Braced Excavation Research Articles

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.

Assessment of Wall Deflections and Ground Settlements for Braced Excavations Subjected to Groundwater Drawdown: Numerical Simulations and Design Charts

Assessment of Wall Deflections and Ground Settlements for Braced Excavations Subjected to Groundwater Drawdown: Numerical Simulations and Design Charts

3D numerical analysis of pile response due to soil movements induced by an adjacent deep excavation

Purpose This paper aims to investigate single pile and pile group responses due to deep braced excavation-induced soil movement in soft clay overlying dense sand. The analysis focuses first on the response of vertical single pile in terms of induced bending moment, lateral deflection, induced axial force, skin resistance distribution and pile settlement. To better understand the single pile behaviour, a parametric study was carried out. To provide further insights about the response of pile group system, different pile group configurations were considered. Design/methodology/approach Using the explicit finite element code PLAXIS 3 D, a full three-dimensional numerical analysis is carried out to investigate pile responses when performing an adjacent deep braced excavation. The numerical model was validated based on the results of a centrifuge test. The relevance of the 3 D model is also judged by comparison with the 2 D plane strain model using the PLAXIS 2 D code. Findings The results obtained allowed a thorough understanding of the pile response and the soil–pile–structure interactions phenomenon. The findings reveal that the deep excavation may cause appreciable bending moments, lateral deflections and axial forces in nearby piles. The parametric study showed that the pile responses are strongly influenced by the excavation depth, relative pile location, sand density, excavation support system and pile length. It also showed that the response of a pile within a group depends on its location in relation to the other piles of the pile group, its distance from the retaining wall and the number of piles in the group. Originality/value Unlike previous studies which investigated the problem in homogeneous geological context (sand or clay), in this paper, the pile response was thoroughly studied in a multi-layered soil using 3 D numerical simulation. To take into account the small-strain nonlinear behaviour of the soil, the Hardening soil model with small-strain stiffness was used in this analysis. For a preliminary design, this numerical study can serve as a practical basis for similar projects.

Investigation of Surface Settlement and Wall Deflection Caused by Braced Excavation in Spatially Variable Clays Based on Anisotropic Random Fields

Investigation of Surface Settlement and Wall Deflection Caused by Braced Excavation in Spatially Variable Clays Based on Anisotropic Random Fields

Experimental investigations on a deep excavation support system with adjustable strut length

Conventional deep excavation support systems (diaphragm wall with inner strut) have fixed length of struts, which can lead to a significant discrepancy between deformations of actual and designed diaphragm wall systems. As a result, failure of support system is likely to occur during the deep excavation. In this paper, a novel deep excavation support system with adjustable strut length was proposed, whereby the construction risk during deep excavation can be effectively reduced. An intelligent adjustment method of coordinated deformation of deep excavation support system was proposed as well. Model tests were carried out to investigate the influence of inner strut length on the mechanical and deformation characteristics of the system consisting of inner strut, diaphragm wall and soil behind diaphragm wall. The inner strut forces, lateral earth pressure on the diaphragm wall, the ground settlement and the deformation of the diaphragm wall were measured and analyzed. Results show that control the horizontal displacement of the braced excavation strictly will lead to the increment of axial force and the axial compression ratio of strut, which pose a threat to the safety of the strut system. By contrast, a reasonable adjusted length of inner strut is favorable to the safety of construction.

Probabilistic back analysis for improved reliability of geotechnical predictions considering parameters uncertainty, model bias, and observation error

The predicted performance of a geotechnical system may deviate from the in situ observation due to the uncertainties in the input geotechnical parameters, solution model, and observation error. A precise characterization of these uncertainties is a significant challenge primarily because of limited data availability. The Bayesian theory provides a means for updating these uncertainties by incorporating prior statistical information and observations. However, conventional Bayesian inference focuses on limited sources of uncertainties. This paper presents a probabilistic back analysis method for improved reliability of subsequent predictions that considers all the uncertainties. Three distinct features of this new method include: (1) multiple observations are incorporated into the Bayesian updating, (2) the statistical information of the uncertain variables is updated in a stage-by-stage manner, and (3) the posterior distributions of uncertain variables are derived with Markov Chain Monte Carlo (MCMC) simulation that is based on the Hamiltonian Monte Carlo (HMC) algorithm. Two case histories, including a braced excavation problem and a tunnel excavation problem, are analyzed to demonstrate the effectiveness of the new method. The advantages of this new back analysis method over the conventional Bayesian updating analyses are documented.

