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Cutting shoe design for open caissons in sand: influence on vertical bearing capacity
Abstract
Open caisson shafts are a widely adopted solution for a range of geotechnical applications. An external ‘cutting shoe’ is a common construction feature used to reduce the soil frictional resistance acting on the caisson during sinking. This forms an annular void encircling the caisson which is filled with a support fluid to maintain excavation stability. The primary aim of this paper is to explore the influence of the cutting shoe geometry on the resulting vertical bearing resistance in sand. Finite element limit analysis is adopted for this purpose. Additional parameters considered in the modelling include the roughness of the cutting face and cutting shoe, and caisson radius and embedment depth. The results show that the influence of the cutting shoe is highly dependent on the caisson cutting face roughness and the soil friction angle, illustrated using detailed soil failure mechanisms. The roughness of the cutting shoe is also shown to cause a significant increase in the vertical soil reaction for large caisson embedment depths. By way of example, a recent UK case study, involving the construction of a 32 m diameter caisson, is used to highlight the potential influence of the cutting shoe on the bearing resistance during caisson sinking.
... Previous research on vertical shafts has mainly favoured theory, numerical analysis, monitoring or experiments regarding the general method [25][26][27][28][29][30][31][32][33]. However, there are currently few systematic presentations of new shaft technology based on actual projects [16]. ...
Using the Vertical Shaft Sinking Machine (VSM) for shaft construction is an emerging modern technology, which is also currently one of the most advanced techniques in the field of shaft sinking. Current research on VSM technology primarily focuses on mechanical and technical issues, neglecting the impact of the construction on the surrounding soil and the structure itself. This oversight leaves structural design lacking a reliable foundation. Additionally, there is insufficient information on the role of reinforcement design during construction in soft soils. These engineering challenges have hindered the widespread implementation of this new technology. Therefore, a series of numerical models were used to analyse the mechanical behaviour of shaft and soil during or after the sinking process, with the aim of addressing these gaps by investigating the influence of shafts constructed using VSM technology, and providing a scientific basis for reinforcement design in soft soil. The case study shows that increasing the soil-cement strength does not have a significant effect on the overall deformation of the soil surrounding the shaft, but leads to a notable reduction in the plastic zone volume, subsequently enhancing the overall stability of the neighbouring soil. The ring bottom beam design effectively reduces convergence deformation by about 50%, while also improving the horizontal internal force distribution near the cutting edge. However, this approach significantly escalates local vertical bending moments, necessitating thorough consideration in the design stage.
... 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. ...
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
... The vertical bearing capacity of the ring footing and the cutting face of the caisson has been extensively studied using various techniques, including experimental tests (Chavda et al. 2020;Chavda and Dodagoudar 2022a;Royston et al. 2016) and numerical methods such as the slip line method (Yan et al. 2011), finite element method (Chavda and Dodagoudar 2022b), and finite element limit analysis (Chavda et al. 2023;Nguyen et al. 2023a, b;Templeman et al. 2023). Recently, Royston et al. (2022b) presented updated design charts for the bearing capacity of a caisson-cutting face. ...
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.
... 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). ...
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.
... One aspect has focused on the sinking process of the open caisson, and the other, on the deformation behavior of the open caisson after sinking to the desired depth. The sinking process of open caissons has been studied by theoretical analysis, laboratory tests, numerical analysis, and the analysis of field monitoring data to determine the mechanical responses and deformation characteristics of the surrounding soil that are induced by the installation of open caissons (Yan and Shi, 2013;Jiang, et al., 2019;Zhou et al., 2019;Royston et al., 2020;Guo et al., 2021;Templeman et al., 2021). Due to this focus on settlement deformation after sinking to the desired depth, studies on the sinking process of open caissons are not described in detail in this paper. ...
With the increasing size of open caissons in large-span bridge projects, the overall settlement of giant open caissons is vital to the safe construction of bridge superstructures. Taking the engineering case of the Changtai Yangtze River Bridge, the overall deformation of an open caisson was studied during the construction stage of the bridge’s superstructures. First, the theoretical layer-wise summation method was utilized to analyze the settlement of the open caisson. Then, a 3-D finite element model was established to simulate the installation stage of the bridge superstructure. Finally, a large centrifuge model test was performed to obtain the deformation of the open caisson at each step of bridge’s construction. The results of these approaches demonstrated that final settlements were quite consistent—approximately 225 mm when the bridge superstructure was completely installed—and the settlement deformation curve could be divided into three stages: slowly increasing, rapid, and stabilizing. This study can provide significant guidance for the construction of the Changtai Yangtze River Bridge and be a reference for similar open caisson engineering projects.
