Incorporating installation effects into the probability analysis of controlled modulus columns

This technical report presents the probabilistic analysis which integrates the Monte Carlo simulation (MCS) with random field theory to model the load–displacement behavior of Controlled Modulus Columns (CMCs) in overconsolidated Poznań clay. Presented study focuses on the practical aspects of statistical analysis of geotechnical data, numerical model development, and results evaluation. Variability and spatial distribution of geotechnical parameters are based directly on field and lab testing. The inherent variability of soil parameters obtained from geotechnical investigation at the site is similar to the values reported in worldwide datasets for clays. The extensive discussion about incorporation of installation effects into numerical modelling is made. It was found that proper incorporation of installation effects is governed by correct estimation of initial stress level and interface shear strength parameters. The Anisotropic Undrained Shear Strength (AUS) model which captures nonlinear behavior and anisotropy of soil (Krabbenhøft et al., 2019) is a good choice to model overconsolidated clay in intact and interface zones. The application of total stress approach, the AUS model, installation effects, and natural (inherent) variability of soil and interface parameters is sufficient to explain differences in CMC load – displacement behavior observed in the field.     

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.     

Probabilistic design of ground improvement by vertical drains for soil of spatially variable coefficient of consolidation

The design of soil consolidation via prefabricated vertical drains (PVDs) has been traditionally carried out deterministically and thus can be misleading due to the ignorance of the uncertainty associated with the inherent variability of soil properties. To treat such uncertainty in the course of design of soil improvement by PVDs, more rational probabilistic methods are necessary. In this paper, a simplified probabilistic method is proposed in which the inherent variability of the coefficient of consolidation, which is the most significant uncertain soil parameter that affects the consolidation process, is considered. An easy-to-use design procedure and charts are provided for routine use by practitioners.     

Time-variant reliability of retaining walls and calibration of partial factors

A methodology for the time-variant reliability analysis of geotechnical structures, particularly retaining walls, is presented. Calculation of the reliability of a wall, at any time during its lifetime, is enabled through the consideration of the degradation of soil shear strength parameters with time. In addition, the effects of spatial variability and model uncertainty are accounted for. Following an initial application with regard to British and European design standards, the methodology is extended in order to enable the calibration of partial factors for design to Eurocode 7. It is concluded that the values recommended for these factors are excessively high, at least for temporary structures that should be designed to withstand loads for just a few years. Therefore, a procedure for calculating reduced values of the partial factors that correspond to an appropriate safety level is further proposed.     

岩土样本数目对边坡可靠度分析的影响研究

 工程实践中岩土取样数目通常较少,定量分析样本数量对边坡可靠度分析结果的影响大小,以保证分析结果的可靠性对工程实践有一定参考价值。考虑边坡饱和土体的不固结不排水抗剪强度,从2个角度出发探讨不同岩土样本数目对边坡可靠度分析结果的影响：(1) 假设参数的变异性已知,考虑统计不确定性,研究其对边坡可靠度分析结果的影响;(2) 提出一种模拟实际边坡工程取样试验的直观分析方法,根据多组参数抽样结果对边坡进行可靠度分析,得到边坡破坏概率的频数分布,进而由实际需要确定所需样本数量。分析结果表明,当参数变异性较大时,所需样本数较少,当参数变异性较小时,所需样本数较多,这与传统的概率方法确定样本数目结果不同。模拟边坡取样试验的分析结果表明：从边坡实际设计上看,对于变异系数为0.2～0.5的不排水强度参数所需样本数目一般在10～15范围内。     

滨海吹填造陆地区岩土工程勘察问题分析与评价

围海造陆是当今沿海国家及城市开发土地资源的重要措施之一。以天津滨海某工程勘察为例,根据建设项目的构筑物特点、地基基础方案及场地土工程性质,采用了钻探取样、标准贯入试验、剪切波测试、十字板剪切测试、浅层平板载荷试验、室内试验等多种勘察手段获得了准确详实的地质资料,并对勘察过程中涉及到的冲填土、液化土、软土震限、负摩阻力、大面积群桩施工影响等关键技术问题进行了分析与评价,为该工程的设计及施工提供了可靠的岩土参数及建议。     

