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.     

考虑沉桩效应的群桩非线性荷载-沉降解析

考虑沉桩效应对桩周土体力学特性的影响,采用指数函数型荷载传递曲线分别建立了静压桩的桩侧和桩端荷载传递模型。在此基础上,根据群桩加载过程中桩周土体的变形模式,基于荷载传递法描述桩-土界面的非线性行为,采用剪切位移法考虑群桩之间的相互作用,提出了考虑沉桩效应的群桩非线性荷载-沉降混合计算方法。通过开展离心模型试验对该计算方法解答进行了验证,研究了沉桩效应和桩-土界面非线性特征对群桩承载特性的影响。研究结果表明,沉桩效应对桩周土体起到挤密作用,使得桩周土体的强度和刚度增大,从而提高了群桩的承载特性。群桩加载过程中桩-土界面刚度随沉降变形而逐渐减小,使得群桩荷载-沉降曲线呈现出明显的非线性特征。     

Non-uniform granular pile–mat foundations: analysis and model tests

Granular piles are commonly used to improve bearing capacity and reduce settlements of soft soil in coastal and lowland areas. Due to various reasons, granular piles tend to acquire a non-uniform tapered-down cross-section in actual construction process. In this paper, a simple theoretical approach to predict deformation behaviour of soft ground reinforced with non-uniform granular pile–mat system is presented. A small-scale experimental set-up is fabricated in the laboratory and tests performed. Results predicted by the proposed approach compare well with the laboratory tests, and the field tests and other numerical test results available in literature. A parametric study has been carried out to evaluate the relative influence of granular mat thickness and relative stiffness on various responses of non-uniform granular piles.     

PHC管桩荷载传递的试验研究和数值分析

通过在预应力高强混凝土管桩(PHC桩)的桩顶、桩端及桩周各主要土层的分界面埋设应变计的静荷载试验，研究了PHC管桩的荷载传递机理，分析了轴力和桩侧摩阻力的变化规律。并以工程中PHC管桩的竖向抗压静载试验为基础，运用有限单元法对软土地区的PHC管桩桩-土相互作用进行模拟，在分析中采用弹塑性模型，引入了非线性接触面单元，并且考虑了土体的材料非线性。分析结果表明，计算值和实测值有一定的差别，但是变化趋势基本一致。     

群桩荷载位移特性研究

将杆系结构有限单元法与荷载传递迭代法相耦合,形成一桩基沉降分析计算的混合法。采用近似解析解中Randolph and Wroth’Model与双曲线模型相结合,模拟桩身与桩周介质边界上剪切滑移的非线性。桩间相互作用在采用弹性理论Mindlin方程解答计算,并考虑了桩间“加筋与遮帘”作用。桩周土介质非均质性特征,采用指数函数模拟。分析了刚性承台下群桩桩数、桩长、桩间距和桩土模量比等群桩工作特性的影响。尤其是桩台基础沉降对群桩相互作用影响机制的研究,对桥梁拼接等对桩基础沉降要求严格的工程有重要的借鉴意义。     

Multiple response surfaces for slope reliability analysis

This paper develops a multiple response surfaces approach to approximate the limit state function for slope failure by second‐order polynomial functions, to incorporate the variation of the most probable slip surfaces, and to evaluate the slope failure probability p_f. The proposed methodology was illustrated through a cohesive soil slope example. It is shown that the p_f values estimated from multiple response surfaces agree well with those p_f values that have been obtained by searching a large number of potential slip surfaces in each Monte Carlo simulation (MCS) sample. The variation of number of the most probable slip surfaces is studied at different scale of fluctuation (λ) values. It is found that when full correlation assumed for each of random fields (i.e., spatial variability is ignored), the number of the most probable slip surfaces is equal to the number of random fields (in this study, it is 3). When the spatial variability grows significantly, the number of the most probable slip surfaces or number of multiple response surfaces firstly increases evidently to a higher value and then varies slightly. In addition, the contribution of a specific most probable slip surface varies dramatically at different spatial variability level, and therefore, the variation of the most probable slip surfaces should be accounted for in the reliability analysis. The multiple response surfaces approach developed in this paper provides a limit equilibrium method and MCS‐based means to incorporate such a variation of the most probable slip surfaces in slope reliability analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

