Excavation In Soft Clay Research Articles

Database for Deep Excavations in Soft Clay with Focus on Groundwater Drainage and Installation Effects

Database for Deep Excavations in Soft Clay with Focus on Groundwater Drainage and Installation Effects

Centrifuge study on behavior of raft foundation after tunnelling in soft clay

Ground losses due to tunneling would induce settlement of nearby raft foundations. To study the change in behavior of the raft foundations over time due to tunnel excavation in soft clay, a series of centrifuge model tests were conducted. The results reveal that the raft stiffness has a significant influence on the development of the gap between the raft and the ground. The width of the gap beneath the flexible foundation would increase over time, leading to a further increase in tensile strain after excavation, whereas the gap for raft foundations with a large stiffness would reduce with time, causing a gradual decrease in tensile strain. The modification factor (MF) design approach is also evaluated with the test results and demonstrates that the MF design approach would underestimate the tensile strain of the flexible raft and provide a relatively conservative prediction for larger stiffnesses.

Monitoring the construction of a deep energy-from-waste bunker in soft clay and peat

This paper describes the monitoring of the construction of a deep underground waste storage bunker for the PROTOS energy-from-waste facility in Chester, UK. A key element of the construction process was a 12 m deep excavation in soft clays and peats which was supported by a complex ‘combi-wall’ cofferdam comprising alternate tube and sheet piles, a reinforced concrete (RC) capping beam and two levels of internal bracing. The permanent bunker RC structure was constructed within the excavation using a bottom-up approach. The primary aim of this monitoring programme was to assess the performance of the composite support system through measured cofferdam wall deflections, bracing member loads, and site and bunker water levels. An additional objective of the monitoring was to provide the site team with real-time feedback to inform the construction process. Interpretation of the monitored data allows for a quantitative assessment of the influence of various construction activities on the responses of the support system. The monitored behaviour shows that the high stiffness and embedment depth of the supporting structure, combined with careful control of groundwater levels, significantly reduced risks associated with deep excavation construction in challenging ground conditions.

A parametric study on deformation behaviour for design of braced excavation in soft clay

ABSTRACT Adequate prediction of surrounding ground movement during braced excavation is critically important as excessive soil movement damages adjacent structures. The magnitudes and patterns of ground movement and wall deflection largely depend on excavation parameters like thickness of diaphragm wall, wall embedment depth, strut locations and soil parameters such as soil strength, compressibility and creep parameter. In the present paper a thorough, parametric study has been conducted using finite element (FE) analysis to address the influence of various parameters on deformation characteristics of braced excavation in soft clayey deposits. The importance of correct estimation of soil parameters for braced excavation design is also documented. The analysis of typical braced excavations in soft clay is carried out using PLAXIS 2D software where soft soil creep constitutive model is used. On the basis of numerical study a handy design guideline is recommended. Further multivariate regression models are developed incorporating various important excavation parameters for the adequate prediction of maximum wall and ground displacement along with wall and ground surface deformation profile. Here large numbers of data reported in case histories and generated artificially from FE analysis are used for formation of regression equations. The proposed model is validated comparing results from literatures not used for the development of the model.

An experimental study on time dependent ground settlement behind a braced excavation in soft clay using geotechnical centrifuge

An experimental study on time dependent ground settlement behind a braced excavation in soft clay using geotechnical centrifuge

A Simplified Analytical Method for the Deformation of Pile Foundations Induced by Adjacent Excavation in Soft Clay

The development of foundation excavations in congested urban areas inevitably induces stress release and soil movements, resulting in additional lateral deflections in adjacent pile foundations, commonly referred to as passive loading piles. Prior research on pile deflection resulting from nearby excavations primarily focused on single-pile behavior and paid little attention to the characteristics of pile groups. This paper presents a two-step approach to predicting the behavior of pile deformation resulting from nearby excavation. Firstly, Mindlin solution in combination with the double Gauss-Legendre formula is employed to calculate the additional lateral stress acting on the centerline of the passive pile resulting from nearby excavations. Secondly, the equation governing the deflection of the passive pile is determined using the Pasternak foundation model and solved using the finite difference method. On the basis of the Mindlin equation, shielding influence among piles is developed and applied to the analysis of a laterally loaded passive pile group. Last but not least, the accuracy of the presented approach is validated by comparing the results from two published centrifuge model tests with single piles and pile groups, and good agreements are obtained. Generally, the recommended two-step approach presents effective insight into the interaction between the excavation, soil, and pile, taking into account the influence of the shielding effect between piles. It can be applied as an alternative method for the conservation evaluation of the deformation tendency of pile foundation deformation in the pre-design of nearby excavations.

