Reliability assessment of serviceability performance of braced retaining walls using a neural network approach

In urban environments, one major concern with deep excavations in soft clay is the potentially large ground deformations in and around the excavation. Excessive movements can damage adjacent buildings and utilities. There are many uncertainties associated with the calculation of the ultimate or serviceability performance of a braced excavation system. These include the variabilities of the loadings, geotechnical soil properties, and engineering and geometrical properties of the wall. A risk‐based approach to serviceability performance failure is necessary to incorporate systematically the uncertainties associated with the various design parameters. This paper demonstrates the use of an integrated neural network–reliability method to assess the risk of serviceability failure through the calculation of the reliability index. By first performing a series of parametric studies using the finite element method and then approximating the non‐linear limit state surface (the boundary separating the safe and ‘failure’ domains) through a neural network model, the reliability index can be determined with the aid of a spreadsheet. Two illustrative examples are presented to show how the serviceability performance for braced excavation problems can be assessed using the reliability index. Copyright © 2005 John Wiley & Sons, Ltd.     

Probabilistic infinite slope analysis

Research activity in the mechanics of landslides has led to renewed interest in the infinite slope equations, and the need for a more general framework for giving insight into the probability of failure of long slopes involving non-homogeneous vertical soil profiles and variable groundwater conditions. This paper describes a methodology in which parameters such as the soil strength, slope geometry and pore pressures, are generated using random field theory. Within the limitations of the infinite slope assumptions, the paper clearly demonstrates the important “seeking out” effect of failure mechanisms in spatially random materials, and how “first order” methods that may not properly account for spatial variability can lead to unconservative estimates of the probability of slope failure.     

Probabilistic analysis of responses of cantilever wall-supported excavations in sands considering vertical spatial variability

The influence of vertical spatial variability of sands on the excavation-induced lateral wall deflection and bending moment of excavations supported by cantilever retaining walls is investigated in this paper. Herein, the random finite element method (RFEM) is adopted to explicitly study the effect of one-dimensional spatial variability of internal friction angle of sands on the predicted wall and ground responses. The RFEM analysis consists of three components: (1) finite element method for analyzing lateral wall deflection and bending moment, (2) random field theory implemented with Monte Carlo simulation (MCS), and (3) statistical interpretation of MCS results through confidence intervals. This study reveals the importance of random field modeling in coping with the spatial variability of sands in the problem of supported excavations: (1) neglecting spatial variability of soil property will cause an overestimation of the variation in the predicted wall deflection and bending moment; (2) the estimated probability of failure based on a well-established serviceability limit state may be overestimated or underestimated depending on the chosen limiting lateral wall deflection. This study further investigates the effect of the number of MCS on the confidence intervals of the predicted statistics of the maximum lateral wall deflection and the maximum bending moment. The results also demonstrate that the confidence interval analysis of the predicted statistics of the maximum lateral wall deflection and the maximum bending moment provides a rational tool for interpreting the statistical data from RFEM.     

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.     

Reliability analysis of piles in spatially varying soils considering multiple failure modes

In most limit state design codes, the serviceability limit checks for drilled shafts still use deterministic approaches. Moreover, different limit states are usually considered separately. This paper develops a probabilistic framework to assess the serviceability performance with the consideration of soil spatial variability in reliability analysis. Specifically, the performance of a drilled shaft is defined in terms of the vertical settlement, lateral deflection, and angular distortion at the top of the shaft, corresponding to three limit states in the reliability analysis. Failure is defined as the event that the displacements exceed the corresponding tolerable displacements. The spatial variability of soil properties is considered using random field modeling. To illustrate the proposed framework, this study assesses the reliability of each limit state and the system reliability of a numerical example of a drilled shaft. The results show the system reliability should be considered for the serviceability performance. The importance measures of the random variables indicate that the external loads, the performance criteria, the model errors of load transfer curves and soil strength parameter are the most important factors in reliability analysis. Moreover, it is shown that the correlation length and coefficient of variation of soil strength can exert significant impacts on the calculated failure probability.     