System reliability analysis of slopes using multilayer perceptron and radial basis function networks

This paper presents a system reliability analysis method for soil slopes on the basis of artificial neural networks with computer experiments. Two types of artificial neural networks, multilayer perceptrop (MLP) and radial basis function networks (RBFNs), are tested on the studied problems. Computer experiments are adopted to generate samples for constructing the response surfaces. On the basis of the samples, MLP and RBFN are used for establishing the response surface to approximate the limit state function, and Monte Carlo simulation is performed via the MLP and RBFN response surfaces to estimate the system failure probability of slopes. Experimental results on 3 examples show the effectiveness of the proposed methodology.     

考虑水泥土桩增强作用的灌注桩水平承载性能现场试验研究

结合淤泥土地基中的三洋港挡潮闸工程,研究水泥土桩增强灌注桩水平承载特性的效果,在现场进行水平静载荷试验。试验时,为量测钢筋应力,在钻孔形成灌注桩时将钢筋计焊接在钢筋笼的不同高程,测得在施加水平荷载时桩身的应力分布情况,从而得到弯矩的分布;将土压力盒埋入桩侧土体中,测试在水平荷载下桩周土体的土压力分布规律。试验结果表明,打设水泥土桩能够控制灌注桩水平位移的发展,并能提高灌注桩水平承载能力;灌注桩桩身弯矩值和桩周土体的土压力的分布情况都呈现先增加后减小的变化规律,并且都主要集中在上部桩体和土体中,其最大值约为泥面以下3 m左右的位置;水泥土桩的打设能够有效地减小桩身弯矩值,并且可以减弱底部反弯矩的出现;打设有水泥土桩的灌注桩桩周土体能够提供更大的土压力。另外,灌注桩的水平承载力受上部土体的影响较大,即提高上部土体的物理力学性质可以有效增大灌注桩水平承载力。     

基于刚性承台群桩沉降的可靠度研究

针对在总结目前的群桩可靠度设计理论完全是基于承载力,提出了对于超大超长群桩基础采用基于沉降的可靠度设计概念,其中沉降计算模式考虑了桩身压缩以及桩端刺入量。在讨论了计算参数的分布规律后,用蒙特卡罗方法进行了可靠度指标的计算。通过一个算例表明,荷载水平增加会导致群桩可靠度指标急剧降低;可接受位移增加,可靠度指标增大,但这种趋势逐渐变缓。     

地震荷载下长短桩复合地基的群桩效应系数分析

长短桩复合地基中,地震发生时,短桩在降低长桩所受剪力和弯矩影响中起到重要作用。本研究对弹性地基中的钢管桩基础实行了静态有限元分析,以及动态离心模型试验,通过桩径、桩长、桩间距等参数进行剪切波速,长桩剪力弯矩的对比分析,得到一种新的水平向群桩效应系数计算方法,并通过对模型建筑物的模拟,验证了该方法的有效性与实用性。结果表明,静态有限元数值模拟分析和离心模型试验结果有较好拟合,不同震级作用下通过该方法均能给出较为合理的系数。本文结果可为长短桩复合桩基的抗震性能提供重要参考,实际工程中,通过调整桩径,桩长,短桩数量等参数,可以提出更为合理的设计施工方案。     

