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.     

Probability of serviceability failure in a braced excavation in a spatially random field: Fuzzy finite element approach

A simplified framework is proposed for evaluating the probability of “serviceability failure” in a braced excavation in a spatially random field. Here, the “serviceability failure” is said to occur when the excavation-induced wall or ground movement exceeds specified limiting values. Knowledge of this probability can aid in engineering decision-making to prevent damage to adjacent infrastructures. The proposed framework consists of five elements: (1) finite element method (FEM) for analyzing wall and ground responses in a braced excavation, (2) fuzzy set modeling of parameter uncertainty, (3) spatial averaging technique for handling spatial variability, (4) vertex method for processing fuzzy input through FEM model, and (5) interpretation of fuzzy output. The proposed framework is demonstrated through a well-documented case history. The results show the proposed framework is simple and effective for assessing the probability of serviceability failure in a braced excavation in a spatially random field. To focus on the proposed fuzzy FEM approach, the scope of this paper is limited to one-dimensional modeling of spatial variability with an assumed exponential autocorrelation function.     

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.     

基坑垮塌的离散元模拟及冗余度分析

目前基于构件设计的基坑设计理论使得很多支护体系缺乏必要的冗余度,并导致了众多基坑事故。将离散元方法引入到基坑垮塌模拟和冗余度研究中,提出了基于局部破坏的冗余度分析模拟方法。进而以探究支撑端部连接对支护结构冗余度的影响为例,对两种连接情况的基坑的破坏过程进行了模拟,对其中的连续破坏现象进行了分析,并将模拟结果与实际工程进行了对比。结果表明,支护体系冗余度的提高可以有效地抵抗连续破坏的发生,防止基坑在局部损伤情况下,由一种形式的破坏引发其他形式的破坏,由局部破坏导致整体崩溃。另外,模拟结果也验证了离散元法在基坑破坏模拟中的适用性与可行性,为类似地下工程的破坏模拟和基坑冗余度的深入研究提供了参考。     

Load-displacement uncertainty of vertically loaded shallow footings on sands and effects on probabilistic settlement estimation

This article focuses on the statistical characterisation and stochastic modelling of the load-displacement behaviour of shallow footings on cohesionless soils and on the probabilistic estimation of settlement for serviceability limit state design (LSD). The study relies on a field database of 30 full-scale footings subjected to vertical loading with cone penetration testing data available for each site. The performance of three load-displacement models in replicating field data is assessed comparatively through statistical analysis. Load-displacement uncertainty is subsequently modelled probabilistically to perform Monte Carlo Simulation (MCS)-based estimation of footing settlement using the best-performing power law model. The dependence among load-displacement model parameters is investigated and replicated using copula theory. Samples are generated to account for parametric uncertainties in model inputs. The simulation output samples of settlement are examined statistically in order to assess the relevance of parametric and load-displacement uncertainties in settlement estimation, as well as the importance of accounting for correlation between power law model parameters. A simple analytical model for the estimation of settlement at any target reliability level is obtained on the basis of the outputs of MCS. The model can be practically implemented in geotechnical LSD at serviceability limit states.     

柱下独立基础稳健性设计与分析

岩土工程随机参数统计特征的不确定性,使得岩土工程可靠度设计存在一定风险。岩土工程稳健性设计能够充分考虑随机参数的不确定性结合结构安全性、稳健性和经济性实现最优设计。针对随机参数统计特征的不确定性对柱下独立基础设计的影响,基于可靠度理论和岩土工程稳健性设计方法,考虑岩土参数、混凝土和钢筋材料力学参数统计特征不确定性的影响,以独立基础几何尺寸作为可控设计参数进行设计分析。将独立基础地基承载力、地基变形、基础结构冲切破坏和基础弯曲破坏4种失效模式视为串联系统,进行多失效模式下的结构体系稳健性设计,分析了多失效模式下结构几何参数与结构体系可靠度的关系。结合稳健性和经济性,进行了独立基础多目标优化设计,确定柱下独立基础设计的最优解。     

