Effectiveness of deep cement mixing walls with top-down construction for deep excavations in soft clay: case study and 3D simulation

This paper presents the observed and simulated effectiveness of deep cement mixing walls created using top-down (DCM-TD) construction techniques for a deep excavation in soft Bangkok clay. The wall system consisted of four rows of 0.7-m-diameter DCM columns, and the bracing system consisted of two 0.25-m-thick basement slabs and seven temporary struts. The effectiveness of the wall system compared to that of other wall systems was evaluated using the measured results of previous case studies. A 3D numerical analysis was performed to calculate forces in the basement slabs and bending moments in the DCM wall. Finally, series of parametric analyses of both DCM-TD and deep cement mixing walls created using bottom-up (DCM-BU) construction techniques were carried out, and their results were compared to highlight the effectiveness of DCM-TD and its applicability to excavations at greater depths. The field and numerical results show that DCM-TD is more effective than DCM-BU in terms of the limitations of lateral wall movement, the bending moment in a DCM wall and the thickness of a DCM wall for various depths because of a larger system stiffness. Therefore, DCM-TD is very effective and suitable for use in potential future deep excavations in urban areas.

     

A parametric study of wall deflections in deep excavations with the installation of cross walls

Several case studies have revealed that the installation of cross walls in excavations can effectively reduce the amount of wall deflection and ground settlement. However, the behaviour of the diaphragm wall due to the installation of the cross walls is still unclear. This study performed a series of 3D numerical studies of wall deflections for deep excavations with cross walls and studied the effects on the wall deflection of several parameters, including the number of cross walls, the distance to the cross wall, the cross wall interval, the cross wall height and the cross wall embedment. The results presented in this study can be used as a first approximation for cases in which cross walls are designed to reduce the wall deflection induced by deep excavation.     

Finite-element modeling of a complex deep excavation in Shanghai

The excavation of the north square underground shopping center of Shanghai South Railway Station is a complex deep excavation using the top-down construction method. The excavation has a considerable size and is close to the operating Metro Lines. In order to predict the performance of the excavation more accurately, 3D finite-element analyses are conducted to simulate the construction of this complex excavation. The effects of the anisotropic soil stiffness, the adjacent excavation, and zone excavation on the wall deformation are investigated. It is shown that the numerical simulation with anisotropic soil stiffness yields a more reasonable prediction of the wall deflection than the case with isotropic soil stiffness. The deformation of the shared diaphragm wall between two excavations is influenced by the construction sequence of the two excavations. The zoned excavation can greatly reduce the diaphragm wall deformation. However, only the zoned excavation at the first excavation stage affects the deformation of the walls significantly. When the depth of the excavation increases, the zoned excavation has minor effect on the deformation of diaphragm walls.     

Simplified approach to estimate the maximum wall deflection for deep excavations with cross walls in clay under the undrained condition

Previous studies have shown that use of cross walls in deep excavations can reduce the wall deflection to a very small amount. However, design of cross walls is costly because the deflection behavior of the diaphragm wall with cross walls is in nature three dimensional. The objective of this study was to establish a simplified approach used as a first approximation to design cross walls such that the lateral wall deflection can satisfy a design criterion. A series of parametric studies using a three-dimensional numerical method was performed to obtain the influence factors on wall deflections, including excavation geometry, wall system stiffness, axial stiffness of strut, axial stiffness of the cross wall, normalized undrained shear strength of clay and the cross wall depth. Then, a simplified formula for predicting the wall deflection for excavations without and with cross walls was established using multivariate regression analysis, respectively. The formulas were validated through 36 excavation cases without cross walls and 12 cases with cross walls. The simplified formulas can be used to develop a spreadsheet that estimates the cross wall sizes and intervals based on the entered excavation geometry, material properties of retaining-strut system, in situ undrained shear strength and tolerable wall deflection. The estimated cross wall sizes and intervals should be verified by an appropriate full numerical analysis.     

Numerical analysis of lateral movements and strut forces in deep cement mixing walls with top-down construction in soft clay

This article presents the observed and simulated lateral movements and strut forces induced in deep cement mixing walls under deep excavation using top-down construction techniques in soft Bangkok clay. The walls are supported laterally by permanent concrete slabs and temporary struts. A three-dimensional numerical model is first calibrated with observed data from a case study. Then, a parametric study is performed to compare this construction method with the bottom-up method and investigate the influence of the DCM wall thickness on lateral movements and strut forces of the wall.     