软土基坑变形失稳形态模拟试验研究

通过不同土性、地下水条件下软土基坑开挖变形失稳的模拟试验, 研究了软土基坑开挖变形发展直至失稳破坏的全过程。通过试验研究, 初步认为软土基坑坑壁在无支护或支护刚度较小的情况下, 其坑壁破坏形态呈抛物线型; 在基坑开挖范围内若存在有砂性土, 且地下水位较高时, 易于发生流砂渗透破坏, 并导致了地表沉陷, 但侧向变形相对较小; 而对于因承压水引起的坑底土体隆起变形, 若不考虑土体的强度特性, 计算结果是偏于安全的。     

Modeling small-strain behavior of Taipei clays for finite element analysis of braced excavations

We examine the small-strain behavior of Taipei clays in braced excavation through a detailed analysis of a well-documented case history. Specifically, we analyze the case of the Taipei National Enterprise Center (TNEC) excavation using two soil models, the Modified Cam-clay model (MCC) and the three-Surface Kinematic Hardening model (3-SKH). Our finite element analysis includes a consideration of the over-consolidated stress state and the high initial shear modulus of the clay. Results show that the observed wall deflection and surface settlement can be satisfactorily predicted simultaneously using the 3-SKH model. This is an improvement on the MCC model, for which only wall deflection, not ground settlement, can be accurately predicted. This study re-confirms the importance of considering small-strain non-linear behavior for the over-consolidated stress state in finite element analyses of braced excavation responses.     

节理分布空间变异的地下洞室稳定性概率分析

以岩土材料力学参数空间变异性的"点估计-有限元"分析方法为基础,结合节理分析时自身存在几何模型、网格划分等特性,扩展了该方法在节理分布空间变异性分析方面的适用性,明确了具体的研究步骤与方法。以某抽水蓄能水电站为例,通过分析节理空间变异性对围岩变形与塑性区的影响,验证了扩展后该方法的准确性和合理性。对工程案例开挖揭露的1400余条节理进行概率统计,建立了节理空间变异性的有限元分析模型;采用扩展后的概率分析方法,研究了节理分布对地下洞室群围岩开挖稳定性的影响。研究结果表明:(1)对比概率分析得到的围岩变形概率分布与现场监测结果,发现剔除变形异常点后监测变形量值大部分位于得到的位移概率分布范围内,说明节理的空间变异性是导致监测变形波动的主要影响因素;(2)围岩变形概率分布的标准差能有效识别出围岩开挖变形受节理空间变异性的影响程度,对于所给出的案例依次为:机窝>边墙>顶拱;(3)围岩塑性区的概率分区能合理判断地下洞室群开挖时受节理影响较大的区域和范围,为工程施工的支护设计提供依据。     

Efficient stochastic analysis of unsaturated slopes subjected to various rainfall intensities and patterns

Rainfall infiltration poses a disastrous threat to the slope stability in many regions around the world. This paper proposes an extreme gradient boosting (XGBoost)-based stochastic analysis framework to estimate the rainfall-induced slope failure probability. An unsaturated slope under rainfall infiltration in spatially varying soils is selected in this study to investigate the influences of the spatial variability of soil properties (including effective cohesion c′, effective friction angle φ′ and saturated hydraulic conductivity k_s), as well as rainfall intensity and rainfall pattern on the slope failure probability. Results show that the proposed framework in this study is capable of computing the failure probability with accuracy and high efficiency. The spatial variability of k_s cannot be overlooked in the reliability analysis. Otherwise, the rainfall-induced slope failure probability will be underestimated. It is found that the rainfall intensity and rainfall pattern have significant effect on the probability of failure. Moreover, the failure probabilities under various rainfall intensities and patterns can be easily obtained with the aid of the proposed framework, which can provide timely guidance for the landslide emergency management departments.     

Reliability assessment of excavation systems considering both stability and serviceability performance

Excavation projects related to urban redevelopment and infrastructure improvement are often governed by serviceability-based design, rather than failure prevention criteria. Deformation tolerance specifications are often prescribed based on minimizing potential damage to adjacent structures. A risk-based approach to serviceability performance that systematically incorporates design parameter uncertainty will allow engineers to address soil uncertainty in performance-based design. This paper demonstrates the use of various kinds of reliability methods, such as response surface method (RSM), first-order reliability method (FORM), second-order reliability method (SORM), adaptive importance sampling (AIS), Monte Carlo simulation (MCS) and system reliability, to assess the risk of stability and/or serviceability failure of an entire excavation support system throughout the entire construction process. By considering multiple failure modes (including serviceability criteria) of an excavation, the component and system reliability indices for each excavation step are assessed during the entire excavation process. Sensitivity analyses are conducted for the system reliability calculations, which demonstrate that the adjacent structure damage potential limit state function is the dominant factor for determining excavation system reliability. An example is presented to show how the serviceability performance for braced excavation problems can be assessed based on the system reliability index.     

Assessment of strut forces for braced excavation in clays from numerical analysis and field measurements

One important consideration in the design of a braced excavation system is to ensure that the structural bracing system is designed both safely and economically. The forces acting on the struts are often determined using empirical methods such as the Apparent Pressure Diagram (APD) method developed by Peck (1969). Most of these empirical methods that were developed from either numerical analysis or field studies have been for excavations with flexible wall types such as sheetpile walls. There have been only limited studies on the excavation performance for stiffer wall systems such as diaphragm walls and bored piles. In this paper, both 2D and 3D finite element analyses were carried out to study the forces acting on the struts for braced excavations in clays, with focus on the performance for the stiffer wall systems. Subsequently, based on this numerical study as well as field measurements from a number of reported case histories, empirical charts have been proposed for determining strut loads for excavations in stiff wall systems.     