砂土液化势剪切波速简化判别法的改进

结合压电陶瓷弯曲元波速测试技术开展了饱和标准砂的不排水循环三轴试验,并根据试验结果改进了以往提出的利用剪切波速进行砂土液化势判别的简化方法。这种方法本质上基于砂土抗液化剪切强度与弹性剪切模量之间良好的相关性。用改进后的方法全面评价了26次地震、70多个液化场地的液化势,并与国内外其他液化势简化判别法的判别结果作了比较。分析结果表明,改进后的简化方法的评价结果与现场震害调查数据更趋一致。最后,通过一个实例分析]示了利用该改进方法进行土层液化势判别的一般步骤。该改进方法仍有待于深入研究,尤其是对密实砂土场地在强震下的液化评价,需要进一步的试验和现场数据加以佐证。     

用有限元方法分析基坑开挖的可靠度

基坑稳定性评价是岩土工程中的重要问题,基坑土体参数通常表现出较强的变异性,因此可靠度分析方法能更合理评价基坑的稳定状态。采用有限元方法对基坑开挖、支护进行模拟,并对开挖后的基坑进行强度折减计算,得到基坑稳定性安全系数,然后结合Rosenblueth统计矩法对基坑稳定性进行可靠度分析。分析方法简单,能够获得合理的结果,可为基坑工程设计和施工提供参考。     

考虑地层变异性和土体参数空间变异性的边坡可靠度全概率设计方法

目前边坡稳定设计研究中大多数考虑了土体参数的空间变异性,但忽略了地层变异性的影响。为此,提出了一种同时考虑这两类变异性的边坡可靠度全概率设计方法。在全概率设计框架内,将广义耦合马尔可夫链模型与随机场模型相耦合用于同时表征地层变异性和土体参数空间变异性,给出了所提方法的计算流程。利用澳大利亚珀斯市钻孔资料,以某边坡为例进行可靠度设计,为说明在边坡可靠度设计中同时考虑地层变异性和土体参数空间变异性的重要性,分析了仅考虑土体参数空间变异的情况,进一步分析了同时考虑两类变异性的情况,并对二者进行了比较。结果表明:所提出的边坡可靠度设计方法能够有效地考虑边坡中存在的地层变异性和土体参数空间变异性。当仅考虑土体参数空间变异性时,边坡可靠度设计结果很大程度上取决于所采用地层的分布情况,特别是地层分布中抗剪强度较强土体类型占比高于真实情况时,将导致得到的最优设计方案偏于危险。反之,若地层分布中抗剪强度较弱的土体类型占比高于真实情况,得到的最优设计方案偏于保守。因此,为准确地得到最优设计方案,在边坡可靠度设计中应同时考虑地层变异性和土体参数空间变异性的影响。     

Robust design and optimization procedure for piled-raft foundation to support tall wind turbine in clay and sand

A geotechnical design and optimization procedure for piled-raft foundations to support tall wind turbines in clayey and sandy soil are presented in this paper. From the conventional geotechnical design, it was found that the differential settlement controlled the final design and was considered as the response of concern in the optimization procedure. A parametric study was subsequently conducted to examine the effect of the soil shear strength parameters and wind speed (random variables) on the design parameters (number and length of piles and radius of raft). Finally, a robust design optimization procedure was conducted using a Genetic Algorithm coupled with a Monte Carlo simulation considering the total cost of the foundation and the standard deviation of differential settlement as the objectives. This procedure resulted in a set of acceptable designs forming a Pareto front which can be readily used to select the best design for given performance requirements and cost limitations.     

