Load and resistance factor design (LRFD) calibration for steel grid reinforced soil walls

This paper reports the results of load and resistance factor design (LRFD) calibration for pullout and yield limit states for steel grid reinforced soil walls owing to soil self-weight loading plus permanent uniform surcharge. The calibration method uses bias statistics to account for prediction accuracy of the underlying deterministic models for reinforcement load, pullout capacity and yield strength of the steel grids, and random variability in input parameters. A new revised pullout design model is proposed to improve pullout resistance prediction accuracy and to remove hidden dependency with calculated pullout resistance values. Load and resistance factors are proposed that give a uniform probability of failure of 1% for both pullout and yield limit states. The approach adopted in this paper has application to a wide variety of other reinforced soil wall technologies.     

Reliability analysis of soil nail walls

In India, soil nail walls are being extensively used for supporting vertical excavations below ground level to accommodate construction of one-or two-storied basements. Generally, the depth of excavations for basement construction ranges from 10 m to 15 m. For such large depth of excavation, variability of in-situ soil properties has significant influence on the stability of the soil nail walls. In the present study, using reliability analysis, an attempt is made to study the influence of variability of in-situ soil properties on the stability of soil nail walls. For better understanding, a case of 10 m high soil nail wall constructed to support a vertical cut is considered for the study and its stability is evaluated for various failure modes. Additionally, the influence of correlation among soil parameters on soil nail wall stability is assessed. In-situ soil friction angle and correlation between in-situ soil cohesion and angle of friction are found to influence soil nail wall stability significantly. In general, reliability analysis provided a better insight into the assessment of stability of soil nail wall.     

一汽乘用车所基坑土钉墙支护优化设计

分析了长春一汽乘用车所基坑工程地质条件,对基坑支护方案进行分析论证,并对所采用的土钉墙支护方案进行设计。采用单因素分析法,在土钉倾角合理区间内,对土钉倾角进行最优设计,得出了土钉倾角对基坑边坡稳定性影响规律。本工程采用土钉墙支护的方案,短时高效地完成了工程任务节约了投资。实践证明,通过土钉倾角优化设计,在工程造价不变的情况下,大幅提升施工安全。为土钉墙支护在长春基坑工程中的应用研究提供了借鉴。     

Soil nailed slope by strength reduction and limit equilibrium methods

A detailed study of nailed slopes under different conditions is reported in this paper. No major difference is found in terms of safety and slip surfaces between the strength reduction method (SRM) and the limit equilibrium method (LEM) in general cases. Appreciable differences between the SRM and LEM appear, however, if the nail load is controlled by the overburden stress. Some special slip surfaces from the SRM obtained by using a very fine mesh are discussed. Field tests demonstrated that the nail head is important in determining the failure mode and the factor of safety of a nailed slope, while the effect of the nail elastic modulus is more noticeable only when the slope is very steep. The optimum layout of the soil nail was found to be longer at bottom and shorter at the top, which is contrary to some engineers’ guidelines for soil nail design during top-down construction. The distribution of tensional force along the soil nail is influenced by the state of the slope (service state, limit state) and the failure modes (external failure, internal failure). In general, the line of maximum tension may not correspond to the critical slip surface as commonly believed, except for the case where the failure mode is an internal tensile failure.     

元宝山露天煤矿3~#排土场工程地质条件与稳定性分析预测

在分析矿区地形地貌、地层岩性、水文地质条件和影响排土场稳定因素的基础上,结合区内2#排场的监测资料,采用FLAC3D模拟软件和刚体极限平衡法中的毕肖普条分法,对排土场边坡变形、破坏的主要影响因素进行了分析,揭示了其滑坡机理;对3#排土场的稳定性做出预测,认为排土场边坡变形破坏模式是上部排弃物沿圆弧滑面剪切滑移与下部沿粉土、粉质粘土平面滑移的圆弧—平面组合滑移形式;基底土层的内摩擦角对排土场允许排高较为敏感,是决定允许排高的内在因素,基底粉质粘土的极限承载力是决定排土场允许排高的外在因素;基底土体极限承载力所决定的排土场的允许排高为90.60～91.60m。提出了填沟压脚、降低第一排台阶高度和基底排水等措施来满足排土场边坡稳定的要求。     

