Maximum Wall Deflection Research Articles

Large deformation problems arising from deep excavation in silt strata: A case study in Shenzhen, China

Deep excavations in silt strata can lead to large deformation problems, posing risks to both the excavation and adjacent structures. This study combines field monitoring with numerical simulation to investigate the underlying mechanisms and key aspects associated with large deformation problems induced by deep excavation in silt strata in Shenzhen, China. The monitoring results reveal that, due to the weak property and creep effect of the silt strata, the maximum wall deflection in the first excavated section (Section 1) exceeds its controlled value at more than 93% of measurement points, reaching a peak value of 137.46 mm. Notably, the deformation exhibits prolonged development characteristics, with the diaphragm wall deflections contributing to 39% of the overall deformation magnitude during the construction of the base slab. Subsequently, numerical simulations are carried out to analyze and assess the primary factors influencing excavation-induced deformations, following the observation of large deformations. The simulations indicate that the low strength of the silt soil is a pivotal factor that results in significant deformations. Furthermore, the flexural stiffness of the diaphragm walls exerts a notable influence on the development of deformations. To address these concerns, an optimization study of potential treatment measures was performed during the subsequent excavation of Section 2. The combined treatment approach, which comprises the reinforcement of the silt layer within the excavation and the increase in the thickness of the diaphragm walls, has been demonstrated to offer an economically superior solution for the handling of thick silt strata. This approach has the effect of reducing the lateral wall displacement by 83.1% and the ground settlement by 70.8%, thereby ensuring the safe construction of the deep excavation.

Centrifuge modelling of supported excavations using an improved in-flight excavation tool

A modified tool for simulating the entire process of supported excavation in centrifuge modelling was developed based on a Cambridge type of excavating system. Utilising the modified tool, a series of centrifuge tests of braced excavations in sand and clay soils were conducted to investigate the impacts of staged excavation on the characteristics of retaining wall displacement and ground settlement. The experimental results show that before the struts were installed, the movement of the retaining wall follows a cantilever movement pattern, in which the wall rotates about a fixed point near the toe and the maximum wall deflection occurs at the wall crest. For the strut installation process of a multi-strutted excavation, the wall deformation pattern changes to a concave profile. The maximum wall displacement in a multi-strutted excavation in sand is located slightly above the bottom of the excavation; while it is slightly below the bottom of the excavation in clay. The differences between the deformation pattern for multi-strutted excavations in sand and clay are clarified using the profiles of the ground settlement against distance from the wall. The results from this study can provide useful references for predictions of excavation-induced deformations and understanding the associated deformation mechanisms.

Buttress wall in limiting wall deformation caused by deep excavation: A case study for colluvial soil in Vietnam

The objective of this study is to assess the impact of utilizing a BW (Buttress wall) to control the deflection of a diaphragm wall in colluvial soil conditions in Vietnam. The physical and mechanical properties of the colluvial layers are evaluated using data closely monitored during a specific project, serving as validation for 3D numerical simulations utilizing the Hardening Soil Model. The analysis results closely match the field monitoring data, which has tested the accuracy of the simulation model. This forms the basis for further investigations into the dimensional parameters of BW walls, including length, thickness, and spacing between them. The results obtained from the parametric study demonstrate that altering the wall length and spacing between BWwalls significantly limits the deflection of the diaphragm wall, while changes in thickness have a negligible effect. Through the 3D numerical simulations, a linear relationship between the maximum wall deflection and parameters such as wall length and spacing between BW walls has been established.