... Deep, large-diameter caisson shafts are popular as underground storage, attenuation tanks and pumping stations for the water and wastewater industry (see Fig. 1; Phillips et al., 2019;Royston et al., 2022a;Templeman et al., 2023). Given their large depth and internal volume, groundwater-induced uplift pressures acting on the caisson base can be significant. ...
Deep, large-diameter caisson shafts are a popular means of constructing underground storage and attenuation tanks and pumping stations for the water and wastewater industry. One of the key design concerns for these structures is resistance to flotation during periods when the tanks are only partially filled or empty. In this paper, two-dimensional numerical analysis is used to explore the undrained uplift resistance provided by under-reaming the walls of the caisson shaft to create an enlarged base. The primary aim of the study is to assess the influence of the taper angle of the anchor (i.e. the protruded base) on the resulting uplift resistance. The effects of the anchor–soil interface roughness factor, soil weight, surcharge pressure and caisson radius are also investigated. The results indicate that the effect of the taper angle on both the uplift bearing capacity and the developed horizontal reaction can be very significant. The numerical output informs the development of a closed-form approach for application in routine design. The new design method is shown to provide an excellent agreement with both finite-element and additional finite-element limit analysis calculations. By way of example, the proposed design method is applied to a hypothetical design scenario.
The development of underground spaces inevitably poses significant risks to nearby infrastructure due to construction-induced ground displacements. While our understanding of tunnel-induced ground movements is now relatively mature, there is a distinct lack of literature on large-diameter open caisson shafts. This paper fills this gap by describing results from a small-scale laboratory study exploring soil deformation mechanisms during caisson construction in dry sand. Results from seven tests are analyzed to identify the influence of key caisson geometric properties as well as the effectiveness of external cofferdams in minimizing soil displacement. The results show that the primary mechanisms driving ground movements are a compressive ‘bearing’ front beneath the cutting face and a ‘frictional’ contribution above the cutting face. The normalized radial settlement profile is also shown to be insensitive to the normalized caisson embedment depth, and the settlement zone of influence extends up to 0.25 diameters below the caisson cutting edge. Furthermore, the presence of an external cofferdam is shown to be highly effective in reducing soil settlements. Quantitative analysis reveals a significant decrease in soil settlement with an increase in cofferdam depth from 0.25 to 0.5 of caisson depth, with good consistency between results for different soil elevations. In addition, larger cofferdam diameters provide maximum benefits in minimizing ground displacements.
The use of supporting fluids to stabilise excavations is a common technique adopted in the construction industry. Rapid detection of incipient collapse for deep excavations and timely decision making are crucial to ensure safety during construction. This paper explores a hybrid framework for forecasting the collapse of fluid−supported circular excavations by combining physics-based and data-driven modelling. Finite element limit analysis is first used to develop a numerical database of stability numbers for both unsupported and fluid−supported circular excavations. The parameters considered in the modelling include excavation geometry, soil strength profile and support fluid properties. A data-driven algorithm is used to ‘learn’ the numerical results to develop a fast ‘surrogate’ amenable for integration within real−time monitoring systems. By way of example, the proposed forecasting strategy is retrospectively applied to a recent field monitoring case history where the observational method is used to update the input parameters of the data-driven surrogate.
Large-diameter 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. A ‘cutting face’ at the base of the caisson wall, resembling an inclined ring footing, is typically used to aid the sinking phase. This paper describes a suite of over 15 000 finite-element limit analyses exploring the bearing capacity of a caisson cutting face, partially or wholly embedded in undrained soil. The primary aim of the study is to assess the influence of the cutting face inclination angle on the vertical bearing capacity. The effects of cutting face roughness, internal overburden and surcharge, and caisson radius are also investigated. In particular, the results indicate that a steepening of the inclination angle may not always reduce the bearing capacity, if the cutting face is rough. The numerical output informs the development of a closed-form approach for application in routine design. The new design method is shown to provide an excellent representation of the numerical output.
Open caissons are sunk into the ground by their own weight. A cutting edge of the caisson having a tapered inner face on loading – that is, raising of the steining – results in bearing failure by displacing the soil which is in contact with the cutting edge. The bearing capacity of the cutting edge and the soil flow mechanism depend on the configuration of the cutting edge, sinking depth and soil type. This paper presents the results of a series of 1g model tests, which investigate the effect of varying tapered angles of the cutting edge on the penetration resistance of the open caisson. The vertical failure load and corresponding vertical bearing capacity factor, N′γ, and the soil flow mechanism around the cutting edge are investigated. The soil flow mechanism and the influence of surcharge formed at the top level of the cutting edge due to advancement of the caisson in the ground are examined using the image-based deformation measurement technique. The results highlight that the cutting angle of the cutting edge and sinking depth play important roles in the load–penetration response and soil flow mechanism.
