Expanded reliability-based design of piles in spatially variable soil using efficient Monte Carlo simulations

Inherent spatial variability is considered as a major source of uncertainties in soil properties, and it affects significantly the performance of geotechnical structures. However, research that considers, directly and explicitly, the inherent spatial variability in reliability-based design (RBD) of geotechnical structures is limited. This paper develops a RBD approach that integrates a Monte Carlo Simulation (MCS)-based RBD approach, namely the expanded RBD approach, with random field theory to model, both directly and explicitly, the inherent spatial variability of soil properties in RBD of drilled shafts. The proposed approach is implemented in a commonly-available spreadsheet environment to effectively remove the hurdle of reliability computational algorithms and to provide a user-friendly graphical user interface to practicing engineers. To improve the efficiency and resolution of MCS at small probability levels, the expanded RBD approach is enhanced with an advanced MCS method called “Subset Simulation”. Equations are derived for the integration of the expanded RBD approach and Subset Simulation. The proposed approach is illustrated through a drilled shaft design example, and is applied to explore the effects of inherent spatial variability (including the scale of fluctuation and correlation structure) and to evaluate systematically the equivalent variance technique that is commonly used to indirectly model inherent spatial variability in current RBD approaches. It is found that inherent spatial variability significantly affects the RBD of drilled shafts, and its effects are considered in RBD using the proposed approach in a direct and explicit manner. In addition, the results show that the indirect modeling of inherent spatial variability using the equivalent variance technique with the simplified form of variance reduction function in RBD might lead to relatively conservative designs in design practice.     

Unified description of different soils based on the superloading and subloading concepts

Geotechnical engineering often involves different types of geomaterials, such as sandy soil and clayey soil. Existing studies have confirmed that these soils have some common features, i.e. their mechanical behaviors depend not only on the inherent characteristics but also on their initial states. To describe the main mechanical behaviors of different soils within a simple and reasonable constitutive framework is of great significance for the numerical analysis on geotechnical engineering. This paper first introduces a model based on the concepts of superloading and subloading, which considers the “state dependence” (effects of overconsolidation and structure) of soil and only adds two material parameters compared with the Cam-Clay model. Secondly, conventional triaxial tests are systematically carried out on four types of soils (i.e. sand, silty clay, clay, and intermediate soil) with different initial void ratios, and the mechanical similarities and differences of these soils are discussed uniformly. After that, six material parameters of these soils are uniformly determined based on the concepts of superloading and subloading, and then used in constitutive calculations to verify the feasibility. The calculated results show a good agreement with test data, indicating that the model based on the concepts of superloading and subloading has great potential for describing the general mechanical behaviors of different soils within a unified framework. This work is expected to be applied to constitutive selection and parameter determination in the geotechnical numerical analysis of complex soil profiles.     

UH模型系列研究

岩土材料的本构模型是岩土工程学科的重要理论基础。合理的本构模型既能定性地揭示岩土的变形强度机制,也能定量地进行岩土体强度和变形计算。笔者20余年来潜心于土的本构模型研究,取得了以下3个方面的理论成果:1在修正剑桥模型的基础上,通过引入统一硬化(unified hardening,UH)参数,建立UH模型,该本构模型能够反映饱和超固结土的剪缩、剪胀、硬化、软化和应力路径相关性等特性,模型所用土性参数与修正剑桥模型完全相同;2扩展UH模型,使其考虑多种外部因素(温度、时间和基质吸力)、复杂特性(各向异性、结构性和小应变特性)和复杂加载条件(循环荷载、部分排水即渐近状态)等的影响;3提出广义非线性强度准则和满足热力学定律的变换应力三维化方法,从而实现了本构模型的合理三维化。UH模型已被嵌入到数值计算软件中,并被用于分析岩土工程问题。以上研究包括本构建模、强度准则、三维化方法和数值分析等方面,形成了独具特色的岩土本构理论和应用体系。     

软弱地层中深开挖产生隆起的迷思

一个位於台湾台北市软弱地层之深开挖基地,依据防止隆起规范及公式分析均合乎安全要求,仍然产生全面性破坏的实例检讨中,发现目前部分土壤力学理论与软弱地层的实际应力应变行为不一致的迷思。并确定软弱地层於国际间尚是一种地层的模糊名词,至今未有明确“定性”的定义与适当“定量”的方法确认,亦缺少完整理论基础。软弱地层基本上是沿用由过压密粘土发展之“弹塑性理论”,於实际开挖的原型量测中发现甚多不能合理解释的矛盾点,必须另建立较合乎软弱地层实际力学特性、具有三线性之“弹变塑性理论”。     