路堤荷载作用下柔性桩复合地基的沉降分析

为了寻求路堤荷载下柔性桩复合地基沉降计算的简便方法,基于已有的研究成果,将桩侧摩阻力分布简化为分段线性模式,根据桩长与临界桩长的大小关系,结合桩-土-垫层三者在交界面上的应力与压缩变形协调条件,运用单位元法推导了柔性桩复合地基加固区的沉降计算公式,并采用分层总和法计算复合地基下卧层的沉降量。结合工程实例对计算结果进行验证,结果表明:采用理论方法计算的柔性桩复合地基沉降量与现场实测沉降结果吻合较好,证明理论计算方法的合理性,且能较好地反映路堤荷载作用下柔性桩复合地基的工作性状。进一步分析表明:在临界桩长范围内,桩与桩间土相互作用,最大限度发挥了复合地基桩间土的承载能力;此外,由于桩侧负摩阻力对桩体有拖拽作用,桩身轴力在桩体中性面位置处达到最大值。因此,在工程设计中要高度重视和运用临界桩长和中性面的概念。     

刚-柔性桩复合地基中桩荷载传递规律试验研究

为了解刚-柔性桩复合地基中刚性桩和柔性桩的荷载传递规律,在温州地区选择了2幢采用刚-柔性桩复合地基的建筑物进行原位试验。试验前在钻孔灌注桩桩身埋设了钢筋应力计,在水泥搅拌桩桩身埋设了振弦式应变计,在建筑物施工过程中同步检测传感器的变化情况。试验结果表明,不同部位及不同类型桩荷载传递规律不同,中部刚性桩摩阻力相对于边部刚性桩重心下降,刚性桩荷载传递长度大于柔性桩。无褥垫层刚-柔性桩复合地基中部刚性桩在层数低时会出现负摩阻力,有褥垫层刚-柔性桩复合地基中在施工期间始终存在负摩阻力。中部桩端承力要高于边部桩端承力。     

蒙特卡罗模拟在区域地震滑坡灾害评价中应用

2004年10月23日,日本新泻中越地区发生史上最强地震之一,震级达到Mw6.6。这次地震诱发上千个规模大小不一、形式各样的滑坡,造成一定的灾害损失和人员伤亡。因此,为了有效地避免和减轻这方面的灾害,有必要有效地、科学地预测和评价地震诱发滑坡的空间发生规律。为此以一定的地区作为研究对象,提出了蒙特卡罗随机模拟和Newmark滑动体位移分析法在区域地震斜坡失稳概率分析中的应用。该法充分考虑到岩土参数和相关地震参数空间变异性,结合地理信息系统空间分析的优势,以网格模型为基础,分析该区斜坡滑动体位移分布规律,并以2cm为失稳临界值探讨网格斜坡破坏的概率。实例证明,所提出的模型有效地预测了滑坡发生的空间分布规律。     

钻孔灌注桩压浆后承载性能的可靠度分析

钻孔灌注桩在压浆后的承载特性得到明显改善,离散程度明显降低,但目前对压浆后钻孔灌注桩的可靠度还缺乏系统研究。收集了122根未压浆的钻孔灌注桩和147根压浆后的钻孔灌注桩静载试验资料,结合可靠度分析方法中的确定性和不确定性分析方法,利用近似概率法的基于一次二阶矩法的验算点法（简称JC法）和Monte Carlo法对钻孔灌注桩压浆前和压浆后的可靠度指标进行了对比分析。结果表明：压浆后桩的可靠度指标有很大幅度提高,压浆可减少持力层对可靠度的影响,同时后压浆桩的可靠度有随桩径增大而提高的趋势,且不同荷载效应比值下可靠度随桩径变化的趋势趋于一致。     