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。     

Effect of soil properties on pavement failures along the F209 highway at Ado-Ekiti,south-western Nigeria

A case of of long-term and frequent highway pavement failure induced by poor soil properties, at a locality along the F209 highway at Ado-Ekiti was investigated. Visual observation revealed an extensive pavement failure right down to the subgrade level. Laboratory test results of the disturbed soil samples, collected from the failed sections of the road showed that the natural soil moisture ranged from 2.5% to 8.3%, the liquid limit from 43% to 60.50%, the linear shrinkage from 1 to 10.10 and the specific gravity values from 2.57 to 2.67. X-Ray diffraction analysis confirmed the presence of abundant kaolinite and illite in the soil sample. A CBR value of 20% indicated a strong reduction in the strength of the soil materials used in the construction of the road. Excess fines and very high liquid limit values combined with a very low CBR value to instigate the failure of the pavement at this locality. The geotechnical properties indicated substandard paving properties for Nigerian roads. Therefore, some method of soil improvement, such as soil stabilisation is required to improve the strength of the soil and prolong the useful life and durability of the road.     

Sensitivity analysis of vertically loaded pile reliability

The purpose of this paper is to examine the influence of geotechnical uncertainties on the reliability of vertically loaded pile foundations and the use of this information in decision-making support, especially when gathering the information necessary for reliability analyses. Two case studies of single pile foundations were selected, and each uncertainty source was investigated to identify which are the most important and influential in the evaluation of vertical pile resistance under axial loading. Reliability sensitivity analyses were conducted using FORM (the first-order reliability method) and MCS (Monte Carlo simulations). The characterisation of uncertainties is not an easy task in geotechnical engineering. The aim of the analyses described in this paper is to optimise resources and investments in the investigation of the variables in pile reliability. The physical uncertainties of actions, the inherent variability of soil and model error were assessed by experimental in situ standard penetration tests (SPT) or from information available in the literature. For the cases studied, the sensitivity analysis results show that, in spite of the high variability of the soils involved, model error also plays a very important role in geotechnical pile reliability and was considerably more important than soil variability in both case studies. From a comparison of the two reliability methods (FORM and MCS), it was concluded that FORM is applicable in simple cases and as a first approach because it is an approximate method and sometimes does not have the capability to incorporate every detail of the problem, namely a specific probability density function or more specific limit conditions.     

Reliability-based design in rock engineering: Application of Bayesian regression methods to rock strength data

Reliability-based design(RBD) is being adopted by geotechnical design codes worldwide, and it is therefore necessary that rock engineering practice evolves to embrace RBD. This paper examines the Hoek-Brown(He B) strength criterion within the RBD framework, and presents three distinct analyses using a Bayesian approach. Firstly, a compilation of intact compressive strength test data for six rock types is used to examine uncertainty and variability in the estimated He B parameters m and sc, and corresponding predicted axial strength. The results suggest that within-and between-rock type variabilities are so large that these parameters need to be determined from rock testing campaigns, rather than reference values being used. The second analysis uses an extensive set of compressive and tensile(both direct and indirect) strength data for a granodiorite, together with a new Bayesian regression model, to develop joint probability distributions of m and scsuitable for use in RBD. This analysis also shows how compressive and indirect tensile strength data may be robustly used to fit an He B criterion.The third analysis uses the granodiorite data to investigate the important matter of developing characteristic strength criteria. Using definitions from Eurocode 7, a formal Bayesian interpretation of characteristic strength is proposed and used to analyse strength data to generate a characteristic criterion.These criteria are presented in terms of characteristic parameters mkand s ck, the values of which are shown to depend on the testing regime used to obtain the strength data. The paper confirms that careful use of appropriate Bayesian statistical analysis allows the He B criterion to be brought within the framework of RBD. It also reveals that testing guidelines such as the International Society for Rock Mechanics and Rock Engineering(ISRM) suggested methods will require modification in order to support RBD. Importantly, the need to fully understand the implications of uncertainty in nonlinear strength criteria is identified.     

Utilisation of transparent synthetic soil surrogates in geotechnical physical models: A review

Efforts to obtain non-intrusive measurement of deformations and spatial flow within soil mass prior to the advent of transparent soils have perceptible limitations. The transparent soil is a two-phase medium composed of both the synthetic aggregate and fluid components of identical refractive indices aiming at attaining transparency of the resulting soil. The transparency facilitates real life visualisation of soil continuum in physical models. When applied in conjunction with advanced photogrammetry and image processing techniques, transparent soils enable the quantification of the spatial deformation, displacement and multi-phase flow in physical model tests. Transparent synthetic soils have been successfully employed in geotechnical model tests as soil surrogates based on the testing results of their geotechnical properties which replicate those of natural soils. This paper presents a review on transparent synthetic soils and their numerous applications in geotechnical physical models. The properties of the aggregate materials are outlined and the features of the various transparent clays and sands available in the literature are described. The merits of transparent soil are highlighted and the need to amplify its application in geotechnical physical model researches is emphasised. This paper will serve as a concise compendium on the subject of transparent soils for future researchers in this field.     