An open-dug caisson shaft is a form of top-down construction in which a concrete shaft is sunk into the ground using the weight of the shaft and additional kentledge, if required. Excavation at the base of the caisson shaft wall allows the structure to descend through the ground. A thorough understanding of the interaction between the caisson shaft and soil is essential to maintain controlled sinking of the caisson. In this paper, the failure mechanisms developed beneath caisson blades in sand are investigated. A series of laboratory tests were carried out at the University of Oxford to explore how varying blade angles affect the performance of the bearing capacity beneath the caisson. Cutting angles of 30°, 45°, 60°, 75° and 90° (flat) were penetrated into sand under plane strain conditions; forces were monitored using a Cambridge-type load cell while soil displacements were recorded using Particle Image Velocimetry (PIV) techniques. The aim of this study is to understand how the soil failure mechanism develops and to determine the optimum cutting angle. The results of the laboratory tests can be scaled to predict the likely behaviour in the field. Results show that the bearing capacity is significantly dependent on the cutting angle; in a dense sand a steep cutting angle may be used to aid sinking of the caisson.
A systematic study is reported of the behaviour of graded silica sands and a silica rock flour silt, sheared against steel interfaces. Ring shear experiments investigated the influences of interface position, displacement scale, normal stress level and particle size distribution on ultimate interface friction angle, δ′cv Grain crushing is shown to play an important role within a concentrated shear zone, and this process, along with changes in the roughness of the interface, could lead to δ′ values markedly different from those developed in conventional small-displacement direct shear tests. It is also shown that δ′ measurements at smaller displacements may depend on whether the interface is mounted above or below the sample, but that such differences diminish to zero when displacements are large. The finding that δ′ is strongly displacement-dependent over the single-millimetre to multiple-metre scale has many practical geotechnical engineering implications.
Open caissons are used for many geotechnical engineering applications. Open caissons may be used as
deep foundation elements bypassing weak soils to tip in firm deeper strata, and in rivers and maritime
construction to reduce the risk of scour. Open caissons are also used for collecting sewage water
through gravity sewer pipe networks or from sewer force mains. In such applications, the design and
construction of open caissons require a detailed soil investigation program. In this way, the design and
construction plan of an open caisson can be developed with full knowledge of the prevailing subsoil
conditions. The engineering and construction techniques are key factors to achieve functional caissons.
Based on close observations during construction stages, the current study presents some challenges
that were encountered during the construction of two open caissons of internal diameters 20 m and
10 m (65.6 ft and 32.8 ft). This paper describes the procedure followed to alleviate the construction
difficulties encountered. Site exploration program and control measures required to satisfy design
and construction requirements are crucial aspects. Sinking of open caissons in dense or very dense
sands is risky. Incorrect sinking of open caissons may cause extra cost, delay in construction, and
harm to nearby structures. Air/water jetting near the cutting edge of an open caisson, outside slurry
trench, and/or inside open trench may be used to drive an open caisson downward. Unsymmetrical
work around an open caisson may lead to tilting of the caisson. If this occurs, the tilt should be
immediately corrected before resuming the sinking process. Improper cleaning of fine materials on
the caisson’s excavation bed, and/or inappropriate pouring of underwater concrete may result in a
defective concrete seal. The paper contains a series of practical guidelines to assist those intending to
use open caissons, and shares good caisson sinking practice with practitioners. Finally, the study aims
to understand the difficulties encountered and to anticipate future problems.
Large-diameter open caissons are a widely used construction solution for deep foundations, underground storage and attenuation tanks, pumping stations, and launch and reception shafts for tunnel boring machines. The sinking phase presents a number of challenges during construction including maintaining caisson verticality, controlling the rate of sinking and minimizing soil resistance through the use of lubricating fluids. This paper describes the instrumentation and monitoring of a large-diameter caisson on a UK construction site. The caisson was instrumented for the measurement of (a) settlement and tilt, (b) soil-structure interaction contact stresses and (c) structural performance. A key objective for the monitoring project was to provide real-time feedback to the site engineering team to inform the construction process. The monitored data reveal the occurrence of complex soil-structure interactions during sinking which are not readily captured by existing prescriptive design approaches. This case history provides valuable information for the development of an improved basis for design as well as an important frame of reference for future monitoring projects.