基于随机响应面法的边坡系统可靠度分析

提出了一套基于随机响应面法的边坡系统可靠度分析方法。该方法首先从大量潜在滑动面中筛选出代表性滑动面。针对每条代表性滑动面,采用Hermite多项式展开建立其安全系数与土体参数间的非线性显式函数关系（即随机响应面）。然后,采用直接蒙特卡洛模拟计算边坡系统失效概率。在蒙特卡罗模拟中,采用所有代表性滑动面的随机响应面计算每一组样本所对应的边坡最小安全系数。最后,以两个典型多层边坡系统可靠度问题为例验证了该方法的有效性。结果表明：文中提出的边坡系统可靠度分析方法能够有效地识别边坡代表性滑动面,具有较高的计算精度和效率,并且确定代表性滑动面时无需计算滑动面间的相关系数。同时该方法可以有效地计算低失效概率水平的边坡系统可靠度,为含相关非正态参数的边坡系统可靠度问题提供了一条有效的分析途径。此外,多层边坡可能同时存在多条潜在滑动面,基于单一滑动面（如临界确定性滑动面）或者部分代表性滑动面进行边坡系统可靠度分析均会低估边坡失效概率。     

刚性桩和柔性桩荷载传递性状的比较

为了比较刚性桩和柔性桩的荷载传递规律,在温州地区选择了钻孔灌注桩和水泥搅拌桩进行静荷载试验。试验前在钻孔灌注桩桩身埋设了钢筋应力计,在水泥搅拌桩桩身埋设了振弦式应变计。在静荷载试验过程中同步检测传感器的变化情况,根据试验成果对两种不同类型桩进行对比分析,得出一些有益的结论,供工程设计施工人员参考。     

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.     

锚杆-抗滑桩系统的可靠性分析

锚杆-抗滑桩结构本身以及结构与岩土体的相互作用是一个相当复杂的问题,其影响因素非常多,再加上岩土体参数的变异性,对此结构系统的确定性分析会造成计算结果与工程实际之间较大的不相符.结构力学计算方法是目前锚杆-抗滑桩系统较为合理的计算方法,在此方法的基础上,引入对问题的维数不敏感的蒙特卡罗方法,进行锚杆-抗滑桩系统可靠性的数字模拟.通过工程实例验证,为锚杆-抗滑桩系统的设计方法由确定性分析向可靠性分析的过渡作了有益的工作.     

Lateral load distributions on grouped piles from dynamic pile‐to‐pile interaction factors

The load distributions of the grouped piles under lateral loads acting from one side of the pile cap could be approximately modeled using the elasticity equations with the assumptions that the underground structure is rigid enough to sustain the loads, and only small deformations of the soils are yielded. Variations of the soil–pile interactions along the depths are therefore negligible for simplicity. This paper presents the analytical modeling using the dynamic pile‐to‐pile interaction factors for 2 × 2 and 2 × 3 grouped piles. The results were found comparative with the experimental and numerical results of other studies. Similar to others' findings, it was shown that the leading pile could carry more static loads than the trailing pile does. For the piles in the perpendicular direction with the static load, the loads would distribute symmetrically with the centerline whereas the middle pile always sustains the smallest load. For steady‐state loads with operating frequencies up to 30 Hz, the pile load distributions would vary significantly with the frequencies. It is interesting to know that designing the pile foundation needs to be cautioned for steady‐state vibrations as they are a problem of machine foundation. However, for transient loads or any harmonic loads acting upon relatively higher frequencies, the pile loads could be regarded as uniformly distributed. It is hoped that the numerical results of this paper will be helpful in the design practice of pile foundation. Copyright © 2008 John Wiley & Sons, Ltd.     

Updating reliability of single piles and pile groups by load tests

Pile load tests are used to refine designs and for quality assurance. They can also be used to verify the reliability of piles and pile groups. Stochastic methods have previously been developed to verify the reliability of single piles. A general stochastic method to verify the reliability of pile groups is developed in this paper. The method can be used to assess the reliability of groups where pile tests have been conducted to the ultimate capacity, to below the ultimate capacity but exceeding specified capacity, and where pile tests fail to achieve the specified capacity. In the latter case, the method allows decisions to be made as to whether the reliability of the entire pile group is satisfactory or whether additional piles need to be installed.