Numerical Study of the Effect of Ground Improvement on Basal Heave Stability for Deep Excavations in Normally Consolidated Clays

The ground-improvement technique is commonly used to restrain the wall displacement induced by excavation and resist the basal heave for deep excavations in soft clays. In this paper, a three-dimensional finite-element stability analysis was conducted to evaluate the effect of different ground-improvement properties and patterns on enhancing the factor of safety against basal heave in deep excavations. The results showed that a large downward movement of soil behind the wall could induce a large upward movement below the excavation, which is mainly resisted by the weight of the interior soil mass and the frictional resistance acting on the contact surface area between the wall and the soil inside the excavation. When the ground improvement was not contacted with the wall, the failure initially occurred on the wall interface, causing the soil inside the excavation to move upward along with the ground improvement, which resulted in an insignificant increase in the factor of safety. Hence, the ground improvement should be contacted to the wall to establish a higher basal heave resistance, where the increase of the safety factor was governed by the amount of the ground improvement that contacted the wall. Furthermore, a simplified method was proposed to calculate the factor of safety with and without ground improvement, which was validated by the results from the finite-element method.

Failure Investigation of Braced Excavation in Soft Clays: Case Study on the Collapse of Nicoll Highway

Deep excavation in clay normally causes large soil movement, which may induce basal heave failure. In this study, the three-dimensional finite element analysis with the strength reduction method was employed to evaluate the typical failure mechanism of braced excavation in soft clays. The collapse of Nicoll Highway was selected as a failure case history and further conducted by considering the full elastoplastic structural behavior. The results showed that the computed wall displacements were successfully verified through the observation data. It was also found that the excavation failure was initiated by the yielding of the strut-waler connection, which was similar to that of the field observation. Finally, the proposed finite element model adopted in this study can be used as a design recommendation to evaluate the failure mechanism and further prevent possible braced excavation failure in soft clays.

Method for Estimating Fully Coupled Response of Deep Excavations in Soft Clays

The influence of excavation rate, geometric layout, and hydraulic conductivity of basal and retained soils on the fully coupled solid–fluid behavior of braced excavations in cohesive soils is presented. Fully coupled excavation analyses were performed using the hypoplasticity clay model to reproduce the constitutive soil behavior combined with Biot’s consolidation theory. A method aimed at predicting excavation-induced ground movements using excess pore-water pressure ratios as a function of site-specific groundwater considerations is presented for the determination of the recommended type of excavation analysis (i.e., drained, partially drained, or undrained). Results obtained from the numerical simulations indicated that excavation rate to hydraulic conductivity ratios, ER/k, smaller than 0.1 and larger than 10,000 can be analyzed under drained and undrained conditions, respectively. The proposed method was validated with published case histories, and its ability to estimate excavation performance in terms of excess pore-water pressures and excavation rates is presented in this paper.

Improved inverse analysis methods and modified apparent earth pressure for braced excavations in soft clay

Inverse analysis procedures are used to update project parameters and applied to predict future performance in the later stages. However, the inverse analysis procedure of earth pressure of excavation has received less attention. This paper proposes an improved inverse analysis method and the modified apparent earth pressure model for braced excavations in soft clay. A new procedure for inverse analysis of excavation is suggested based on the quintic spline interpolation function, and a trial algorithm is used to eliminate the errors generated in the inverse process. The proposed inverse solutions are validated by numerical simulations and reported engineering examples and show good agreements. Based on the inverse data and empirical apparent pressure diagram (APD), a modified apparent earth pressure method for braced excavations in soft clay is further suggested. And the relationship between the degree of anisotropy of soft clay and apparent earth pressure coefficients is discussed. The study results provide a reference for dynamic feedback design and adaptive management of the excavation.

Efficient configurations of untreated opening in flat or inverted arch cement-treated soil layer in soft clay excavation

Cement-stabilized soil layer (CSSL) is often created to enhance lateral support to the retaining wall below the excavation level as it is impossible to install struts below the formation level. The CSSL may be subjected to tensile stress or even cracks due to the underlying uplift pressure. This paper reports a study to explore the feasibility of completely eliminating tensile stresses in a flat CSSL and an inverted arch CSSL by location and sizing of untreated openings. A large number of parametric cases were analyzed using fully-coupled effective stress analyses (ABAQUS 6.13). Parameters such as thickness of CSSL, thickness of underlying soft clay, length of CSSL, locations, shapes and sizes of untreated openings vary in the analyses. The results show that for a flat CSSL, three parameters play a vital role in eliminating the tensile stress in CSSL. They are the minimum distance between end of the opening and wall line, the minimum width of the zone between the opening and the lateral edge of the CSSL and the distance between the section of CSSL with the smallest section area and the wall line. For an inverted arch CSSL, the CSSLs without openings are generally effective in reducing tensile stresses. In contrast, the response of inverted arch CSSL with openings is much more complex owing to a departure from arching action, caused by the loading on the berm. The inverted arch profile in CSSL with openings should be used and designed with great care. Otherwise, the overall performance may be degraded rather than improved.