Fuzzy Modeling for Reserve Estimation Based on Spatial Variability

This article addresses a new reserve estimation method which uses fuzzy modeling algorithms and estimates the reserve parameters based on spatial variability. The proposed fuzzy modeling approach has three stages: (1) Structure identification and preliminary clustering, (2) Variogram analysis, and (3) Clustering based rule system. A new clustering index approach and a new spatial measure function (point semimadogram) are proposed in the paper. The developed methodology uses spatial variability in each step and takes the fuzzy rules from input-output data. The model has been tested using both simulated and real data sets. The performance evaluation indicates that the new methodology can be applied in reserve estimation and similar modeling problems     

Studies on Construction Pre-control of a Connection Aisle Between Two Neighbouring Tunnels in Shanghai by Means of 3D FEM,Neural Networks and Fuzzy Logic

The ground freezing construction technique is one of the most effective and widely applied site construction methods in soft soil areas, like Shanghai. Some elevation-inclined refrigeration pipes are arranged for the artificial freezing excavation of the Pudong-side first-storey connection aisle, which is designed to connect two adjacent tunnel lanes of Shanghai East-Fuxing-Road tunnel project. No advanced research results could be found for computing the temperature field of tunnels and aisles frozen with inclined refrigeration pipes. Anyhow the computation of the relevant temperature field is of high importance for the safe and economical excavation of the above-mentioned aisle. In this paper, a method for computing the aisle temperature field using 3D FEM is given, and the computation accuracy is verified by contrasting the computed and site measured results. The back propagation neural networks are also applied to the temperature prediction using self-developed Neural Network-Expert System software, the predicted results are also very satisfactory. The mechanism during freezing and aisle excavation will be discussed on the basis of 3D FEM simulation. The authors believe that studying the parameter-sensitivity of temperature field is very important for the optimum selection of parameter values. So, in this paper, the parameter-sensitivity of temperature field is also discussed. In order to obtain the optimum frozen wall thickness, the relation between the frozen wall thickness and the initial freezing brine temperature is studied. At the end, an excavation pre-control plan is proposed by means of fuzzy logic theory for improving the excavation safety. The research result of the current paper is very helpful for projects that will be excavated by freezing construction technique.     

Optimized functional linked neural network for predicting diaphragm wall deflection induced by braced excavations in clays

Deep excavation during the construction of underground systems can cause movement on the ground, especially in soft clay layers. At high levels, excessive ground movements can lead to severe damage to adjacent structures. In this study, finite element analyses (FEM) and the hardening small strain (HSS) model were performed to investigate the deflection of the diaphragm wall in the soft clay layer induced by braced excavations. Different geometric and mechanical properties of the wall were investigated to study the deflection behavior of the wall in soft clays. Accordingly, 1090 hypothetical cases were surveyed and simulated based on the HSS model and FEM to evaluate the wall deflection behavior. The results were then used to develop an intelligent model for predicting wall deflection using the functional linked neural network (FLNN) with different functional expansions and activation functions. Although the FLNN is a novel approach to predict wall deflection; however, in order to improve the accuracy of the FLNN model in predicting wall deflection, three swarm-based optimization algorithms, such as artificial bee colony (ABC), Harris’s hawk’s optimization (HHO), and hunger games search (HGS), were hybridized to the FLNN model to generate three novel intelligent models, namely ABC-FLNN, HHO-FLNN, HGS-FLNN. The results of the hybrid models were then compared with the basic FLNN and MLP models. They revealed that FLNN is a good solution for predicting wall deflection, and the application of different functional expansions and activation functions has a significant effect on the outcome predictions of the wall deflection. It is remarkably interesting that the performance of the FLNN model was better than the MLP model with a mean absolute error (MAE) of 19.971, root-mean-squared error (RMSE) of 24.574, and determination coefficient (R²) of 0.878. Meanwhile, the performance of the MLP model only obtained an MAE of 20.321, RMSE of 27.091, and R² of 0.851. Furthermore, the results also indicated that the proposed hybrid models, i.e., ABC-FLNN, HHO-FLNN, HGS-FLNN, yielded more superior performances than those of the FLNN and MLP models in terms of the prediction of deflection behavior of diaphragm walls with an MAE in the range of 11.877 to 12.239, RMSE in the range of 15.821 to 16.045, and R² in the range of 0.949 to 0.951. They can be used as an alternative tool to simulate diaphragm wall deflections under different conditions with a high degree of accuracy.     