Prediction of nonlinear vertical settlement of a pile group consisting of new and existing displacement piles in clay strata

Reusing existing displacement piles is economical, time-saving, and environment-friendly. This paper presents an analytical approach for predicting the load carrying behavior of pile groups consisting of new and existing displacement piles. The evolutions of the undrained shear strength and shear modulus of clay adjacent to the piles from installation through consolidation to long-term ageing are investigated to determine the load carrying behavior of displacement piles. The nonlinear load-settlement behavior of an individual pile is modelled by load-transfer method, where the exponential function-based load-transfer models integrating the two developed soil parameters are employed to represent the nonlinear behavior at the pile-soil interface. The pile-pile interaction in the pile group is explored based on the shear displacement method. Combining the load-transfer method and the shear displacement method, an analytical framework is proposed for predicting the load-settlement behavior of pile groups consisting of new and existing piles. The proposed framework is validated by predicting the vertical settlement of a high-rise apartment built on a pile group consisting of 74 new displacement piles and 22 existing displacement piles during construction. Good agreement is achieved between the predicted and measured results. A parametric study is performed to explore the stiffness efficiency and the load-settlement behavior of pile groups with different layouts of new and existing piles. The results indicate that the pile group with larger ratio of the number of existing piles to the number of total piles shows both stronger stiffness and a higher load carrying capacity.     

砂土不同粒径对邓肯—张双曲线模型参数的影响

邓肯-张双曲线模型能较好地反映土体的非线性性态,概念清楚,易于理解,在岩土工程和地下工程的数值分析中得到广泛应用.在作具体数值计算时,要用到具体的参数值.一些文献、书籍和资料中,给出了一般性的砂土、粘土和黄土等的邓肯-张模型参数.通过常规三轴试验,得到了砂土的不同粒径对邓肯-张模型参数a,b值的影响.     

基于下加载面概念的饱和黏土温度–应力耦合弹塑性模型

温度对黏土力学特性具有重要影响,温度变化将引起黏土的体积变形发生变化,并对其前期固结应力、剪切强度、弹性模量等具有重要影响。基于黏土"热陷"特性,引入热屈服面以描述不同超固结比的黏土在温度升高时所产生的塑性变形,并进一步开展以下工作:(1)建立考虑温度影响的下加载面模型,由于保留了下加载面模型对超固结黏土应力–应变特性优异的描述能力,并考虑了温度对黏土的2种作用(即温度使超固结黏土的超固结比降低和温度塑性应变对屈服面的硬化效应),该模型可以描述温度变化对不同黏土力学特性的复杂影响;(2)证明了所提出的模型严格满足热力学第一、第二定律;(3)采用该模型模拟Pontida clay和MC clay在不同压力下的排水加热试验和在不同温度下的三轴排水/不排水试验,模拟结果与试验结果对比分析表明,模型能合理地描述不同超固结比的黏土在温度变化时产生的体积变形以及温度对黏土强度的影响等。     

往复移动荷载下超固结软黏土重力模型试验研究

研发了能够进行往复移动荷载下超固结软土地基的室内重力模型试验装置,并基于该试验平台开展了上海浅层超固结饱和软黏土的重力模型试验研究。通过一系列试验,揭示了超固结饱和软黏土地基在往复移动荷载下的累计孔压及累计变形随往复次数及动载幅值的时空演化规律。试验表明,深层土体处的超孔压动力响应明显较浅层的动力响应低,但其动力响应更为迅速。同时,土体在交通荷载行进方向上的动力响应比其他方向上更为强烈,其孔压衰减也最不明显。此外,动载幅值越大,初期的塑性变形和后期的位移增量也明显变大,并且不同水平位置处的差异沉降也越大。     

