Deep Braced Excavation Research Articles

Numerical analysis of one-strut failure in deep braced excavation considering anisotropic clay behavior

Numerical analysis of one-strut failure in deep braced excavation considering anisotropic clay behavior

Seismic responses, damage mechanisms and retrofitting methods for deep braced excavation: Centrifuge test and numerical analysis

The current study investigates the nonlinear seismic responses and damage mechanisms of the deep braced excavation, which has rarely been inspected in previous studies. An effective numerical model is developed based on dynamic centrifuge tests to properly capture the responses and damage of the structure under earthquakes. The model is established by using the nonlinear properties of the material (for both soil and structure) and contact. The procedure of constructing the numerical model including determining the material parameters, boundary conditions, and contact properties is elaborated. The seismic responses of the deep braced excavation during earthquakes with increasing levels of earthquake intensity are clearly investigated and compared with those in the static condition. Then, the damage mechanisms of the structure are stated. Finally, different seismic retrofitting methods are proposed; and, the most effective method is suggested. The findings from this study can provide a clear understanding of the seismic responses, damage mechanisms, seismic retrofitting methods, and also recommendations for seismic design applications of deep braced excavations.

Analysis of observed performance of a deep excavation straddled by shallowly buried pressurized pipelines and underneath traversed by planned tunnels

The objective of this paper is to analyze the observed performance of a deep excavation facing severe restrictions from underground facilities or structures in Suzhou, China. The first source of the restrictions comes from two shallowly buried pressurized pipelines that straddle the excavation at an intersection angle of about 81°. The pipelines need to be well protected at the site during the construction to ensure the normal life of the local resident. The second source of the restrictions arises from the twin tunnels that are planned to be constructed beneath the excavation in the future for Suzhou Metro Line 14. The planned tunnels will be constructed using a tunnel boring machine, which necessitates the reservation of favorable conditions by the present excavation project. Because of these restrictions, the form of the excavation support system becomes complicated, and the performance of the excavation will inevitably differ from that of the common excavations. Based on a thorough analysis of the extensive field performance data collected by means of settlement points and inclinometers located within and adjacent to the excavation, the characteristics of the ground surface settlements, lateral ground movements, ground water table variations, vertical and lateral displacements atop diaphragm walls, vertical displacements atop lattice column, axial forces in concrete strut, and vertical and lateral pipeline deformations are captured. Despite the discontinuity of the peripheral retaining wall and the shortened wall height on the discontinuous sides, the ground and structural deformations are well controlled by the specially designed excavation support system. The pipelines are slightly uplifted by the suspension system, with the maximum vertical pipeline displacement being lower by an order of magnitude than the commonly observed maximum pipeline settlements in the literature. The existing empirical envelops are found to be site-specific in applicability, and are recommended to incorporate the field observations of this unusual deep braced excavation case history.

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.

Analysis and prediction of diaphragm wall deflection induced by deep braced excavations using finite element method and artificial neural network optimized by metaheuristic algorithms

The construction of metropolises in smart cities is the trend of developed countries. However, it may cause damages to the surrounding structures. For this reason, the diaphragm wall has been applied to prevent the deformation or collapse of the surrounding structures. Diaphragm walls can be deflected due to the swelling pressure or other geotechnical properties during construction. Therefore, the accurate prediction of diaphragm wall deflection (DWD) is challenging in construction aiming to ensure the safety of the surrounding structures. This study is, therefore, to propose two intelligent models for predicting DWD induced by deep braced excavations based on finite element method (FEM) and metaheuristic algorithms. Accordingly, a total of 1120 finite elements were analyzed to investigate the behaviors of DWD. Finally, a neural network with multiple layer perceptron (MLP) was optimized by two metaheuristic algorithms for predicting DWD, including whale optimization (WO) and Harris hawks optimization (HHO), called MLP-HHO and MLP-WO, respectively. The results indicated that the proposed MLP-HHO and MLP-WO provided high accuracy in predicting DWD. A comparison of the proposed models in this study and previous studies was also discussed to highlight the superiority of the proposed MLP-HHO and MLP-WO models.

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.

Estimating the subgrade reaction at deep braced excavation bed in dry granular soil using genetic programming (GP)

Modulus of subgrade reaction (Ks) is a simplified and approximated approach to present the soil-structure interaction. It is widely used in designing combined and raft foundations due to its simplicity. (Ks) is not a soil propriety, its value depends on many factors including soil properties, shape, dimensions and stiffness of footing and even time (for saturated cohesive soils). Many earlier formulas were developed to estimate the (Ks) value. This research is concerned in studying the effect of de-stressing and shoring rigidity of deep excavation on the (Ks) value. A parametric study was carried out using 27 FEM models with different configurations to generate a database, then a well-known “Genetic Programming” technique was applied on the database to develop a formula to correlate the (Ks) value with the deep excavation configurations. The results indicated that (Ks) value increased with increasing the diaphragm wall stiffness and decreases with increasing the excavation depth.

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.

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.

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.

3D numerical analysis of passive pile groups adjacent to deep braced excavation in soft clay

3D numerical analysis of passive pile groups adjacent to deep braced excavation in soft clay

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.

Estimation of strut forces for braced excavation in granular soils from numerical analysis and case histories

Construction work of many projects in urban areas requires deep braced excavations, under which circumstances, appropriate design method to assure both the safety and economy of the retaining walls and supporting systems is therefore of paramount importance. For multi-propped retaining walls, Apparent Pressure Diagram (APD) method developed by Peck (1969) is commonly adopted to determine the forces acting on the struts. Unfortunately, most previous studies focused on the performance of braced excavations in soft to stiff clays, where limited publications involve braced excavations in dense sand and gravel with medium rigid walls. In this study, both 2D and 3D finite element analyses were carried out to examine the strut forces for braced excavations in granular soils. Subsequently, based on the numerical results as well as the field measurements from a number of reported case histories, empirical charts have been proposed to determine strut loads for deep excavations in granular soils with medium rigid walls.