Mechanical responses of shallowly buried pipelines to adjacent deep excavations considering heavy rainfall influence

Climate change has resulted in a high global incidence of heavy rainfall, severely damaging the urban infrastructure. The analytical method used to predict the mechanical response of pipelines under the influence of rainfall influence could serve as a valuable design basis in the preliminary stage. This study developed an analytical solution based on the stratified Green-Ampt model and displacement-controlled two-stage method. The first stage evaluates the greenfield soil displacement at pipeline elevation by employing the Mindlin solution and considering the superposed effects of excavation (unloading) and rainfall infiltration. The second stage predicts the mechanical responses of the pipeline by simplifying the pipeline–soil interaction into a continuous Euler–Bernoulli beam resting on a Winkler foundation model. The analytical solution was calculated using the finite difference method and verified against a published study. Finally, parametric studies were conducted to investigate the rainfall– excavation–pipeline interaction, including the rainfall intensity and duration, maximum lateral wall deflection, and pipeline location. The results showed that a 48-8-mm/h rainfall increased the maximum displacement and maximum bending moment of the pipeline by 6.54% and 7.93%, respectively. The prediction results provide practical guidance for designing deep excavations adjacent to pipelines buried under heavy rainfall conditions.

Performance of an Asymmetric Pit-in-Pit Excavation Supported by Diaphragm Wall and Multi Uplift Piles System in Coastal Areas

The performance of asymmetric pit-in-pit excavation supported by diaphragm wall and multi uplift piles in coastal areas has been rarely reported. In this work, case study is conducted to investigate the mechanical characteristics of such excavation. A numerical model is established using the ABAQUS finite element platform, and its effectiveness is evaluated by comparing with the field monitoring data. After evaluation, the effects of uplift piles on excavation-induced deformations are investigated. Results show that the maximum wall deflections (δhm) are 0.02%~0.22% of the excavation depth (He), with the ground surface settlements of less than 0.6%·He and the settlement influence zone extending beyond 4He. As the soil is excavated, the top wall shows outward deformations to the active zone, and the transverse support is under tension. The use of combined diaphragm wall and multi uplift piles reduces the embedment ratio of diaphragm wall to a minimum value of 0.14, and decreases the δhm and rebound of base soil (δvrm) by 42% and 63%, respectively. A design suggestion is proposed for pile diameter (D), pile length (L) and pile spacing (S) to fall within the range of 0.4~0.8 m, 10~20 m and 6D~8D, respectively.

Exploration of maximum wall deflection and stability for deep excavation in loose to medium-dense sand

Exploration of maximum wall deflection and stability for deep excavation in loose to medium-dense sand

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.

Optimized extreme gradient boosting machine learning for estimating diaphragm wall deflection of 3D deep braced excavation in sand

In a deep braced excavation, wall deflection plays an important role in its safety and also adjacent buildings. However, most of previous studies focused on predicting wall deflection caused by excavations in clay. Therefore, this paper intends to develop a machine learning (ML) method to predict the maximum wall deflections of deep braced excavations in sand, which has not yet received much attention. To this end, an advanced ML model of extreme gradient boosting (XGBoost) is employed. The performance of XGBoost model is optimized by using Bayesian optimization (BO) algorithm and fivefold cross-validation technique. For the development of the ML model, an extensive dataset is generated from large numbers of three-dimensional (3D) numerical simulations. The validation of the numerical model is conducted by using a centrifuge test. The wall deflections predicted by the optimized ML model indicate good agreement with the numerical results. The feature importance and feature interactions of the model are clearly assessed. Parametric study is also implemented to quantitatively investigate the effects of the input parameters on the maximum wall deflection. A web application for predicting the maximum wall deflection induced by excavations in sand is deployed and made available to the public.

Displacement Characteristics of a Deep Excavation in Hangzhou Soft Clay

Excavations in a soft soil area are usually associated with substantial difficulties. Taking a special‐shaped deep foundation pit in Hangzhou soft clay as the research object, the excavation performances, including groundwater level height, axial force, lateral wall, and soil deflection, and ground surface settlement were monitored and summarized based on the data published in the literature on similar excavations in Hangzhou, P. R. China. The following conclusions are drawn: (1) The axial forces of the struts dynamically change during the excavation and construction or removal of adjacent braces. (2) The ratio between the measured maximum wall deflection and excavation depth (δh−max/He) is 0.14–0.17%, larger than those in Shanghai. (3) The surface settlement behind the wall has an obvious primary influence zone and secondary influence zone, characterized by a “groove shape” and “triangle shape,” respectively. The maximum ground surface settlement δv−max ranges from 0.29% to 0.5% of the excavation depth. (4) The distribution of the ground settlement was analyzed. The relationship between the maximum settlements δv−max is between 1.28 δh−max and 3.72 δh−max. Moreover, ABAQUS software with Mohr–Coulomb soil models was used for model analysis of the construction process. The research results have important significance for the effective prevention of foundation pit accidents and the optimal design of deep foundation pit projects.

