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Bearing capacity of open caissons embedded in sand

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Geotechnique
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Brian Sheil at University of Cambridge
Brian Sheil
Jack Templeman at University of Oxford
Jack Templeman
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Open caissons are an increasingly common means of constructing underground storage and attenuation tanks as well as launch and reception shafts for tunnel-boring machines. The caisson walls typically feature a tapered base, referred to as the ‘cutting face’, to aid the sinking process by reducing the vertical soil reaction. The primary aim of this paper is to explore the influence of the caisson cutting face inclination angle on the vertical soil reaction in sand. Both finite-element limit analysis and finite-element analysis are adopted for this purpose. The effects of cutting face roughness, external embedment depth and caisson radius are also investigated. The results show that the influence of the cutting face inclination angle on the bearing capacity is highly dependent on both the soil friction angle and the roughness of the cutting face. A reduction in the caisson radius is also shown to cause a significant increase in the vertical soil reaction. The numerical output is used to inform the development of a new closed-form analytical approach amenable for use in routine design. The design method is shown to provide a high-fidelity representation of the numerical output.
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Bearing capacity of open caissons embedded in sand
BRIAN SHEILand JACK TEMPLEMAN
Open caissons are an increasingly common means of constructing underground storage and attenuation
tanks as well as launch and reception shafts for tunnel-boring machines. The caisson walls typically
feature atapered base, referred to as the ‘cutting face’, to aid the sinking process by reducing the vertical
soil reaction. The primary aim of this paper is to explore the influence of the caisson cutting face
inclination angle on the vertical soil reaction in sand. Both finite-element limit analysis and finite-
element analysis are adopted for this purpose. The effects of cutting face roughness, external
embedment depth and caisson radius are also investigated. The results show that the influence of the
cutting face inclination angle on the bearing capacity is highly dependent on both the soil friction angle
and the roughness of the cutting face. A reduction in the caisson radius is also shown to cause a
significant increase in the vertical soil reaction. The numerical output is used to inform the development
of a new closed-form analytical approach amenable for use in routine design. The design method is
shown to provide a high-fidelity representation of the numerical output.
KEYWORDS: bearing capacity; caissons; finite-element analysis; sands; shafts
INTRODUCTION
Deep shafts and caissons have a wide range of geotechnical
applications, including foundation solutions, storage and
attenuation tanks (e.g. Royston et al., 2020; Sheil, 2021), and
launch and reception shafts for tunnel-boring machines (e.g.
O’Dwyer et al., 2018, 2020; Phillips et al., 2019; Cheng et al.,
2020). A common construction procedure is to ‘sink’
monolithic reinforced concrete caissons into the ground
using the self-weight of the caisson walls. A key feature of the
caisson wall cross-section is a tapered base, typically known
as the ‘cutting face’. This resembles an inclined ring footing
and is used to promote sinking by reducing the vertical
bearing resistance of the soil (Royston et al., 2021). An
accurate estimation of this resistance is an essential element
of the design process: overestimation can lead to uncontrolled
and dangerous sinking, whereas underestimation can result
in failure to reach formation level (Sheil et al., 2018).
The widely used Terzaghi (1943) bearing capacity theory
provides the following expression for calculating the bearing
capacity of a strip footing
V=A¼cNcþqNqþ1
2γBNγð1Þ
where Vis the vertical bearing capacity force; Ais the footing
area; cis the soil cohesion; qis the surrounding surcharge; γis
the unit weight of the soil; Bis the width of the footing; and
N
c
,N
q
and N
γ
are the dimensionless bearing capacity factors
providing the contributions of c,qand γ, respectively. Many
investigators have sought to develop more accurate solutions
for the bearing capacity factors using numerical techniques
such as finite-element analysis (FEA; Goss & Griffiths, 2001;
Gourvenec, 2007; Georgiadis, 2010), method of character-
istics (e.g. Houlsby & Martin, 2003; Smith, 2005) and
finite-element limit analysis (FELA; Merifield et al., 1999;
Salgado et al., 2004; Lyamin et al., 2007; Sloan, 2013).