Assessment of basal stability for braced excavation systems using the finite element method

Conventional methods of predicting the basal stability of braced excavations are unable to take into consideration the stiffness of the retaining wall and the depth of penetration of the wall below the bottom of the excavation. A simple and improved procedure for predicting the stability of strutted excavations using the finite element method is presented. Detailed studies were carried out to assess the effects of the wall properties and soil geometry on the stability of the excavation.     

Three‐dimensional inverse analyses of a deep excavation in Chicago clays

Numerical models are commonly used to estimate excavation‐induced ground movements. Two‐dimensional (2D) plain strain assumption is typically used for the simulation of deep excavations which might not be suitable for excavations where three‐dimensional (3D) effects dominate the ground response. This paper adapts an inverse analysis algorithm to learn soil behavior from field measurements using a 3D model representation of an excavation. The paper describes numerical issues related to this development including the generation of the 3D model mesh from laser scan images of the excavation. The inverse analysis to extract the soil behavior in 3D is presented. The model captures the measured wall deflections. Although settlements were not sufficiently measured, the predicted settlements around the excavation site reflected strong 3D effects and were consistent with empirical correlations. Copyright © 2010 John Wiley & Sons, Ltd.     

Probabilistic Inverse Analysis of Excavation-Induced Wall and Ground Responses for Assessing Damage Potential of Adjacent Buildings

This paper presents an approach for the probabilistic inverse analysis of braced excavations based on the maximum likelihood formulation. Here, the soil parameters are updated using the observations of the maximum ground settlement and/or the maximum wall deflection measured in a staged excavation. The updated soil parameters are then used to refine the predicted wall and ground responses in the subsequent excavation stages, as well as to assess the building damage potential at the final excavation stage. Case study shows that the proposed approach is effective in improving the predictions of the excavation-induced wall and ground responses. More-accurate predictions of the wall and ground responses, in turn, lead to a more accurate assessment of the damage potential of buildings adjacent to the excavation. The proposed approach offers an effective means for a probabilistic inverse analysis of braced excavations.     

A simple prediction model for wall deflection caused by braced excavation in clays

Deep excavations particularly in deep deposits of soft clay can cause excessive ground movements and result in damage to adjacent buildings. Extensive plane strain finite element analyses considering the small strain effect have been carried out to examine the wall deflections for excavations in soft clay deposits supported by retaining walls and bracing. The excavation geometry, soil strength and stiffness properties, and the wall stiffness were varied to study the wall deflection behavior. Based on these results, a simple Polynomial Regression (PR) model was developed for estimating the maximum wall deflection. Wall deflections computed by this method compare favorably with a number of field and published records.     

A Numerical Comparison of the Behavior of a Braced Excavation Using Two and Three-Dimensional Creep Plastic Analyses

Current study deals with investigating the effects of both time factor and the selection of a constitutive model type on predicting deformations of an excavation braced by nailing using two and three-dimensional finite element analyses. In addition, the effects of stress path and the type of defined initial conditions of the analytical model on deformations of the floor and walls of the excavation are also studied. Time factor, in the form of earth materials’ creep, can largely be entered into calculation of deformations of excavations by conducting viscoelastic and viscoplastic analyses. On the other hand, there hasn’t been done a comprehensive study regarding the creep behavior of excavations through comparing the results of two-dimensional and three-dimensional numerical analyses so far. The results showed that it’s largely possible to approach the actual deformation behavior of an excavation by considering the constitutive model of soft soil creep, SSC model, in the numerical plastic analyses. The effects of stress path on the deformation behavior of the excavation walls and excavation floor are investigated by using OCR stress ratio and POP stress difference; These two factors, both of which are also analogous, represent a boundary value for swelling behavior of the excavation floor and an increasing rate for the deformation behavior of the excavation walls since the increase in OCR or POP is equal to the increase in the soil lateral pressure coefficient at rest.     