Closed-form solution for excavation-induced ground settlement profile in clay

Prediction of ground movement adjacent to an excavation is critically important, especially in urban areas, to prevent any potential damage of neighboring buildings. In the present study, a closed-form solution for prediction of excavation-induced ground settlement profile is proposed based on elastic displacement theory. Superposition method is employed to adapt the basic solution for translation wall deflection mode to general wall deflection mode. By assuming the wall deflection in braced excavation to be subject to a piecewise quadratic function, the displacement boundary of the problem is determined. An easy-to-use explicit solution is then developed. The validity of and applicability of the proposed solution is verified with existing analytical solution, finite-element model and six well-documented case records. The ground settlement profiles predicted on the basis of the new approach are in good agreement with reported settlement data. The results show that the proposed solution can be used as a feasible approach to obtain ground settlement profile in clay.

Numerical investigation on performance of braced excavation considering soil stress-induced anisotropy

Numerical investigation on performance of braced excavation considering soil stress-induced anisotropy

Effect of arching on active earth pressure distribution against braced wall

ABSTRACT Active earth pressure distribution against the braced wall depends on the mode of wall movement, which may be translational or rotational. The pressure distribution is generally non-linear resulting from arching effects. Here new formulations are proposed for calculation of active earth pressure above and below cut level of braced excavation considering arching effects. Horizontal translation as well as rotation about the top of wall is accounted for above excavation level whereas in case of below cut level it is assumed that the wall is tilting around its lower edge. Expressions for earth pressure coefficient and soil deformation profile are also provided analysing the equilibrium condition of soil within failure zone. In order to find out effects of major soil parameters like soil friction angle ‘ϕ’, wall friction angle ‘δ’ and cohesion ‘c’, parametric studies are carried out. The prposed formulations are validated with existing case studies and found satisfactory results.

Assessment of apparent earth pressure for braced excavations in anisotropic clay

The evaluation of strut forces is critical to ensure the system stability of braced excavation in urban areas. The most common empirical approach to determine the forces in the struts is the empirical apparent pressure diagram (APD) method. Although the importance of soft clay anisotropy on the stability of excavation systems has been demonstrated by various researchers, there are only limited studies on the influence of anisotropy of soft clay on the forces in the struts. In this paper, extensive plane strain finite element studies have been carried out to analyze the strut forces and the apparent earth pressures for braced excavations in anisotropic clay deposits. The NGI-ADP soil constitutive model was adopted in the finite element analyses to assess the influence of various soil properties and wall stiffness properties on the strut forces. Based on the numerical results, modified design pressure envelopes which consider the anisotropic behavior of clays are proposed for flexible and stiff walls.

Allowable wall deflection of braced excavation adjacent to pile-supported buildings

To protect adjacent buildings is a major concern for the construction of excavation in urban areas. In practice, the impacts on neighboring pile-supported buildings are normally minimized by limiting the deflection of earth retaining wall. Existing deflection criteria, however, are often empirical. In this work, we employ an analytical model to relate the wall deflection of braced excavation to the response of adjacent pile-supported buildings and thus forming a theoretical tool for determining the allowable deformation of excavation support structures based on the tolerance of buildings to distortion. The model combines closed-form excavation-induced free-field soil movements with elastic continuum solution that explicitly accounts for the interactions between raft, piles, and soils. Following validation against field model test and finite element simulation, the model is utilized to reveal the correspondence between the angular distortion of pile-supported buildings and the maximum retaining wall deflection under different combinations of excavation geometry, soil properties, and parameters of pile foundation potentially encountered in practice. A dimensionless factor composed of these influencing variables is proposed, and its correlation with the ratio of the building angular distortion over the maximum retaining wall deflection provides a rational way to determine the serviceability limit states of braced excavation.