Equation (1) is typically extended to footings founded
below the ground surface and/or subjected to oblique loads
through the use of additional depth and load inclination
factors, respectively –for example, Meyerhof (1951, 1963),
Hansen (1970), De Beer (1970), Vesic (1973), Sastry &
Meyerhof (1987) and Van Baars (2014). Similarly, ‘shape’
factors have been used to compute the bearing capacity of
foundations with alternative geometries –for example,
Michalowski (2001), Lyamin et al. (2007), Gourvenec
(2007) and Taiebat & Carter (2010).
The vertical bearing capacity of ring footings has been
studied using various experimental and numerical techniques
in both cohesionless (e.g. Saha, 1978; Boushehrian & Hataf,
2003; Saran et al., 2003; Kumar & Ghosh, 2005; Zhao &
Wang, 2008; Choobbasti et al., 2010; Benmebarek et al.,
2012) and cohesive soils (e.g. Lee et al., 2016; Benmebarek
et al., 2017). However, the influence of the footing inclination
angle on the bearing capacity has received much less
attention. Hansen (1970) documented one of the earliest
uses of ‘base factors’to modify the Terzaghi bearing capacity
equation to account for the influence of an inclined footing
base. More recently, Royston et al. (2016) and Chavda et al.
(2019) have explored the influence of the base inclination
angle on ring footings embedded in sand for open caisson
applications, using small-scale laboratory testing. While
Royston et al. (2021) presented updated design calculations
for the bearing capacity of a caisson cutting face in
clay, similar design methods for cohesionless soils do not
exist.
To address these uncertainties, the current paper presents
the results of a suite of FELA calculations of the vertical
penetration behaviour of a caisson cutting face in drained
soil. Parameters considered in the modelling include the soil
friction angle, the inclination and roughness of the caisson
cutting face, the external embedment depth and the caisson
radius. A limited number of FEA calculations were also
undertaken to explore the influence of soil non-associativity
on the vertical bearing capacity. The FELA output is used to
inform the development of closed-form analytical
expressions amenable for use in routine design.
Department of Engineering Science, University of Oxford,
Oxford, UK.
Manuscript received 4 May 2021; revised manuscript accepted
20 August 2021. First published online ahead of print 15 November
2021.
Discussion on this paper closes on 1 October 2023, for further details
see p. ii.
Sheil, B. & Templeman, J. (2023). Géotechnique 73, No. 6, 495–505 [https://doi.org/10.1680/jgeot.21.00089]
495
... Based on the identical framework, the impact of different penetration types and the sinking depth of the caisson on the ultimate load of the open caisson were also investigated (Chavda and Dodagoudar 2022a). Later on, a novel numerical method namely the finite element method (Sheil and Templeman 2023;Chavda and Dodagoudar 2022b) and finite element limit analysis (Templeman et al. 2021;Royston et al. 2022;Sheil and Templeman 2023) were used to compute the bearing capacity factors of cutting edge. Recently, the undrained stability solution of the circular open caisson was evaluated by Chavda et al. (2023) by employing the finite element limit analysis (FELA) techniques under different sinking conditions. ...
... Based on the identical framework, the impact of different penetration types and the sinking depth of the caisson on the ultimate load of the open caisson were also investigated (Chavda and Dodagoudar 2022a). Later on, a novel numerical method namely the finite element method (Sheil and Templeman 2023;Chavda and Dodagoudar 2022b) and finite element limit analysis (Templeman et al. 2021;Royston et al. 2022;Sheil and Templeman 2023) were used to compute the bearing capacity factors of cutting edge. Recently, the undrained stability solution of the circular open caisson was evaluated by Chavda et al. (2023) by employing the finite element limit analysis (FELA) techniques under different sinking conditions. ...