Large-diameter open caissons are a widely-adopted solution for deep foundations, underground storage and attenuation tanks, pumping stations, and launch and reception shafts for tunnel boring machines. The sinking process presents a number of challenges including maintaining verticality of the caisson, controlling the rate of sinking, and minimizing soil-structure frictional stresses through the use of lubricating fluids. A bespoke monitoring system has been developed at University of Oxford to provide early warning of adverse responses during the sinking phase (e.g. excessive soil-structure interface friction). The monitoring system was trialled on a recent pilot project in the UK involving the construction of a 32 m internal diameter, 20 m deep reinforced concrete caisson. This paper describes the monitoring system that was developed and its impact on the construction process of the pilot project. Early indications are that real-time feedback of live construction data has a major impact on the efficiency and safety of the construction process.
Based on the assumption of pointed blade of circular cassion, the formulas of the bearing capacity of soil beneath the blade are presented by the slip line method. The values of Nγ are not too sensitive to change for the soil’s internal friction angle, and are significantly greater than those obtained by Terzaghi’s and Berezantsev’s formulas. They decrease with an increase in the value of blade embedment depth and the taper angle.
Design of shallow foundations relies on bearing capacity values calculated using procedures that are based in part on solutions obtained using the method of characteristics, which assumes a soil following an associated flow rule. In this paper, we use the finite element method to determine the vertical bearing capacity of strip and circular footings resting on a sand layer. Analyses were performed using an elastic–perfectly plastic Mohr–Coulomb constitutive model. To investigate the effect of dilatancy angle on the footing bearing capacity, two series of analyses were performed, one using an associated flow rule and one using a non-associated flow rule. The study focuses on the values of the bearing capacity factors Nq and Nγ and of the shape factors sq and sγ for circular footings. Relationships for these factors that are valid for realistic pairs of friction angle and dilatancy angle values are also proposed.
Open caissons are a very efficient method of deep foundation, provided the ground conditions are favorable. Design and construction of a 60‐m deep shaft, 27.0‐m ID for a pumped storage hydroelectric power station is described. The soil consists of about 8 m of soft fluvial deposits on a 200‐m thick layer of homogeneous over consolidated stiff marly clay. The upper 8 m of soft soil were excavated and replaced by 5 m of compacted soil on which the leading ring of the shaft was concreted. Above the leading ring a 20‐cm wide slit filled with stabilized bentonite slurry was provided in order to eliminate skin friction during sinking. The shaft was concreted using slip form work. The method of excavation and verticality control of the shaft axis during excavation and concreting and the construction procedure of the shaft bottom are described in some detail. The shaft was successfully completed within 120 working days. Two examples of open caisson foundation that had to be changed due to unfavorable soil conditions are discussed. It is emphasized that careful soil exploration is prerequisite to success in open caisson foundations.
Open caisson-sinking techniques permit a shaft structure to be progressively sunk, either under its own weight or with the aid of caisson jacks, in a controlled manner from the surface to a predetermined depth. The technique is suited to shaft construction through weak soils, high-to extremely high-plasticity clays, silts, and sands and gravels, particularly below the water table. In the tunnelling and public works engineering sectors, open caissons are typically circular in cross-section; those used in harbour works are commonly square or rectangular in plan. The paper briefly describes the components and method of sinking dry and wet open caissons, highlighting good practice. It then examines in detail four recent, highly successful caisson-sinking operations carried out under a framework agreement for Scottish Water Solutions in their wastewater treatment works at Crianlarich, Doune, Killearn and Drymen. Although modest in size, each contractor-designed caisson was sunk through a variety of ground and water conditions, illustrating the versatility of the technique and its advantages over more conventional methods of shallow-depth shaft construction. The importance of a comprehensive site investigation programme and the control measures required to satisfy verticality and structural integrity requirements are discussed. The paper concludes with a series of practical guidelines designed to assist those contemplating using the technique, and to remind practitioners of good caisson-sinking practice.
Fundamental conceptionsAssumptions involved in the theories of consolidationDifferential equation of the process of consolidation of horizontal beds of ideal clayThermodynamic analogue to the process of consolidationExcess hydrostatic pressures during consolidationSettlement due to consolidationApproximate methods of solving consolidation problemsConsolidation during and after gradual load applicationEffect of gas content of the clay on the rate of consolidationTwo- and three-dimensional processes of consolidation
A method is outlined for determination of the bearing capacity of the beds of caissons beneath various shapes of blades implanted
into sandy and clayey soils.
Investigation of Soil-Structure Interaction for Large Diameter Caissons
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