From discrete to continuum modelling of boundary value problems in geomechanics: An integrated FEM-DEM approach

Double-scale numerical methods constitute an effective tool for simultaneously representing the complex nature of geomaterials and treating real-scale engineering problems such as a tunnel excavation or a pressuremetre at a reasonable numerical cost. This paper presents an approach coupling discrete elements (DEM) at the microscale with finite elements (FEM) at the macroscale. In this approach, a DEM-based numerical constitutive law is embedded into a standard FEM formulation. In this regard, an exhaustive discussion is presented on how a 2D/3D granular assembly can be used to generate, step by step along the overall computation process, a consistent Numerically Homogenised Law. The paper also focuses on some recent developments including a comprehensive discussion of the efficiency of Newton-like operators, the introduction of a regularisation technique at the macroscale by means of a second gradient framework, and the development of parallelisation techniques to alleviate the computational cost of the proposed approach. Some real-scale problems taking into account the material spatial variability are illustrated, proving the numerical efficiency of the proposed approach and the benefit of a particle-based strategy.     

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.

     

Probabilistic analysis of strip footings resting on a spatially random soil using subset simulation approach

The failure probability of geotechnical structures with spatially varying soil properties is generally computed using Monte Carlo simulation (MCS) methodology. This approach is well known to be very time-consuming when dealing with small failure probabilities. One alternative to MCS is the subset simulation approach. This approach was mainly used in the literature in cases where the uncertain parameters are modelled by random variables. In this article, it is employed in the case where the uncertain parameters are modelled by random fields. This is illustrated through the probabilistic analysis at the serviceability limit state (SLS) of a strip footing resting on a soil with a spatially varying Young's modulus. The probabilistic numerical results have shown that the probability of exceeding a tolerable vertical displacement (P _e) calculated by subset simulation is very close to that computed by MCS methodology but with a significant reduction in the number of realisations. A parametric study to investigate the effect of the soil variability (coefficient of variation and the horizontal and vertical autocorrelation lengths of the Young's modulus) on P _e was presented and discussed. Finally, a reliability-based design of strip footings was presented. It allows one to obtain the probabilistic footing breadth for a given soil variability.     

Robust geotechnical design of shield-driven tunnels

This paper presents a fuzzy set-based robust geotechnical design (RGD) methodology for the design of shield-driven tunnels. Here, uncertain geotechnical parameters required for analysis of tunnel performance (referred to herein as the structure safety and serviceability performance of tunnel cross section) are represented as fuzzy sets. Given fuzzy input parameters, the performance of a shield-driven tunnel will be uncertain, which is expressed in this study as a fuzzy factor of safety, according to the analysis of vertex method. Then, the fuzzy factor of safety for a given design is used to evaluate the failure probability and design robustness, which are, in turn, employed in the proposed RGD framework. Note that a design is considered robust if the performance of the shield-driven tunnel is insensitive to the variation of its uncertain geotechnical parameters. Within the RGD framework, each candidate design in the design space is analyzed for its safety state (in terms of failure probability), design robustness, and cost. The goal of the RGD of a shield-driven tunnel is to bring the safety state to an acceptable level, while maximizing the robustness and cost efficiency simultaneously. To this end, a multi-objective optimization is performed and a Pareto front is obtained, which provides a trade-off that may be used to select the most preferred design. Through an illustrative case, the effectiveness and significance of this new robust design methodology is demonstrated.     

内支撑结构基坑的空间效应及影响因素分析

具有内支撑结构的围护系统在基坑边角处具有更大的系统刚度,使得基坑边角附近处土体的位移小于距离边角较远处土体的位移,即基坑的变形问题表现出空间特性。为了更好地研究L/He（L为沿基坑纵向方向上的距离;He为开挖深度）、开挖深度等因素对空间效应的影响,量测了两个狭长形地铁车站深基坑不同位置处土体的侧向位移、土体沉降等。通过对现场监测资料的分析发现,边角效应能够减小侧向位移的平面应变比,灌注桩围护结构、SMW工法桩围护结构和地下连续墙在边角附近处的平面应变比(PSR)分别为0.50、0.61和0.72。当平面应变比(PSR)接近于1.00时,对应的L/He值分别为2.50、6.00和4.00。随着L/He值的增大,土体的纵向最大沉降呈先增大后保持稳定的趋势。随开挖深度的增加,边角效应的影响范围呈增大的趋势。在基坑纵向沉降的空间效应中,灌注桩围护结构、SMW工法桩围护结构的土体最大沉降值达到稳定时对应的L/He值分别为2.50和5.20。土体沉降和侧向位移的空间效应有一定的相关性。