FELA evaluated bearing capacity factors for circular, planar, and interfering cutting edges of open caisson
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In the study, a thorough analysis of the drained bearing capacity factors of cutting edges of open caissons with and without interference has been carried out which helps in planning the controlled sinking of caissons. Five different conditions are analysed in this study: single circular, single planar, V-shape planar, interfering planar, and interfering V-shape planar cutting edges. Implementing upper and lower bound finite element limit analysis (FELA) in accordance with Terzaghi's conventional bearing capacity equation, the bearing capacity factors of the cutting edge of open caissons are evaluated. The design charts for the bearing capacity of cutting edges are determined by accounting for the effects of the radii ratio of the circular caisson, distance ratio of planar cutting edges, cutting angle of cutting edge, soil internal friction angle, and spacing between the cutting edges. The soil failure planes corresponding to the aforementioned aspects of the cutting edges are also evaluated. ARTICLE HISTORY
View
... The evaluation of bearing capacity of the cutting edge will help in planning the controlled sinking. The bearing capacity of cutting edge has been investigated using experiments (Chavda et al. 2020;Chavda and Dodagoudar 2022a), slip line method (Berezantsev 1952;Solov'ev 2008;Yan et al. 2011), finite element method (Sheil and Templeman 2022;Chavda and Dodagoudar 2022b) and finite element limit analysis (Templeman et al. 2021;Sheil and Templeman 2022;Royston et al. 2022a). There are several practical possible cases of caisson sinking in clayey soil starting from only cutting edge embedded in soil, cutting edge and steining complete embedded in soil, embedded caisson with soil removed within caisson, with different roughness conditions (α = 0 to 1) of steining (Chavda and Dodagoudar 2022a). ...
... The evaluation of bearing capacity of the cutting edge will help in planning the controlled sinking. The bearing capacity of cutting edge has been investigated using experiments (Chavda et al. 2020;Chavda and Dodagoudar 2022a), slip line method (Berezantsev 1952;Solov'ev 2008;Yan et al. 2011), finite element method (Sheil and Templeman 2022;Chavda and Dodagoudar 2022b) and finite element limit analysis (Templeman et al. 2021;Sheil and Templeman 2022;Royston et al. 2022a). There are several practical possible cases of caisson sinking in clayey soil starting from only cutting edge embedded in soil, cutting edge and steining complete embedded in soil, embedded caisson with soil removed within caisson, with different roughness conditions (α = 0 to 1) of steining (Chavda and Dodagoudar 2022a). ...
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Circular open caissons are deep foundations sunk into the ground utilising the self-weight of cutting edge and steining as driving force along with the subsequent failure of soil in bearing. During the sinking of the caisson, the clay in contact with the cutting edge is subjected to undrained loading and the controlled sinking of the caisson can be achieved by evaluating the undrained bearing capacity of the cutting edge. In the study, the undrained bearing capacity factor (N) of the cutting edge for varying cutting angle (b), radius ratio of the caisson (r i /r o), full embedment of the caisson (d), removal of soil within the caisson (d'), and different roughness (α) conditions of the steining are evaluated using finite element limit analysis (FELA). The factors affecting the undrained stability of caisson considering all practical scenarios are addressed and presented as charts to be used in practice.
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... In recent years, many studies have investigated open caissons using both experimental tests and numerical simulations to monitor and capture the importance of the effects of these mechanisms on the mechanical behavior of open caissons [8]. Several researchers have focused on two effective factors concerning resistance to side friction [9] and the sloping angle of the cutting edge on caissons [10]. In addition, laboratory tests on external skin friction in sandy soil were conducted by Wang et al. [11] and Zhou et al. [12], and field and model tests on the bearing capacity of such problems in clay were conducted by Houlsby et al. [13], Guo et al. [14], and Chavda et al. [15]. ...