Performance of a 56 m deep circular excavation supported by diaphragm and cut-off double-wall system in Shanghai soft ground

The performance of a 56 m deep circular excavation supported by a double-wall system, consisting of an inner circular diaphragm wall and an outer rectangular cut-off wall, in Shanghai soft ground is studied in this paper. The surveyed data (e.g., lateral wall deflections ( δ h), horizontal displacement of soil ( δ s), ground surface settlement ( δ v), soil rebound ( δ v+), earth pressure ( P), pore water pressure ( Pw), and soil stress path) are systematically investigated. The results show that the maximum lateral wall deflections are 0.002% ∼ 0.03% of the excavation depth ( H e). The maximum ground surface settlement ( δ v m) is generally larger than δ h m, which is located at 0.4 ∼ 0.6 H e. Moreover, an innovative formula is proposed to estimate the ground settlement. It is able to distinguish various characteristics of surface settlement in different zones of the settlement profile. The ground surface settlements are mainly induced by surcharge loading and continuous excavation, and they are related to the change in confined water level. The lateral earth and pore water pressures in the active zone are relatively insensitive to the excavation, while the correlation becomes obvious in the passive zone. The findings from this study can be helpful to the design of other similar deep excavations in soft clay.

Soil and structure deformation during deep foundation pit excavation in soft clay: case study and sensitivity analysis

Soil and structure deformation during deep foundation pit excavation in soft clay: case study and sensitivity analysis

Pareto-optimal performance-based robust design of braced excavations in soft clay with response surface methodology

This paper presents a methodology for robust design optimization of braced excavations in soft clay with response surface methodology based on the pareto-optimal trade-off between system performance and cost. As prediction of maximum ground settlement induced by excavation is generally accompanied by an implicit numerical model, the response surface methodology is employed to build the performance function with respect to both design parameters and uncertain geotechnical parameters. Then a pareto-optimal performance-based robust design framework of braced excavations in soft clay is formulated to find the optimal design that meets the design requirements while optimizing the system performance and construction cost. With the aid of a multi-objective optimization approach, trade-off relationship between performance robustness and cost efficiency are developed to help identify the most robust design based on the targeted performance levels. A design example of braced excavation in Taipei soft clay is used to illustrate the significance of the proposed methodology.

Soil Creep Effect on Time-Dependent Deformation of Deep Braced Excavation

This paper describes recent advances in the effect of soil creep on the time-dependent deformation of deep braced excavation. The effect of soil creep is generally investigated using the observational method and the plain-strain numerical simulation method. The observational method is more applicable for deep braced excavations in soft clays constructed using the top-down method. The plain-strain numerical simulation method can be conveniently used for parametric analysis, but it is unable to capture the spatial characteristics of soil creep effect on lateral wall deflections and ground movements. The additional lateral wall deflections and ground movements that are generated due to the soil creep effect can account for as large as 30% of the total displacements, which highlights the importance of considering the effect of soil creep in deep braced excavations through soft clays. The magnitude of the displacements due to soil creep depends on various factors, such as excavation depth, elapsed period, unsupported length, and strut stiffness. Parametric analyses have indicated several effective measures that can be taken in practice to mitigate the detrimental effect of soil creep on the deformation of deep braced excavation. Based on the literature review, potential directions of the related future research work are discussed. This paper should be beneficial for both researchers and engineers focusing on mitigating the adverse effect of soil creep on the stability of deep braced excavations.

Case study of post uplift in deep excavation of a subway station in thick soft clay using long pile foundations

There is growing engineering concern about the base heave and post uplift phenomena in deep excavation in soft clay, which may pose a risk of instability of retaining systems. The purpose here is to conduct a detailed case study on the post uplift observed in a 17.6-meter-deep braced excavation of a subway station in thick soft clay (total thickness up to 42 m) in Shanghai. In this case, a large uplift up to 87 mm unexpectedly developed for the post founded on a 30-meter-long pile foundation. Efforts were first made to examine the complex relationships between the post uplift with the excavation depth (H), Terzaghi’s safety factor against base heave (Fs) and maximum deflection of retaining wall. A simplified approach for soil-post-strut interaction analysis was then proposed and used for quantitative research. The working characteristics of the long pile foundation under low safety factor against base heave (Fs < 1.5) are summarized as following: (a) the back-analyzed neutral plane, where soil uplift equals the post uplift, lies at approximately 0.68 times the pile length from the pile top; (b) deep soil movement below the neutral plane results in the observed post uplift; (c) strut reaction plays a minor role in the restriction of post uplift. The influence of base treatment and excavation/construction procedures on post uplift and the principles of pile foundation design are also discussed in this paper.