Neural network application in forecasting maximum wall deflection in homogenous clay

An attempt was carried out by using a neural network to predict the maximum deflection and its position caused by braced excavation in homogeneous clay. Six input variables, including excavation depth, Ratio of EI wall/EI of brace, the vertical distance between bracing, Length to width ratio of an excavation, shear strength, and the coefficient of lateral earth pressure, were adopted. Two models were developed, one is to estimate the maximum deflection and the other one to estimate the position of maximum deflection. The ANN models were developed and verified using a database of (169) cases of actual measured and presumptive cases using the analysis with the Finite element of maximum deflection. A sensitivity analysis was accomplished, to examine the relative significance of the parameters that influence the maximum deflection of the wall and its position; it indicates that the Ratio of EI wall/EI of brace has the most significant effect on the maximum wall deflection, while the properties of the soil have the most considerable effects on the position. The results show that the ANN can reasonably forecast the magnitude of the maximum deflection of the wall, as well as its position. Design charts are developed based on the ANN model.

Effect of Wall Deformation on Dewatering-Induced Ground Surface Settlement

In practice, dewatering for pressure relief is commonly undertaken during ongoing excavation to secure bottom stability against basal upheaval. Simultaneously, through unloading, wall deflection is obviously observed. Noticing that both cause soil deformations, this research is to study the effect of wall deformation on dewatering-induced settlement. A coupled numerical analysis of finite-difference software is employed to model Shanghai soft soils under multi-aquifer-aquitard systems (MAASs) by analyzing the results in association with an empirical approach. Consequently, through gradual force reduction, shear strength at soil-wall interface is significantly diminished. As wall deformation increases instantaneously upon lower loading, wall surface becomes deformedly bending; this condition causes the challenge to workability of shear strength. Moreover, wall deformation caused by unloading affects dewatering-induced settlement substantially. Under smaller loading, large wall deflection is observed; soil plane of failure caused by both sliding and compression occurs along slip curve, with weaker shear-strength soils at rD = 0.4 and stronger shear-strength soils between rD = 0.4 and rD = 0.65, where rD is the distance from the wall that is normalized by the depth measured from ground surface. During dewatering, stronger soils tend to drag weaker soils upward to reduce large differential settlements caused by additional compression. Consequently, settlement becomes larger at rD = 0.4 and smaller at rD = 0.65. Remarkably, at rD > 2.3, both settlement curves that result from numerical analysis and empirical method show overlapping; this indicates that the unloading effect on dewatering-induced settlement at rD > 2.3 is insignificant. Furthermore, as wall reaches maximum allowable wall deflection by 67% applied force, additional compression caused by dewatering after loading remains smaller than that under 70% applied force, contributing to smaller dewatering-induced settlement.