Bearing capacity prediction of open caissons in anisotropic clays utilizing a deep neural network coupled with a population based training approach
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Open caissons are increasingly utilized for underground construction due to the increasing demand for aboveground structures, which employ the principle of submersion using the self-weight of the edge cutting face and the applied bearing pressure to mitigate the vertical soil reaction. This paper examines the bearing capacity factor of the edge cutting face in anisotropic clays, approximated using the finite element limit analysis (FELA) method and considering the average results between the upper and lower bounds. The influence of the adhesion factor at the interface of the cutting edge (α), the ratio between the depth of the internal embedment and the embedded width (H/B), the ratio between the radius and the embedded width (R/B), the anisotropic shear strength (re), and the cutting face angle (β) is investigated. The results indicate a significant influence of the anisotropic shear strength on the adhesion factor at the interface of the cutting edge. An increase in re denotes a decrease in the undrained shear strength obtained from the triaxial compression test, resulting in an increase in the value of N. An increase in α influences β, such that when β <90°, the value of N remains constant when β = 90°. In addition, a highly efficient hybrid model called DNN-PBT was established utilizing a deep neural network (DNN) and a population based training (PBT) approach, specifically for the purpose of accurately predicting the bearing capacity factor of circular open caissons positioned in undrained clay. Both computational and comparative outcomes demonstrate that the proposed DNN-PBT can precisely forecast the bearing capacity, achieving an R2 value higher than 0.999 and a mean squared error (MSE) less than 0.007. These findings highlight the accuracy and efficiency of the suggested approach. Furthermore, the sensitivity analysis results demonstrated that the anisotropic shear strength (re) is the most important input variable for estimating the bearing capacity factor of the edge cutting face.
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... During the sinking process, the cutting face of the cutting edge plays a crucial role by diminishing the vertical bearing capacity and promoting the progression of the soil failure mechanism toward the central area of the caisson (Nonveiller 1987;Sheil et al. 2018). Several investigations in the literature have focused on specifying the cutting face angle, denoted as "β", within ranges such as 30°-90° (Sheil and Templeman 2021) or 30°-45° (Chavda and Dodagoudar 2022b), illustrating various approaches. ...
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Open caissons are commonly used in the construction of various underground structures, such as launch and reception shafts for tunnel-boring machines, storage or attenuation tanks, and cofferdams. During the sinking phase, the cutting edge of a caisson wall with a cutting face encounters soil and is subjected to loading to facilitate and control the sinking process. In this regard, the bearing capacity factor (N) of the cutting face of a circular open caisson in heterogeneous clay is evaluated via finite element limit analysis, which accounts for the increase in the undrained shear strength with depth. The parameters considered in this study cover practical aspects, including the excavation geometry, soil strength profile, and caisson geometry. This investigation also explored the impacts of the cutting face angle (β), roughness (α), ratio of the internal embedment depth to the embedment width (H/B), ratio of the internal radius to the embedment width (R/B), and strength gradient ratio (ρB/su0). In particular, when the H/B > R/B ratio, the N value tends to stabilize. Crucially, when H/B > 4, an increasing trend in R/B leads to a rise in N until R/B exceeds 10, i.e., large diameter caissons, stabilizing the N value. Furthermore, the results reveal the significant dependency of the cutting face roughness and strength gradient ratio of clay on N. The artificial neural network model, which is a soft computing-based model, is also developed to present the undrained bearing capacity forecasting equation. Compared with conventional regression, including the multiple linear regression model and the multiple nonlinear regression model, it has excellent performance, as measured by eight indices. In addition, ANOVA and Z-tests can support the research hypothesis and reject the null hypothesis.
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... The clayey backfill under drained conditions was simulated using an elastic, perfectly plastic Mohr-Coulomb (MC) model. The roughness of the wall-soil interface is characterised by a roughness factor μ such that the interface friction angle δ is given by tan δ = μ tan ϕ (Sheil & Templeman, 2021). From the MC failure criterion, the shear strength of the interface satisfies ...
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... For this reason, a key design concern for these structures in the permanent state is flotation when the tanks are emptied due to the significant reduction in downward force. For monolithic caissons, the use of lubricating fluids to minimise external caisson-soil friction during the sinking process (Sheil & Templeman, 2022) means that long-term antiflotation frictional resistance is typically neglected in design. ...
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