Simulating the effects of braced excavation on the adjacent tunnel using numerical method

It is inevitable that the construction of braced excavation is in the vicinity of existing tunnels. Therefore, it is necessary to assess the influence of braced excavation on the nearby tunnel. Using two-dimensional (2D) numerical method, this study is conducted to investigate the response of existing tunnel induced by overlying excavation. A series of scenarios are carried out, with consideration of relative locations of tunnel with respect to overlying excavation. The results demonstrate that different distortion of the tunnel will occur with the variety of relative locations between the tunnel and the braced excavation. Additionally, the excavation-induced tunnel responses show a decrease trend with an increase distance between the tunnel and excavation. In this regard, the study can provide a useful reference for evaluating the influence of braced excavations on nearby structures.

Bottom Reinforcement in Braced Excavations: Coupled Analysis and New Method for Basal-Heave Stability Study

Bottom Reinforcement in Braced Excavations: Coupled Analysis and New Method for Basal-Heave Stability Study

Base Instability Triggered by Hydraulic Uplift of Pit-in-Pit Braced Excavations in Soft Clay Overlying a Confined Aquifer

The aim of this study is to investigate the coupling effects of re-excavation and hydraulic uplift on base instability of pit-in-pit (PIP) braced excavations. The numerical model of PIP braced excavation in Shanghai soft clay overlying a confined aquifer was established by upper-bound finite element limit analysis (UBFELA) method. The effects of the sensitive design parameters (i.e., the artesian pressure, thickness and undrained shear strength of the aquitard and excavation width of inner pit) on failure mechanisms and upper-bound safety factor (FS) against hydraulic uplift were analyzed. The results show that the value of FS increases with an increase in the thickness and undrained shear strength of the aquitard, but decreases with increasing the artesian pressure and excavation width of inner pit. The failure modes can be typically classified into three categories: circular slip surface in outer pit (M1), hydraulic uplift combined with circular slip surface in entire PIP system (M2), and basal hydraulic uplift in inner pit (M3); then the corresponding critical artesian pressure is determined. Finally, the average value of critical artesian pressure used to distinguish the three types of failure modes is recommended as the design value against hydraulic uplift in the PIP system, and validity is verified by the comparison with the current design methods. The proposed stability design by UBFELA contributes to ensure the serviceability and performance of PIP system.

A simplified method for estimating the longitudinal and circumferential behaviors of the shield-driven tunnel adjacent to a braced excavation

The construction of soil excavations will inevitably have an adverse effect on the adjacent tunnels. However, the structural responses of existing shield-driven tunnels adjacent to a braced excavation and their serviceability are still not fully understood. This paper presents a simplified method for evaluating the longitudinal and circumferential behaviors of the shield-driven tunnel adjacent to a braced excavation. To capture both shear and bending deformation of segmental linings, an appropriate model was adopted to study the longitudinal tunnel deformation features based on the Timoshenko beam theory. A continuous ring structure under a plane strain condition was used to determine the circumferential behavior of segmental lining. Case history was used to verify the developed simplified method. Systematic parametric studies were performed to develop a better understanding of the governing factors of the tunnel behaviors, including excavation-tunnel relative position and the retaining wall deflection. Furthermore, the serviceability limit state of shield tunnels was quantified using the defined three factors of safety, which enables a quick assessment of the excavation-induced tunnel damage potential. The contours of factor of safety can also be employed to determine the envelope of the excavation influence zones.