Monitoring systems for warning impending failures in slopes and open pit mines

Slope stability is a critical safety and production issue for mining. Major wall failure can occur seemingly without any visual warning, causing loss of lives, damage to equipment, and disruption to the mining process. Monitoring systems, ranging from simple piezometers and extensometers to highly sophisticated radars and global navigation satellite systems, are employed to predict impending instabilities and failure. Here, we provide a review of the available monitoring systems used in slope management and highlight their major advantages and shortcomings. We propose a simple method for evaluating the effectiveness and reliability of monitoring systems to warn of pending slope failures. The method is based on constructing monitoring reliability maps for the slope by evaluating two slope parameters: Expected deformation to failure and critical reading frequency, which depend on the slope characteristics (e.g., geology and design), service condition (e.g., rainfall, blast), and the economic impact of the failure. The reliability of a deformation monitoring system can be subsequently assessed by identifying three parameters of the system: Coverage area (large or discrete), Deformation monitoring precision, and Measurement frequency. The application of the method to most commonly used deformation monitoring systems is demonstrated. The advantages and implications of the proposed method are highlighted.     

基坑工程对邻近建筑物附加变形影响的分析

随着软土地区城市建筑密度的增加,基坑工程周围通常存在建筑物,基坑工程的开挖受到更严格的环境制约。在深基坑工程的设计中,必须考虑基坑开挖引起的附加变形满足周边建筑物提出的严格要求,基坑设计的关键从强度控制转变为变形控制。结合上海地区大量基坑工程实践,考虑邻近建筑物条件下基坑开挖的数值模拟分析,总结出适用于基坑围护选型阶段的实用计算分析方法,对方案选型进行指导。分别结合邻近桩基础和浅基础的不同基础形式建筑物的工程实例,对基坑开挖产生的邻近建筑物的附加变形进行计算分析,并与实际工程实施的监测结果相比较,验证了计算分析的必要性和可靠性,为类似基坑工程的设计和施工提供了有益的参考。     

Numerical investigation of the failure of a building in Shanghai,China

The overturning failure of a 13 storey residential building in Shanghai, China, has been investigated by plane strain finite element analysis (FEA). The results of the FEA indicate that ultimate failure of the building was probably initiated by the formation of tensile cracking in the reinforced concrete piles located under the side of the building adjacent to an excavation. This eventually led to complete structural failure of the piles located along the excavation side, which probably caused further settlement of the building, leading eventually to a toppling failure resulting in overturning of the entire building. Excessive tensile stress in the piles was probably caused by the combination of excavation of soil at one side of the building and the temporary dumping of the excavated soil on the opposite side of the building. It is likely that the effect of temporary dumping of the excavated soil adjacent to the building was either not considered or not properly taken into account in the foundation design nor the construction operations. A simple but important lesson to be draw from this failure is the need for engineers who design foundations in soft soil regions to consider not only the final loading conditions, but also any temporary and transient loading conditions during the construction process.     

Numerical integration method for computing reliability index of geotechnical system

To account for the uncertainties in the design of a geotechnical system, reliability-based design approach is often adopted, in which the main task is to evaluate reliability index of the system based on a performance function (or limit state function). In this paper, we propose a new method for computing the reliability index, based upon the numerical integration of the cumulative distribution function (CDF) of the performance function. This numerical integration method requires only a deterministic evaluation of the system performance and the joint probability of the uncertain input parameters. The effectiveness and the efficiency of the proposed method, measured in terms of the accuracy and the computational effort, respectively, are demonstrated with two geotechnical problems: a drilled shaft in sand and a semi-gravity retaining wall. The new method is found valid regardless of the type of distribution of uncertain input parameters, whether the correlations exist among these input parameters, whether the system involves single or multiple failure modes, and how the performance function is formulated.     