考虑参数空间变异性的失稳边坡参数概率反分析

概率反分析能够有效地考虑岩土体参数不确定性并融合现场监测数据和观测信息等更新岩土体参数统计特征,进而使得边坡稳定性评价更为符合客观工程实际,然而目前参数概率反分析几乎没有考虑参数固有空间变异性的影响。结合多重响应面和子集模拟提出了考虑岩土体参数空间变异性的边坡参数概率反分析方法,并以芝加哥国会街切坡为例,融合边坡失稳和滑动面入滑点与出滑点的大致位置这两个现场观测信息,概率反分析得到边坡不排水抗剪强度参数的后验统计特征。结果表明:本文提出方法可以有效地解决考虑参数空间变异性的低概率水平边坡参数概率反分析问题,具有较高的计算效率。子集模拟中每层随机样本数目对计算结果具有重要的影响,常用的500组样本点难以获得满意的计算结果。此外,土体参数空间变异性对概率反分析计算结果具有重要的影响,考虑参数空间变异性边坡参数由平稳随机场更新为非平稳随机场,与工程实际相符,然而忽略参数空间变异性更新后的参数仍服从平稳分布。     

改进超固结状态参量次加载面模型数值实现与应用

为合理描述超固结土复杂的弹塑性力学行为,对现有Hashiguchi次加载面模型中的超固结状态参量R进行修正,在硬化方程中,考虑塑性体应变与塑性剪应变的综合作用,提出了修正超固结状态参量的次加载面模型。同时,着重介绍了该模型的隐式积分算法及数值实现过程,编制了对应的接口子程序,实现了该模型在有限元软件ABAQUS中的应用。通过不同工况和加载方式下的数值模拟验证了程序的合理性,最后应用模型研究了Fujinomori黏土的三轴压缩力学特性并与UH模型的模拟结果、室内试验研究进行对比。结果表明,子程序具有较高的计算精度和可靠性,模型能够准确地模拟黏土的超固结特性。     

静压敞口预应力高强混凝土管桩界面剪切性状试验研究

桩土界面剪切行为对静压敞口预应力高强混凝土(PHC)管桩沉贯性状及长期承载力特性具有至关重要的作用。通过成层土地基中桩身预埋光纤光栅(FBG)传感器的静压桩足尺试验,分别对敞口PHC管桩贯入及静载荷试验中的桩土界面剪切行为进行研究。结果表明：在贯入阶段,桩身轴力及侧摩阻力沿桩的深度方向逐渐传递,传力幅值与桩周土体性状密切相关,土层界面处轴力传递效率依次为98.2%、92.2%、96.3%、83.8%、80.5%。随着压桩循环次数的增加,同一深度土层摩阻力呈逐渐减小趋势。经历5个压桩循环后,深度6 m处的砂质粉土层摩阻力减小幅度约为46.25%,深度10m处的粉质黏土层经历3个压桩循环后摩阻力减小幅度约为12.1%;载荷试验过程中,桩侧摩阻力随着桩顶荷载施加自上而下逐步发挥。摩阻力完全发挥所需的桩土相对位移,粉质黏土层的最大,约为6.96~7.46mm,淤泥质黏土层的次之,约为6.05mm,砂质粉土层的最小,约为4.23mm;与原状土相比,重塑区土体含水量、孔隙比参数指标降低,重度、黏聚力及内摩擦角增大。桩周重塑区土体物理力学指标变化是贯入及载荷试验阶段桩土界面剪切行为不同的重要原因。     

SIMULATION OF SHEAR AND ONE-DIMENSIONAL COMPRESSION BEHAVIOR OF NATURALLY DEPOSITED CLAYS BY SUPER/SUBLOADING YIELD SURFACE CAM-CLAY MODEL

Naturally deposited clays exhibit complicated mechanical behavior that differs from that of remolded clays. For example, clay in a normally consolidated state commonly exhibits softening in undrained shear tests or “rewinding” in a heavily overconsolidated state. The Super/subloading Yield Surface Cam-clay model (Asaoka et al., 1998, 2000, 2002) was proposed in an attempt to clarify the complicated mechanical behavior in naturally deposited clays. In this constitutive model, the concepts of “structure,” overconsolidation, anisotropy, and their evolution laws, are introduced into the modified Cam-clay model. In the present study, undrained triaxial compression tests and oedometer tests were carried out on two types of naturally deposited undisturbed clay, Pleistocene clay and Holocene clay, and the behavior was then simulated using the Super/subloading Yield Surface Cam-clay model. The findings of the present study are as follows:

• 1)For the two types of undisturbed clay, the Super/subloading Yield Surface Cam-clay model can simulate undrained triaxial compression behavior ranging from the normally consolidated state to the overconsolidated state, corresponding to various isotropic pressures using a single set of material constants.
• 2)In addition, the model can simulate one-dimensional compression behavior using the same material constants as those used for the simulation of the undrained triaxial compression behavior.
• 3)Through the simulation, the mechanical behavior of Pleistocene clay and Holocene clay, which have different loading histories and have undergone different aging effects, can be described by the different evolution parameters, as well as the elasto-plastic parameters.