Assessment of basal heave stability for braced excavations in anisotropic clay using extreme gradient boosting and random forest regression

A finite-element analysis considering the anisotropy for the undrained shear strength was performed to examine the effects of the total stress-based anisotropic model NGI-ADP (developed by Norwegian Geotechnical Institute based on the Active-Direct simple shear-Passive concept) parameters on the base stability of deep braced excavations in clays. These parameters included the ratio of the plane strain passive shear strength to the plane strain active shear strength suP/suA, the ratio of the unloading/reloading shear modulus to the plane strain active shear strength Gur/suA, the plane strain active shear strength suA, the unit weight γ, the excavation width B, the wall thickness b, and the wall penetration depth D. According to the numerical results for 1778 hypothetical cases, extreme gradient boosting (XGBoost) and random forest regression (RFR) were adopted to predict the factor of safety (FS) against basal heave for deep braced excavations. The results indicated that the anisotropic characteristics of soil parameters need to be considered when determining the FS against basal heave for braced excavation. XGBoost and RFR can yield a reasonable prediction of the FS. This paper presents a cutting-edge application of ensemble learning methods in geotechnical engineering.

Estimation of diaphragm wall deflections for deep braced excavation in anisotropic clays using ensemble learning

This paper adopts the NGI-ADP soil model to carry out finite element analysis, based on which the effects of soft clay anisotropy on the diaphragm wall deflections in the braced excavation were evaluated. More than one thousand finite element cases were numerically analyzed, followed by extensive parametric studies. Surrogate models were developed via ensemble learning methods (ELMs), including the eXtreme Gradient Boosting (XGBoost), and Random Forest Regression (RFR) to predict the maximum lateral wall deformation (δhmax). Then the results of ELMs were compared with conventional soft computing methods such as Decision Tree Regression (DTR), Multilayer Perceptron Regression (MLPR), and Multivariate Adaptive Regression Splines (MARS). This study presents a cutting-edge application of ensemble learning in geotechnical engineering and a reasonable methodology that allows engineers to determine the wall deflection in a fast, alternative way.

A theoretical study on ground surface settlement induced by a braced deep excavation

Ground surface settlement (GSS) is one of the major concerns in design and construction of a deep excavation. This paper proposes an analytical approach for the prediction of GSS induced by a braced deep excavation. Considering wall deformation and stress release, the problem investigated is formulated as a system of two second-order partial differential equations (Lame equations) with mixed boundary conditions based on the elastic theory. Taking advantage of the superposition principle, the mixed boundary conditions are decomposed into displacement and stress boundary. The separation of variables method is applied to solve the governing equations with displacement boundary, while the Fourier Transform Method is employed to derive the solution for the governing equations with stress boundary. A novel least-squares based method is proposed to transform the scatter data of wall deflection into a continuous function, which is used to determine the unknown coefficients in the solution. The validity of the proposed solution is checked by predicting the GSS of two well-documented cases and by comparing with some empirical approaches. Parametric studies are conducted to demonstrate the impact of the modulus ratio on the excavation responses.

Effects of jet grouting slabs on responses for deep braced excavations

Jet grouting slabs are widely used in deep excavations owing to their effectiveness for reducing the deflection of the diaphragm wall and the prop forces acting on the struts and improving the basal-heave stability. In this paper, according to case histories in Singapore, a series of finite-element numerical simulations are performed to evaluate the effects of jet grouting slabs on responses to deep braced excavations. On the basis of a parametric sensitivity study, a reasonable thickness of jet grouting slabs is proposed. The effects of the wall depth, wall stiffness, soft-clay thickness, and stiffness on the performance of the jet grouting slabs are assessed by comparing and analyzing a series of simulation results. It is found that the soft-clay thickness significantly affects the wall deflection and basal heave in deep excavation. During the design of support structures, soil profiles should be considered first. The findings of this study provide a reference and guidance for the support system design of similar projects.

Hybrid Method for Predicting the Response of a Pile-Raft Foundation to Adjacent Braced Excavation

AbstractWith the increasing development of underground spaces in cities, a critical indicator for the design of excavation in urban areas is controlling the damage potential of adjacent buildings. ...