多失效模式下基于区间非概率的岩质隧道稳定可靠度分析

对于岩质隧道工程的稳定可靠度分析,一方面由于岩体参数的统计数据获取困难使得概率可靠度方法难以适用,另一方面其工程稳定与多种失效模式密切相关,须考虑隧道结构体系的可靠度问题。首先,基于区间理论,通过区间变量形式表征不确定性参数;然后,针对岩质隧道工程中多种失效模式并存的情况,引入结构体系可靠度理念,建立基于区间非概率的岩质隧道结构体系可靠度指标计算及其稳定性评价方法;在此基础上,通过工程实例验证了该方法的合理性;最后,定义不确定性参数的波动幅度,进一步分析各失效模式中不同参数对相应可靠度指标以及结构体系可靠度指标的影响。分析结果表明,各失效模式相应的非概率可靠度指标均随区间变量范围的增大而降低,且同一参数在不同失效模式中表现出不同的影响;另外,不确定性参数的变化还将导致影响岩质隧道结构体系稳定的主要失效模式发生相应的变化。     

金川矿区深部巷道开挖损伤数值模拟研究

在地下深处开挖巷道会诱发应力的急剧调整,无疑会使围岩产生不同程度的损伤。开挖的动态作用虽然持续时间较短,但造成的损伤为后续时效变形和工程扰动影响下的失稳破坏提供了基础条件。为了能够得到开挖作用引起的金川矿区深部巷道损伤区演化规律,从而对巷道损伤破坏进行针对性的防治,本文基于矿区的实际巷道埋深和岩体结构种类,采用等效开挖模拟方法和非均质模型进行了巷道开挖损伤问题的数值模拟研究。模拟结果揭示了在不同水平和岩体结构条件组合情况下开挖损伤区的演化过程和规律。根据研究区巷道的损伤破坏规律,本文提出了深部巷道开挖损伤演化的5种模式,并揭示了各种模式的诱发条件、表现特征、力学机制及发展趋势。相关认识和结论期望为理论和工程实践提供一定的借鉴和参考。     

Probabilistic analysis of retaining walls

A methodology for the probabilistic analysis of reinforced concrete cantilever walls is developed and described in this paper. The wall’s external stability under static conditions is addressed and modeled as a series system with correlated failure modes. Computations of reliability are performed using Monte Carlo simulations for assumed probability distributions of the backfill and foundation material engineering properties. A case example is analyzed based on the described methodology. The results indicated that risk, measured by the system probability of failure, is not a linear function of safety ratios. All three safety ratios in question were positively correlated, with bearing capacity being subjected to higher degree of uncertainty. The degree of correlation was found to have an important effect on the system probability of failure. Considering the width of the base as varying design parameter, the study also showed that first-order reliability bounds, which are often applied in practice, may lead to a noteworthy over- or under-estimation of the design.     

基于Copula函数的基桩荷载-位移双曲线概率分析

提出了基于Copula函数的基桩荷载-位移双曲线概率分析方法。首先将基桩标准化荷载-位移双曲线模型不确定性转化为双曲线参数不确定性,然后在Copula理论框架下建立了双曲线参数的联合分布函数。最后以钻孔现浇灌注桩试验数据为例证明了所提方法的有效性,并进行了基桩正常使用极限状态可靠度分析。结果表明：Copula函数是构造基桩标准化荷载-位移双曲线参数联合分布函数一种有效的方法,它能够更加准确地实现基桩荷载-位移双曲线的随机模拟,从而得到更为合理的可靠度结果。钻孔现浇灌注桩双曲线模型中两个参数间具有较强的负相关关系,忽略了这种负相关性将会高估基桩的失效概率。此外,常用的Gaussian Copula函数并不是拟合双曲线模型中两个参数间相关结构最优的Copula函数,采用Gaussian Copula函数将会明显低估基桩的失效概率。     