     

Performance analysis of a jacked-in single pile and pile group in saturated clay ground

When a pile is installed into saturated clay ground, the “setup” effect may occur due to ground consolidation, which changes pile performance. Although this phenomenon has been observed both in the field and laboratory, its numerical simulation is still challenging. In this work, pile installation effects on the behaviors of jacked-in piles were investigated through three simulation techniques by a three-dimensional finite element analysis program, PLAXIS 3D. A constitutive model called the soft soil creep model was used to describe the soil behavior based on soil parameters obtained from laboratory tests. The behaviors of single piles were first investigated with or without the consolidation process after pile installation to evaluate the pile setup effect. Then, a pile group comprising 4 piles was analyzed using the consolidation process to verify the applicability of the three simulation techniques. The calculated results were compared with the corresponding experimental results. The calculated results using three techniques generally agreed well with the experimental results in terms of initial stiffness and pile shaft resistance. Both the measured and calculated results indicate that ground consolidation caused by pile installation significantly increases the pile bearing capacity and especially, the pile shaft resistance. Therefore, the pile setup effect can be reasonably simulated by the three proposed techniques.     

Statistical analyses of inherent variability of soil strength and effects on engineering geology design

Clayey soil strata, as all natural deposits, generally show variability in the values of their geotechnical properties. This is due mainly to geological and environmental processes such as deposition and diagenesis, which introduce heterogeneity, anisotropy and variability to soil properties. Other causes of variability, and thus uncertainty, are the representativeness of samples and errors related to testing procedure, measurement and data processing procedures. To improve our knowledge about the inherent variability in the geomechanical properties of clays, this work presents a case study related to the analysis of the strength variability along a log of marine stiff clay deposits, which are apparently quite homogeneous. The analysis was based on pocket penetrometer strength measurement, performed both punctually and across the whole deposit. The adopted testing procedure, which is fast and reliable, provides a really wide dataset of the investigated soil property, with more than 800 data points. These allow for detailed variability analysis, and a reliable estimation of the coefficient of variation as well as research into the best fitting probability density functions, which are key factors for robust design. The presented case study allows discussion of the inherent variability of soil properties, and its influence on the characteristic values of soil strength in geotechnical design.     

考虑法向应力历史的黏土–混凝土界面弹塑性分析

 土与混凝土接触界面剪切力学特性研究对桩基础设计至关重要,非挤土桩施工过程中由于成孔卸荷或桩身混凝土凝结造成界面附近土体径向卸荷,径向荷载的变化势必改变桩土界面的荷载传递规律。通过大型结构剪切试验发现：法向应力的变化历史对黏土–混凝土接触面剪切变形特性和强度参数影响明显。为给钻孔灌注桩桩土接触界面数值模拟提供合理的剪切力学模型,以接触界面积累的能量为硬化参数,假定接触界面剪切过程为界面积累能量对外做功的过程,考虑法向应力历史对剪切刚度的影响,提出考虑法向应力历史的黏土–混凝土界面模型,并介绍各模型参数的确定方法。模型原理清晰,参数物理意义明确。通过大型直剪试验成果数据对界面模型进行验证。结果表明：提出的模型能准确地再现和预测不同法向应力历史条件下的黏土–混凝土接触界面的力学特性。     

Influence of particle contact models on soil response of poorly graded sand during cavity expansion in discrete element simulation

The discrete element method (DEM) has been extensively adopted to investigate many complex geotechnical related problems due to its capability to incorporate the discontinuous nature of granular materials. In particular, when simulating large deformations or distortion of soil (e.g. cavity expansion), DEM can be very effective as other numerical solutions may experience convergence problems. Cavity expansion theory has widespread applications in geotechnical engineering, particularly to the problems concerning in situ testing, pile installation and so forth. In addition, the behaviour of geomaterials in a macro-level is utterly determined by microscopic properties, highlighting the importance of contact models. Despite the fact that there are numerous contact models proposed to mimic the realistic behaviour of granular materials, there are lack of studies on the effects of these contact models on the soil response. Hence, in this study, a series of three-dimensional numerical simulations with different contact constitutive models was conducted to simulate the response of sandy soils during cylindrical cavity expansion. In this numerical investigation, three contact models, i.e. linear contact model, rolling resistance contact model, and Hertz contact model, are considered. It should be noted that the former two models are linear based models, providing linearly elastic and frictional plasticity behaviours, whereas the latter one consists of nonlinear formulation based on an approximation of the theory of Mindlin and Deresiewicz. To examine the effects of these contact models, several cylindrical cavities were created and expanded gradually from an initial radius of 0.055 m to a final radius of 0.1 m. The numerical predictions confirm that the calibrated contact models produced similar results regarding the variations of cavity pressure, radial stress, deviatoric stress, volumetric strain, as well as the soil radial displacement. However, the linear contact model may result in inaccurate predictions when highly angular soil particles are involved. In addition, considering the excessive soil displacement induced by the pile installation (i.e. cavity expansion), a minimum distance of 11a (a is the cavity radius) is recommend for practicing engineers to avoid the potential damages to the existing piles and adjacent structures.     