Evaluation of excavation-induced movements through case histories in Hangzhou

Purpose Deep excavation in soft clay often causes additional deformations to surroundings. Then, if deformations cannot be predicted reasonably, the adjacent buildings may be threatened by the deep excavation. Based on the good field observations from ten deep excavations in Hangzhou, this paper aims to thoroughly investigate the characteristics of wall deflections and ground settlements induced by deep excavations. Design/methodology/approach On the basis of good field observation of ten deep excavations, the performances of excavations, supported by contiguous pile in Hangzhou, were studied, and also compared with other case histories. Findings The maximum wall deflections (dhm) rang mostly from 0.7 to 1.2 per cent He, where He is the final excavation depth, larger than those in Taipei and Shanghai. The observed maximum ground settlement in the Hangzhou cases generally ranges from 0.2 to 0.8 per cent He. Then, the settlement influence zone extends to a distance of 2.0-4.0 He from the excavation. The relatively large movements and influence zones in Hangzhou may be attributed to low stability numbers, large excavation widths and the creep effect. The excavation width is justified to have a significant influence on the wall deflection. Therefore, to establish a semi-empirical formula for predicting the maximum wall deflection, it is necessary to include the factor of excavation width. Originality/value The relevant literature concentrated on the characteristics of deep excavations supported by the contiguous pile wall in Hangzhou soft clay can rarely be found. Based on the ten deep excavations with good field observation in Hangzhou, the characteristics of wall deflection and ground settlements were comprehensively studied for the first time, which can provide some theoretical support for similar projects.

Numerical Analysis on the Performance of the Underwater Excavation

Based on the Yongdingmen Station of Beijing Metro, the underwater excavation method for deep foundation pit was introduced. This study constructed a numerical analysis model to analyze the performance of surface settlement and lateral wall deflection in the process of underwater excavation. Results showed that this method was better to control the surface settlement and lateral wall deflection compared with other dewatering excavations. In detail, most of the surface settlement was caused during the dry excavation stage and dewatering excavation stage while the deflection caused by underwater excavation only accounted for about 10% of the total settlement. Besides, the maximum settlement occurred 0.25∼0.5 He behind the retaining wall and the value was 0.04% He. Similar to the result of the surface settlement, most of the lateral wall deflection had been completed before the underwater excavation, which only caused about 7% of the total deflection. The maximum wall deflection and its location were approximately 0.06% He and 0.5 He, respectively. Moreover, a series of 3D numerical analyses were studied on the design parameters of the underwater excavation method. This study can be used as a reference for general performance and structural design of foundation pits with underwater excavation.

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.

Finite element analysis of time-dependent behavior in deep excavations

This study investigated the time-dependent behavior of a diaphragm wall and ground surface for a typical excavation in soft clay using two-dimensional finite element analysis. During excavation, deformations normally increase with time, both in the soil excavation time and the elapsed time between excavation stages. The increase may be due to consolidation and/or creep but the mechanism has never been studied comprehensively before. The hardening soil, hardening soil small strain, soft soil, and soft soil creep model were used depending on analysis requirements. Compared to the measurement, the soft soil creep model could simulate the time-dependent wall deflection and maximum ground settlement properly. Creep had a significant contribution to increasing the wall deflection while consolidation had little effect. Creep also caused a significant increase in the ground settlement both in excavation stages and elapsed times while consolidation caused the ground settlement to rebound slightly during elapsed times. The maximum wall deflection and ground settlement induced by the bottom-up method were 0.79 and 0.80 times as large as those by the top-down method, respectively. This was a contribution of a three times smaller support stiffness, elapsed time and unsupported length of the bottom-up method compared to those of the top-down method.

Cause investigation of large deformation of a deep excavation support system subjected to unsymmetrical surface loading

This paper presents a case study to investigate the large deformation of a deep excavation supporting system closed to a railway with high embankment. The in-situ monitoring is used to determine the effects of the unsymmetrical surface loading on the support system. Meanwhile, the optimization of its design schemes is discussed by the finite element method (FEM). The monitoring results present the support system rotates away from the railway side during the excavation process. The deflection of the diaphragm walls significant affects the railway settlement especially when the maximum movement of lateral wall exceeds 21 mm. The observed maximum lateral deflection exceeded 50.9 mm, which caused considerable settlement (106.0 mm) to the nearby railway. The results of the FEM indicate that soil strengthening inside the deep excavation is an optimized scheme. It can reduce 35.8% maximum wall deflection on bias loading side so that largely control the rotate deformation of the support system, and the scheme may also reduce about 70% maximum settlement of the railway bed.