基于遗传算法的隧道围岩变形稳定可靠性分析

由于隧道的开挖,隧道围岩的初始应力状态将发生变化,围岩发生卸荷变形,并可能导致隧道的失稳。为了研究隧道开挖后围岩变形的可靠性,建立了隧道围岩变形可靠度指标计算模型,并用遗传算法进行求解。应用本文方法对一高速公路隧道围岩变形的可靠性进行分析。从结果可以看出,围岩的潜在破坏区域出现在拱顶和两侧拱肩区域,从这些区域的可靠度指标和破坏概率值可以看出,该隧道毛洞的拱顶下沉和拱肩收敛可靠度都满足不了规范的要求,要及时进行支护。     

暗挖隧道与邻近结构物相互作用研究现状及展望

软土中采用暗挖方式开挖隧道往往会引起土体变形。由于城市中暗挖隧道多建在建筑物高度集中的地区,土体变形对邻近既有结构物的损伤不容忽视。同时,既有暗挖隧道周围结构物的施工也不可避免地会引起隧道的变形和受力。文章主要阐述了目前暗挖隧道施工引起的土体变形对邻近地面建筑物、桩基、地下管线、既有隧道和基坑的影响以及邻近结构物施工对既有暗挖隧道影响的研究成果,并提出了需要进一步研究的课题及研究方法。     

Reliability assessment of granular filters in embankment dams

Empirical criteria have been used successfully to design filters of most embankment large dam projects throughout the world. However, these empirical rules are only applicable to a particular range of soils tested in laboratory and do not take into account the variability of the base material and filter particle sizes. In addition, it is widely accepted that the safety of fill dams is mainly dependent on the reliability of their filter performance. The work herein presented consists in a new general method for assessing the probability of fulfilling any empirical filter design criteria accounting for base and filter heterogeneity by means of first‐order reliability methods (FORM), so that reliability indexes and probabilities of fulfilling any particular criteria are obtained. This method will allow engineers to estimate the safety of existing filters in terms of probability of fulfilling their design criteria and might also be used as a decision tool on sampling needs and material size tolerances during construction. In addition, sensitivity analysis makes possible to analyse how reliabilities are influenced by different sources of input data. Finally, in case of a portfolio risk assessment, this method will allow engineers to compare the safety of several existing dams in order to prioritize safety investments and it is expected to be a very useful tool to evaluate probabilities of failure due to internal erosion. Copyright © 2006 John Wiley & Sons, Ltd.     

Rosenblueth方法在深基坑抗隆起可靠度分析中的应用

由于岩土参数具有很大的不确定性,在深基坑抗隆起稳定性分析中高安全系数并不一定安全。为此,提出了深基坑抗隆起稳定性可靠度分析方法,基于Rosenblueth方法开发了相应计算程序,研究了土性参数c, 变异性对可靠指标的影响,并通过实例分别运用可靠度和安全系数分析方法分析深基坑抗隆起稳定性。结果表明:可靠度分析方法具有科学性和合理性,同时体现了Rosenblueth方法在复杂功能函数中进行可靠指标计算的优越性。     

Offshore pile system model biases and reliability

ABSTRACT

Recent hurricanes between 2004 and 2008 in the Gulf of Mexico provide valuable new information about design model biases and uncertainties because multiple offshore platforms were loaded beyond the predicted capacity of their pile systems and because there was a failure of a pile system. A Bayesian calibration of model bias factors based on predicted versus observed performance of pile systems in hurricanes indicates that the conventional American Petroleum Institute design method for pile capacity is slightly conservative by about 10% for base shear (i.e. lateral) failures of pile systems in clay, unbiased for overturning (i.e. axial) failures of pile systems in clay, and conservative by more than 50% for overturning failures of piles systems in sand. The epistemic uncertainty in the updated bias factors is represented by coefficient of variation values of about 0.25 for base shear and overturning failures of pile systems in clay and 0.35 for overturning failures of pile systems in sand. A reliability assessment with the calibrated model bias factors shows that the current design practice produces lower reliability for a pile system with three piles versus one with eight piles and lower reliability for a pile system failing in overturning versus one failing in base shear. Therefore, the current design practice could potentially be improved by taking into account the mode of failure and the redundancy in the pile system to provide for a more uniform level of reliability.