (Mandal and Gupta, 1994; Mhaiskar and Mandal,
1996; Dash et al. 2001, 2003). However, due to constructional difficulties it has been
recommended to make pocket opening of geocell mattress slightly smaller than the footing
area so that the footing can cover at least one full pocket opening. Optimum pocket size
recommended as 0.8D where ‘D’ is the footing diameter (Dash et al. 2003).
Width and height of Geocell Mattress
Researchers have indicated that the performance of the geocell-reinforced system is highly
depended on the width (or length) and height of the geocell-mattress. According to Mhaiskar
and Mandal (1996), Sitharam et al. (2005), Sireesh et al. (2009) and others, the optimum
width of geocell mattress should be 4-6D. Beyond it, the improvement is marginal (Fig. 5a)
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Training Course on ‘Introduction to Geosynthetics and Their Applications’
December 14, 2015, NIT Surat
as farthest rupture planes were observed well within 2B distance at each side of the footing
(Dash et al. 2001). Figure 5b shows the effect of geocell height on the performance model
foundations (Sireesh et al., 2009). The optimum height has been reported to be in the range 1-
1.5D.
(a) (b)
Fig. 5 (a)Effect of geocell width (Latha et al. 2009) (b) Effect of geocell height (Sireesh et al.,
2009)
Placement Depth
Placement depth of the geocell below the footing is the height of sand cushion (u) in between
footing and geocell mattress (Fig. 2). This soil cushion redistributes the incoming load with a
lesser intensity and uniformly to the underlying geocell-mattress and thus prevents the
geocell-wall from early local buckling (Dash et al., 2001; Yoon et al., 2008; Tafreshi and
Dawson, 2010). Placement depth of 0.1D has been recommended as the optimum value to
prevent geocell wall from direct loading as shown in Fig. 6 (Dash et al. 2001).
Relative Density of in-fill Soil
Dash et al. (2001), Latha et al. (2009), Dash (2010) and Pokharel et al. (2010) observed that
with increase in relative density (ID) of in-filled sand, the performance of geocell-foundation
system improves as its stiffness increases (Fig. 7). Thus, it is recommended that the in-filled
soil relative density (ID) should be kept high as much high as possible for achieving
enhanced performance of the geocell-reinforced systems through higher density of infill soil
and soil-geocell interfacial properties.
Geogrid Properties
Stiffness of geogrid, orientation of ribs and aperture opening size (da) also has great influence
in improving the reinforcing effect of geocell (Rajagopal et al., 1999; Krishnaswamy et al.,
2000; Dash et al., 2001, Dash, 2012). It is seen that having larger opening size, the geogrid
develops better interlocking and anchorage with the soil particles than the solid walled or
perforated walled geocells (geoweb) which gives rise in improving the performance. In other
side geogrids having smaller opening sizes has higher improvement capacity, as the
confinement of in filled soil is better in smaller openings and per unit surface area for
frictional resistance and anchorage effect increases. This produces a comparatively stiff
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Training Course on ‘Introduction to Geosynthetics and Their Applications’
December 14, 2015, NIT Surat
geocell-sand mattress and redistributes the loads even better. The orientation of geogrid ribs
is also significant in improving load bearing capacity. Horizontal and vertical orientation of
ribs (square or rectangular openings) gives better resistance against loading than the inclined
orientation (diamond openings).
Influence of Subsoil strength
The performance and behavior of the entire geocell-reinforced foundation system is
affected by the strengths of the underlying subgrade (Fig. 8) i.e. soil-2 in Fig. 3 (Biswas et al.,
2013, 2015). It may happen that sufficiently strong geocell-soil mattress could not able to
derive desired performance due to the underlying stiff subgrade which affected in huge
buckling in geocell-walls resulting reduction in performances (Biswas et al, 2015).
050 100 150 200 250 300 350 400 450
Bearing Pressure (kPa)
-25
-20
-15
-10
-5
0
Footing Settlement, s/D (%)
c
u
= 7 kPa
c
u
= 15 kPa
c
u
= 30 kPa
c
u
= 60 kPa
H = 0.63D
Clay + Sand
Clay + Sand + Geocell
Fig. 8 Subgrade strength on improvement (after Biswas et al. 2013)
It is further reported that the performance of the geocell-reinforced foundation is
magnified with the provision of an additional planar geogrid placed at the bottom of the
Fig. 6 Effect of depth of placement
(Dash et al 2001)
Fig. 7 Effect of relative density of in-filled
sand (after Dash 2010)
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Training Course on ‘Introduction to Geosynthetics and Their Applications’
December 14, 2015, NIT Surat
geocell-mattress (Dash et al., 2001; Sitharam et al., 2007, Biswas et al., 2015). Table 1 is
summarizing the optimum values after normalized with footing diameter (D) or footing width
(B). Table 1 Optimum value of the parameters
Parameters
Values (Range)
Formation Pattern
Chevron
Rib Orientation
Horizontal & Vertical
Geogrid Opening (d
a
/D
50
)
80
Stiffness of Geogrid
As high as possible
Pocket size (d)
0.8 - < 1D (B)
Width (b)
4 – 6D (B)
Height of geocell (h)
1.5 < 2D (B)
Depth of placement (u)
0.1 – < 0.33D (B)
ID of in-filled sand (%)
As high as possible
CONCLUSIONS
This paper briefly discussed about geocell reinforced foundation systems and various
parameters influencing the performance of the same. The developments and parametric
studies on geocell foundation performances are discussed giving the optimum values of
various parameters. To have a better performance of geocell-reinforced soil, one must
consider the different influencing parameters and their working mechanism. The behaviour
and design of geocell reinforced soil structures is yet to be fully explored for developing the
design guidelines.
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Training Course on ‘Introduction to Geosynthetics and Their Applications’
December 14, 2015, NIT Surat
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