Method for estimating wall deflection of narrow excavations in clay

The support system stiffness is crucial to designing the excavation support system by displacement control, but the current methods of estimating the system stiffness are inapplicable to narrow excavations. The field data from narrow excavations in the same site in Shanghai soft clay were analyzed and it was found that the deformations of these excavations were significantly affected by the width-depth ratio (B/H). 111 numerical simulations were carried out to study the contributions of wall bending stiffness (EI), average vertical support spacing (Sv) and width-depth ratio (B/H) to the support system stiffness. A parameter was proposed to quantify the support system stiffness for narrow excavations. It can consider the coupled effects of EI, Sv and B/H. An equation for the maximum wall deflection was presented based on the parameter and it was applicable to the cases with B/H less than 1.5. The proposed method was verified against the measured data from several narrow excavations in clay and the overall agreement was satisfactory.

System reliability assessment on deep braced excavation adjacent to an existing upper slope in mountainous terrain: a case study

Due to rapid urbanization, the land resources available for construction are becoming increasingly scarce in many built-up environments, especially for infrastructure development in mountainous terrain. For deep excavations developed for basements of high-rise buildings in cities with mountainous terrains, the excavation activities may have an influence on the stability or performance of existing nearby upper slopes. Based on a case study in Chongqing, this paper numerically investigates the effects of the excavation geometry, the retaining wall system stiffness and the distance between the excavation and slope on the performances including the global factor of safety and retaining wall deflection of the excavation-slope system. Subsequently, simplified ultimate and serviceability limit state response surface models have been developed and implemented into the First-Order Reliability Method to determine the probability that the ultimate or serviceability limit state is exceeded by performing probabilistic analysis on the global factor of safety through setting the threshold maximum wall deflection as an optimization constraint.

Deformation Characteristics of Retaining Structures and Nearby Buildings for Different Propped Retaining Walls in Soft Soil

Abstract This article reports the field performance of deep excavations of two subway station cases, including the lateral wall deflection behavior and settlement trends of the surrounding soil and nearby buildings. The retaining structures employed in these cases were contiguous pile walls (CPW), soil-mixing walls, and diaphragm walls (DW), all of which were embedded in soft clay. The measured wall deflection profiles exhibited typical bulging behavior at the end of the excavation. The ratios of the measured maximum wall deflection to the excavation depth were found to be similar for all three types of retaining wall. Furthermore, the maximum and minimum corner effects on the wall deflection development were obtained for the DW and CPW, respectively. The measured ground surface settlement increased linearly with increased maximum lateral wall deflection, while the settlement magnitude became extraordinarily large because of the presence of sludgy soil. A concave pattern was proposed for the surface settlement profiles for all three types of retaining wall. The building settlement was quantified, with the value lying between those of the surface settlement and soil settlement at 10-m depth. The soil displacement field induced changes in the side and end resistance behaviors of the loaded piles, along with additional settlement of pile-foundation buildings. In addition, the pile-foundation building settlement was influenced by the corner effect. These research results will enhance our understanding of the deformation characteristics of the retaining structure and nearby buildings. Meanwhile, the findings will provide guidance for the optimal design of the retaining structure in soft soil.

Wall Deflection and Ground Surface Settlement due to Excavation Width and Foundation Pit Classification

Foundation pits have obvious spatial effect, and the lateral wall deflection, ground movement and basal heave stability are closely related with the excavation width. A database of 92 case histories with extensive excavation widths in soft soil in China are collected and analyzed. The maximum lateral wall deflections and maximum ground surface settlements measured are smaller than the case histories there before. With the increase of excavation width, the maximum lateral wall deflections and the maximum ground surface settlements increase firstly and then tend to stable. The database indicates that the type of retaining structure has little relevance to the excavation width of the foundation pit. There are two types of basal heave according to the excavation width, single-peak and double-peak. Three types of foundation pit considering excavation width, named wide foundation pit, normal foundation pit and narrow foundation pit, are proposed based on the slip circle basal heave analysis approach. The classification method is consistent with the measured data and the simulated results.