Three-dimensional numerical analysis of deep excavations with cross walls

Previous plane strain analysis of a case history has shown that cross walls in an excavation can effectively reduce movements induced by deep excavation. This study performed three-dimensional numerical analyses for 4 deep excavation cases with different installations of cross walls, including different excavation depths, cross wall intervals and cross wall depths. Both the observed and computed wall deflections for the 4 cases were compared with those of the same excavations that were assumed with no cross walls installed to demonstrate the effectiveness of cross walls in reducing lateral wall deflections. The results show that the cross wall also had a corner effect similar to that of the diaphragm wall. The deflection of the diaphragm wall was smallest at the location of the cross wall installed and then increased with the increasing distance from the cross wall, up to the midpoint between two cross walls. Many factors such as in situ soil properties, diaphragm wall properties, construction procedure, cross wall depth and so on may affect the amount of reduction in lateral wall deflections due to the installation of cross walls. Under the same condition, the amount of reduction was highly dependent on the depth of cross walls, distance to the cross walls and the cross wall interval.     

Influence of construction technique on the performance of a braced excavation in marine clay

Finite element analyses are used to quantify the effects of construction technique on the performance of braced excavations. It is first shown that the computations can be continued to ‘collapse’ and that the results agree with limit plasticity solutions. A case study involving stratified deposits of marine clay and sand is used to carry out a Class C1 ‘prediction’ of field performance. The influence of construction technique on deformations and strut loads is shown. The results of the computations are in agreement with field observations, assuming a relatively ‘flexible’ construction technique. However, even if much ‘stiffer’ techniques had been used, large field deformations would have been unavoidable under certain circumstances.     

Numerical studies of anchored sheet pile wall behavior constructed in cut and fill conditions

The construction of sheet pile walls may involve either excavation of soils in front or backfilling of soils behind the wall. These construction procedures generate different loading conditions in the soil and therefore different wall behavior should also be expected. The conventional methods, which are based on limit equilibrium approach, commonly used in the design of anchored sheet pile walls do not consider the method of construction. However, continuum mechanics numerical methods, such as finite element method, make it possible to incorporate the construction method during the analyses and design of sheet pile walls. The effect of wall construction type for varying soil conditions and wall heights were investigated using finite element modeling and analysis. The influence of construction method on soil behavior, wall deformations, wall bending moments, and anchor forces were investigated. The study results indicate that walls constructed by backfill method yield significantly higher bending moments and wall deformations. This paper presents the results of the numerical parametric study performed and comparative analyses of the anchored sheet pile walls constructed by different construction methods.     

两侧铰接地下连续墙的试验研究及数值分析

地下连续墙用于平面不规则形状基坑支护时,对任意直线段墙体,其受力与变形实际上是三维的,而不是一般经验简化方法假设的二维变形与受力,并应考虑相邻墙体之间的相互作用。采用考虑墙土相互影响的地下连续墙与土共同作用的三维有限元方法,研究了墙端铰接和墙端自由两种边界条件对等刚度及变刚度墙体内力与变形的影响,计算结果表明,建议的理论计算方法与模型试验结果吻合较好。通过与模型试验实测值的对比,指出了以往采用平面有限元进行分析的方法的不足,并重点分析了墙端铰接对墙体横向弯矩的影响,研究结果表明,三维变形产生的横向弯矩是可观的,必须加以考虑。     

State-of-the-art review of soft computing applications in underground excavations

Soft computing techniques are becoming even more popular and particularly amenable to model the complex behaviors of most geotechnical engineering systems since they have demonstrated superior predictive capacity,compared to the traditional methods.This paper presents an overview of some soft computing techniques as well as their applications in underground excavations.A case study is adopted to compare the predictive performances of soft computing techniques including eXtreme Gradient Boosting(XGBoost),Multivariate Adaptive Regression Splines(MARS),Artificial Neural Networks(ANN),and Support Vector Machine(SVM) in estimating the maximum lateral wall deflection induced by braced excavation.This study also discusses the merits and the limitations of some soft computing techniques,compared with the conventional approaches available.     

Evaluation of buttress wall shapes to limit movements induced by deep excavation

Three-dimension finite element analyses of deep excavations with buttress walls were performed to evaluate the effect of buttress wall shapes on limiting movements induced by deep excavation. Results showed that a combination of the rectangular and the capital L-letter shapes (RL-shape) yielded the greatest performance in reducing wall deflections and ground surface settlements. The main deformation-control mechanism mainly came from the horizontal and vertical frictional resistances of buttress walls against adjacent soils which were pushed by wall deflections and the soil heave at the excavation bottom, respectively. Besides, the RL-shape buttress walls were successfully verified through a well-documented case history.     