强度参数统计特性对边坡稳定可靠性的影响

由于对边坡强度参数统计特性考虑不全面,使得边坡可靠性指标计算值偏小,对应的失效概率偏大,常常超过10％,设计中难以采纳,造成地基可靠度规范的制定和执行进展缓慢。以某高速公路全风化花岗岩土质高边坡为工程背景,在分析土质参数统计特性的基础上,采用极限平衡理论和蒙特卡罗模拟法,系统地分析了土质强度参数的均值不定性、变异性、相关性、区间特性和空间变化性等对边坡稳定可靠性的影响。结果表明：土性参数的各种统计特性对边坡稳定可靠性均具有不同程度的影响。可靠性指标计算值随抽样距离的减小而增加,随 c 或φ均值的增大而增加,随c 和φ的变异系数的增大而减小,随 c 和φ相关系数绝对值的增大而增加,考虑区间性后可靠性指标计算值也明显变大。即变异性对边坡稳定不利,而相关性、区间性和空间变化性对边坡稳定有利。因此,准确而全面地考虑土质参数的统计特性,尤其是在参数变异性和相关性的基础上加以考虑土性参数空间变化性和区间性会更加符合工程实际,且计算结果趋于安全,有利于地基可靠度规范的推广运用和边坡工程的安全评价。     

Sources of variability in laboratory rock test results

Appropriate rock characterization is beneficial in providing a reliable judgment on rock properties which is crucial for the design process of rock engineering applications. However, it can be difficult to obtain consistent mechanical parameters due to substantial variations in rock properties. In this research,uniaxial compression tests on dolerite specimens collected from a gold mine in Western Australia showed substantial scatter in the results. Rock categorization based on the P-wave velocities is as accurate as the thin section analysis, which suggests that they can be used together to gain a more accurate initial understanding of the rock types before any laboratory testing. The quality of specimen preparation and rockemachine interaction greatly affect the test results. For instance, non-parallelness of loading platens can lead to considerable scatter of the testing results, which would be perceived as rock variability. It is suggested that the current testing standards should be modified towards a better control of the loading machine performance and equipment precision. Finally, the possibility of pre-existing microcracks in rock, neither detected by the thin section analysis nor by the ultrasonic measurement,must be examined by computed tomography(CT) scanning as they can affect the test results. This study will enhance our knowledge about the sources of variability in laboratory test results of rock which is essential for obtaining reliable data.     

土动力学与土工抗震研究进展综述

综述当前国内外土动力学与土工抗震方面的研究进展,包括土体动力特性与本构关系、土工地震反应分析、土体动力测试、土体振动液化、土体地震永久变形、交通荷载作用下土体动力特性和土工抗震措施等内容。对各种方法的优缺点进行比较和评述。进一步阐述今后的研究方向。     

苏通大桥岩土特性的扁铲侧胀试验研究

随着工程建设规模的不断扩大,岩土工程设计中对岩土参数的精度和可靠性要求越来越高。由于岩土性质对其所赋存环境的强烈依赖性,仅仅依靠室内测试方法已远远不能满足岩土工程设计的要求,因此岩土性质的现场原位测试技术和方法在岩土工程,特别是大型岩土工程的勘察和设计中具有越来越重要的地位和作用。本文结合苏州—南通(苏通)长江公路大桥建设的实际,运用扁铲侧胀仪(DMT),对桥梁地基的有关岩土参数进行了现场试验研究。通过与常规室内外试验结果的对比分析发现,这种试验不仅具有快速、简便、经济、重复性好的特点,而且其所提供的岩土信息丰富、结果可靠。