Numerical investigation of pile responses caused by adjacent braced excavation in soft clays

ABSTRACT The pile responses to soil movements induced by adjacent braced excavation have attracted increasing attention from geotechnical design engineers. The lateral displacement of soil caused by deep braced excavation could produce significant effect on adjacent piles according to field measurement data. In this paper, the finite element (FE) analyses considering soil of the hardening small strain (HSS) constitutive model were carried out to establish the excavation-induced pile behaviours by varying the depth of excavation, pile diameter, pile length, pile distance away from the excavation, pile-head fixity, unsupported depth of excavation and axial loadings exerted on the pile head. A set of design charts is developed to estimate the behaviours of piles subjected to adjacent braced excavation in soft clay, targeting to provide design guidelines for the lateral wall deflection assessment of pile foundations and stability evaluation.

NUMERICAL SIMULATION OF STAGED BRACED EXCAVATION IN SAND ~ O6 MRT STATION

A numerical model for long-term drained analysis of staged excavation is developed and validated with a case history in this paper. Kaohsiung O6 station in Taiwan is selected for this purpose. A complete soil-structure interaction is considered in the current analysis which involves structural elements for diaphragm wall and strutting with preloading. FISH scripts are used to simulate staged strutted excavation using the finite difference program FLAC. Within the framework of drained analysis for excavation in the sand, dewatering is performed for each stage of the excavation work. Field monitoring data from other published sources are used to compare with the results produced by the numerical model. The main purpose of this paper is to demonstrate the usefulness of the development in assisting design and monitoring construction of deep braced excavation. The developed model can be used to assist future design and construction projects.

2D and 3D numerical analysis on strut responses due to one-strut failure

In deep braced excavations, struts and walers play an essential role in the whole supporting system. For multi-level strut systems, accidental strut failure is possible. Once a single strut fails, it is possible for the loads carried from the previous failed strut to be transferred to the adjacent struts and therefore cause one or more struts to fail. Consequently, progressive collapse may occur and cause the whole excavation system to fail. One of the reasons for the Nicoll Highway Collapse was attributed to the failure of the struts and walers. Consequently, for the design of braced excavation systems in Singapore, one of the requirements by the building authorities is to perform one-strut failure analyses, in order to ensure that there is no progressive collapse when one strut was damaged due to a construction accident. Therefore, plane strain 2D and three-dimensional (3D) finite element analyses of one-strut failure of the braced excavation system were carried out in this study to investigate the effects of one-strut failure on the adjacent struts.

Multivariate Adaptive Regression Splines Approach to Estimate Lateral Wall Deflection Profiles Caused by Braced Excavations in Clays

Based on a database including a total of 30 case histories for braced excavation in stiff, medium and soft clays, a multivariate adaptive regression splines (MARS) approach for estimating wall deflection profile caused by deep braced excavations is presented in this study. For each soil type, ten case histories with information on subsurface soil conditions, geometry characteristics, excavation support system details, the maximum wall deflections and the wall deflection profile against embedded depth are provided. Seven input variables, including wall length, excavation depth, excavation length, system stiffness, average unit weight and undrained shear strength of the soil, and the depth below the ground surface, are adopted as inputs to the MARS deflection profile model. Comparison with three more excavation case histories indicates that the developed MARS model can give an accurate graphical representation of the wall deflection profile. It is capable of not only predicting the value of maximum wall deflection but also estimating the possible depth at which maximum lateral deformation occurs.

Glyph and hyperstreamline representation of stress and strain tensors and material constitutive response

AbstractResults of numerical analyses of boundary value problems in geomechanics include output of three‐dimensional stress and strain states. Two‐dimensional plots of stress–stress or stress–strain quantities, often used to represent such output, do not fully communicate the evolution of stress and strain states. This paper describes the use of glyphs and hyperstreamlines for the visual representation of three dimensional stress and strain tensors in geomechanics applications. Glyphs can be used to represent principal stress states as well as normal stresses at a point. The application of these glyphs is extended in this paper to represent strain states. The paper introduces a new glyph, called HWY glyph for the representation of shear tensor components. A load step‐based hyperstreamline is developed to show the evolution of a stress or strain tensor under a general state of loading. The evolution of stress–strain states from simulated laboratory tests and a general boundary value problem of a deep braced excavation are represented using these advanced visual techniques. These visual representations facilitate the understanding of complex multidimensional stress–strain soil constitutive relationships. The visual objects introduced in this paper can be applied to stress and strain tensors from general boundary value problems. Copyright © 2003 John Wiley & Sons, Ltd.

Ground movements from a deep excavation in sands and interbedded clay : O'Rourke, T D Proc Symposium on Failures in Earthworks, London, 6–7 March 1985 P371–381. Publ London: Thomas Telford, 1985

The paper presents and discusses the ground movements caused by an 18 m deep braced excavation in sands and interbedded stiff clay. The excavation was part of the construction of a metro station in Washington, DC. The maximum horizontal and vertical ground movements were relatively small, being of the order of 18 mm. Nevertheless, significant damage was observed in adjacent buildings, one of which was a four-storey masonry structure which was underpinned before excavation was started. The characteristics of the ground movements and their effects on adjacent structures are evaluated. For the covering abstract of the symposium see IRRD 286603. (Author/TRRL)