Efficiency of excavations with buttress walls in reducing the deflection of the diaphragm wall

Installation of buttress walls against diaphragm walls has been used as an alternative measure for the protection of adjacent buildings during excavation, but their mechanism in reducing movements has not yet been fully understood. This study performs three-dimensional finite element analyses of two excavation case histories, one in clay with T-shape buttress walls and another in dominant sand with rectangular buttress walls, to establish analysis model. Then, a series of parametric study were performed by varying soil types, types and length of buttress walls based on the above-mentioned excavations. Results show that the mechanism of buttress walls in reducing wall deflections mainly came from the frictional resistance between the side surface of buttress wall and adjacent soil rather than from the combined bending stiffness from diaphragm and buttress walls. The buttress wall with a length <2.0 m had a poor effect in reducing the wall deflection because the soil adjacent to the buttress wall had almost the same amount of movement as the buttress wall, causing the frictional resistance little mobilized. Since the frictional resistance of buttress walls in a deep excavation has fully been mobilized prior to the final excavation depth, the efficiency of buttress walls in reducing the wall deflection in a deep excavation was much less than that in a shallow excavation. Rectangular shape of buttress walls was of a better effect than T-shape in the shallow excavation because frictional resistance between buttress walls and adjacent soil played a major role in reducing the wall deflection rather than bearing resistance of the flange. When the excavation went deeper, the difference in reducing the wall deflection between the R-shape and T-shape became small.     

Earth pressure diagram and field measurement of an anchored retention wall on a cut slope

To utilize space more effectively for constructing apartments, roads, infrastructure, etc., excavation work is typically found in slope areas. An anchored retention wall has been installed because of the presence of soil slopes behind the walls and unsymmetrical excavation sections. An instrumentation system is normally applied on the anchored retention walls of slopes to observe and estimate lateral earth pressure acting on anchored walls. The earth pressure acting on the wall is decreased with increasing the deformation of the wall during the progress of excavation work. An earth pressure diagram acting on the anchored walls can be presented approximately as a trapezoid. The earth pressure at the ground surface is larger than zero. Also, the earth pressure is increased linearly from the ground surface to 15% of total excavation depth and then keeps constant. The earth pressure acting on the anchored retention walls installed on the cut slope is higher than that of the horizontal ground surface behind the wall, owing to the surcharge load of the slope soils.     

Deep Cement Mixed Walls with Steel Inclusions for Excavation Support

Deep cement mixed (DCM) walls are widely used in supporting excavations in many parts of the world. In this paper, a case study of an excavation supported by a DCM wall with steel inclusions is analysed using a three-dimensional finite element model and based on the coupled theory of nonlinear porous media. The DCM wall is constructed with wide flange steel inclusions. The stress–strain behaviour of the DCM wall section is simulated using an extended version of the Mohr–Coulomb model, which considers the strain-softening behaviour of DCM columns beyond yield. The computed lateral deformations are compared with the field measurements to validate the numerical modelling procedure. Using the same case study, the internal stability of the wall against bending and shear failure modes is investigated. In addition, the lateral pressure distribution along the wall length is investigated because in practice design is carried out considering a uniform pressure distribution assuming rigid wall movements. A parametric study was carried out to investigate the viability of DCM walls in supporting excavations by varying the spacing between steel inclusions, wall thickness and initial lateral earth pressure. Based on the results of the parametric study, guidelines are proposed to select the most efficient geometric arrangement of steel inclusions within DCM walls.     

Damage Estimation in Masonry Buildings Based on Excavation-Induced Ground Movements

Excavation-induced ground movements and the resulting damages to adjacent structures and facilities is a source of concern for excavation projects in urban areas. The concern will be even higher if the adjacent structure is old or has low strength parameters like masonry building. Frame distortion and crack generation are predictors of building damage resulted from excavation-induced ground movements, which pose challenges to projects involving excavations. This study is aimed to investigate the relation between excavation-induced ground movements and damage probability of buildings in excavation affected distance. The main focus of this paper is on masonry buildings and excavations stabilized using soil nail wall method. To achieve this purpose, 21 masonry buildings adjacent to 12 excavation projects were studied. Parametric studies were performed by developing 3D FE models of brick walls and excavations stabilized using soil nail wall. Finally, probability evaluations were conducted to analyze the outputs obtained from case studies. Based on the obtained results, simple charts were established to estimate the damage of masonry structures in excavation affected distance with two key parameters including “Displacement Ratio” and “Normalized Distance”. The results also highlight the effects of building distance from excavation wall on its damage probability.