Analytical Study on the Transverse Internal Forces of Shield Tunnel Segments due to Adjacent Excavations in Soft Clays

Deep excavations unavoidably cause the changes in stress and displacement of surrounding soils, which further generate additional internal forces on the adjacent existing tunnel segments, and compromise the performance and stability of the tunnels. In this study, two-stage analysis method is used to theoretically calculate the additional internal forces in the tunnel segments due to laterally adjacent excavation. Based on an empirical deformation curve of diaphragm walls, the excavation-induced stress in a linear elastic soil is formulated using source-sink imaging method. A distribution model of additional external load acting on the tunnel segments due to adjacent excavation is further established, and the additional internal forces in the tunnel segments under the corresponding additional loads are estimated using elastic equation method. Parametric studies are then conducted to investigate the effects of excavation procedure, buried depth of shield tunnel, and excavation-tunnel horizontal distance on the additional internal forces in the tunnel segments. The results show that the studied parameters have significant effect on the additional internal forces in the tunnel segments due to laterally adjacent excavation. When the excavation-tunnel horizontal distance is less than the excavation depth, the accompanied effect on the additional internal forces in the tunnel segments is highly sensitive. This study can provide theoretical insights into the estimation of tunnel segment responses due to laterally adjacent excavation.

Numerical analysis on zone-divided deep excavation in soft clays using a new small strain elasto–plastic constitutive model

Accurate prediction of displacements associated with deep excavations is essential to ensure safety and stability of the excavation and to prevent any damage and distress to the adjoining infrastructures. This paper presents a numerical approach for prediction of ground displacements related to a zone-divided deep excavation construction executed in Shanghai soft clays based on a new elasto–plastic constitutive model (small-strain Shanghai model) that incorporates small strain stiffness. This model can describe the mechanical properties and structural and over-consolidated characteristics of natural clays. The model is implemented into a finite element analysis software. Numerical analysis on the deep excavation in Shanghai using zone-divided method is conducted. A comparison between monitored and simulated results of horizontal displacements along the diaphragm wall, the settlements in the surroundings, and the effects on the adjoining metro tunnel due to excavation construction is carried out. Special attention is paid to the stiffness degradation of representative elements in the ground. The simulated displacements show a good agreement with the monitored data. Overall, this study provides an integrated solution for predicting displacements related to deep excavation in soft clays.

Experience from short-and long-term performance of deep excavations in soft sensitive clays

Design of excavations and permanent underground structures requires accurate predictions of e.g. deformations and earth pressures for both the short-and long-term. As excavation depths increase and/or the proximity to adjacent infrastructure decreases, there is a need to improve and develop existing design methods and validate numerical models. The first part of this paper revisits the measurement data from a previous excavation, the Göta Tunnel in Gothenburg, Sweden, in order to benchmark a contemporary constitutive soil model, called Creep-SCLAY1S. This study looks into time series including e.g. final dewatering of the excavation, followed by the development of pore water pressures, earth pressures and deformations over time (until 2 years after excavation). The model predictions are in general in good agreement with the measurement data up to final dewatering. However, installation effects due to drilling are believed to have caused continued deformations which are difficult to capture in the numerical model. Part two of the paper presents details of a recently instrumented excavation in soft clay in Central Gothenburg. The measurement data comprises e.g. pore water pressures, deformations as well as vertical and horizontal earth pressures at three locations under the permanent structure. Continued long-term measurements are planned and the existing and future data are believed to provide valuable insights on the development of the stress state and earth pressures under permanent structures in soft clay.

Small strain stiffness within logarithmic contractancy model for structured anisotropic clay

Stiffness of soils in the small strain region is high and it decays nonlinearly with increasing shear strains or with mobilization of shear stresses. However, the commonly used critical state based constitutive models use a simple elastic formulation at small strains that falls short in the prediction of the small strain nonlinearity and anisotropy. This paper proposes a simple way for rendering the existing constitutive models with the capability to capture the small strain behaviour of soils. This is illustrated by proposing a new model for structured anisotropic clay extending an existing model that uses the framework of logarithmic contractancy called E-SCLAY1S. The proposed model is implemented into a Finite Element program as a user-defined soil model. The model predictions are compared with experimental data for various clays. Furthermore, the effect of nonlinearity is investigated for an excavation in soft clay.