Effective installation of micropiles to enhance bearing capacity of micropiled raft

While micropiles are used in many geotechnical projects, as ground reinforcement rather than as structural elements, field engineers have reported that the bearing capacity of micropiled rafts greatly exceeds the range of common ground reinforcement. This is known to be due to the confining effects of micropiles from the interaction between the ground and the micropiles, which extends the failure area of the ground significantly. Utilizing micropiles as ground reinforcement can excessively underestimate the structural contribution of the footing in a micropiled-raft system to the bearing capacity. This study investigates the support characteristics of a micropiled raft through model tests and a numerical analysis. The support behavior of the micropiled raft is evaluated for various conditions, such as soil type, pile length, and installation angle. It is found that the micropiles modify the failure behavior of the ground considerably, and that the bearing resistance can be enhanced by considering the appropriate failure mode, installation angle, and pile length.     

Full-scale model tests of load transfer in geogrid-reinforced and floating pile-supported embankments

Well-designed field full-scale model tests were carried out to enhance the understanding of geogrid-reinforced and floating pile-supported (GRFPS) embankments constructed on medium compressibility soil (MCS). Two comparative test sections of GRFPS embankments with and without pile caps were built over silty clay with medium compressibility for field monitoring, over 25 months. The heavily instrumented embankments produced comprehensive high-quality data. Settlement, earth pressure, and geogrid strain measurements during embankment filling stages and the postconstruction placement stage were conducted. The influence of pile cap installation on the differential deformation and load transfer behaviour of the GRFPS embankment was evaluated. The results demonstrate the installation of pile caps can significantly improve the evolution characteristics of the stress increment ratio on the pile, facilitating a change in load sharing of the pile top from a “softening” feature to a “hardening” feature. The state of the “arching structure” heavily depends on the relative displacement. After the maximum arching is formed, although the subgrade load continuously increases, the arching enters the damage and recovery state, and the transfer of the overburden load increment is largely conducted by the tensioned membrane effect.     

超固结非饱和土的本构关系

Alonso等提出的巴塞罗那(Barcelona)弹塑性本构模型是非饱和土本构模型中的代表,将巴塞罗那本构模型与姚仰平等所提出的超固结土UH本构模型相结合,使之适用于超固结非饱和土。该模型在吸力等于零的时候就退化成饱和土的UH本构模型;在吸力不为零且无超固结的情况下,就退化成巴塞罗那本构模型;该模型不仅使超固结状态下的湿化模拟更为合理,而且也能够反映超固结非饱和土的硬化、软化、剪缩、剪胀特性和不同应力路径对超固结非饱和土变形特性的影响,同时能够反映湿化使超固结程度降低甚至使超固结消失的特性。与巴塞罗那模型相比,本文所提出的模型没有增加任何新的材料参数,且与已有的试验结果对比分析,表明该模型能够较为合理地描述超固结非饱和土的基本力学特性。     

三维应力空间下改进状态变量的上下负荷面模型及其数值实现

为准确描述结构性土、超固结土的复杂弹塑性力学行为,对上下负荷面模型中的超固结状态变量R与结构性状态变量R*的演化规律进行改进,运用Sheng建议的破坏准则把模型的应用范围拓宽到三维应力空间,建立了三维应力空间下改进状态变量的上下负荷面模型。详细介绍了该模型隐式应力更新算法的数值实现过程,编制了对应的接口子程序,实现了对有限元软件的二次开发,并通过一系列数值模拟验证了模型的合理性、程序的精度与稳定性。最后应用本文模型模拟Boom黏土和Fujinomori黏土并与试验数据对比,结果表明,模型能够准确描述结构性土、超固结土的力学特征。