铁路台阶式加筋土挡墙潜在破裂面特征模型试验

铁路台阶式加筋土挡墙的设计方法尚不成熟,现有设计方法不能满足铁路边坡工程实践的需求。潜在破裂面的确定是加筋土挡墙设计的关键,但现行规范仅对10 m以下单级加筋土挡墙的潜在破裂面有了明确的规定。为了研究台阶式加筋土挡墙在铁路荷载作用下潜在破裂面的特征,设计了1:4的大比例尺二级台阶式加筋土挡墙室内模型试验,以周期性加卸载的方式模拟铁路荷载,通过监测墙面水平位移、墙顶沉降及土工格栅筋带变形,分析确定潜在破裂面的位置和形状。试验结果表明：I级墙（下级墙）潜在破裂面形状与现行规范中的0.3H法破裂面类似,但位置更深,且下部破裂面更缓,表明潜在不稳定范围更大;II级墙（上级墙）潜在破裂面形状与朗肯主动破裂面基本一致,但并未从II级墙坡脚剪出,而是内移刺入I级墙体;研究结果可为铁路台阶式加筋土挡墙的设计提供理论参考。     

The effect of pressure-grouted soil nails on the stability of weathered soil slopes

Pressure-grouted soil nails have been increasingly used for stabilizing slopes. The pullout resistance of a soil nail is the main factor for reinforcing the slope stability. In this study, a two-dimensional axisymmetric finite element model is developed to simulate the pullout behavior of a pressure-grouted soil nail. This model is verified with field pullout tests result of a pressure-grouted soil nails by comparing with gravity-grouted soil nails. Based on the analysis, a three-dimensional finite element model is proposed for stability analysis of a slope reinforced with pressure-grouted soil nails using the shear strength reduction method. A series of numerical slope stability analyses for a slope composed of weathered soil are performed to investigate the effects of grouting pressure on the slope stability and the behavior of the soil nails. Special attention is given to the installation effect of a pressure-grouted soil nails. It is found from the result of this study that the pressure-grouted soil nails increase the safety factor by fifty percent in a slope by increasing the stiffness of the nailed slope system. Numerical analysis results confirm the fact that the pullout resistance of a soil nail is the main factor for stabilizing slopes rather than the shear resistance of the soil nail.     

Hydraulic failure of diaphragm walls: a possible methodology for safety improvement

The paper presents a methodology aimed at reducing—or even avoiding—the risk of heave and uplift failures of reinforced concrete diaphragm walls. The method is based on the simple concept of increasing the drainage capacity of the embedded portion of the retaining walls. The behaviour of a strutted excavation in a cohesionless soil below groundwater is examined by means of two distinct series of numerical analyses, respectively focused on the hydraulic and mechanical behaviour of the retaining system. It is shown that, for some frequent cases, such methodology is capable to improve the safety of the retaining system with respect to both hydraulic and geotechnical limit states.     

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.     

Design Charts for the Stability Analysis of Unsaturated Soil Slopes

Simple limit equilibrium analyses can be performed to determine the Factor of Safety (FOS) against slope failure of unsaturated soil slopes. However, many of the input parameters needed for these analyses are highly variable, and the FOS value obtained is critically dependent on assumptions made by the designer. This paper describes a suite of reliability analyses on unsaturated soil slopes performed using an invariant reliability model. The results are presented in design charts from which a designer can choose the FOS value required to ensure a given target reliability index for a slope. The approach ensures that despite the variability of input parameters the slope will have a probability of failure of 2.23% or less.     

基于尖点突变理论的浆砌块石边坡稳定性研究

根据影响浆砌块石挡土墙边坡稳定性的主要控制变量：挡土墙的垂直度i、挡土墙高度h、填料的内摩擦角 、墙后边坡土的重度 、土对挡土墙基底的摩擦系数 、土对挡土墙墙背的摩擦角 等6个因素变量,利用正交设计原理优化试验设计方案,结合库仑理论与力的多边形法,分析计算浆砌块石挡土墙边坡的安全系数,最后用尖点突变理论的突变级数法进一步判断边坡的稳定性。研究结果表明：影响挡土墙边坡稳定性的6个主要控制变量因素的主次关系为 ;在分析挡土墙边坡稳定性时,采用突变级数法进行计算判断,可以直观地判断边坡的稳定情况,从而弥补工程中运用传统的最小安全系数法进行判断出现的模糊不确定的情况。     

临近边坡的条形基础地基极限承载力数值分析

建筑物或构筑物基础临近边坡置放的情况在实际工程中十分普遍,但目前对于临近边坡基础的地基承载力及破坏模式尚缺乏深入研究。采用不连续布局优化（DLO）极限分析法建立数值模型,分析边坡几何尺寸、土体参数和基础位置对临坡条形基础的极限承载力和边坡破坏模式的影响,并对国内外现行规范推荐的计算方法进行评价。结果表明：极限承载力随边坡高度和边坡倾斜角的增大而减小,当坡高超过临界高度后,极限承载力将不受其影响;极限承载力随土体黏聚力和内摩擦角的增大而提高,滑动面随黏聚力的增大而变浅,随内摩擦角的增大而变深;极限承载力随基础与坡肩相对距离的增大而提高,当基础置放位置超过某临界距离后极限承载力不受边坡影响。在土体强度高、坡角较大时,《建筑地基基础设计规范》规定的临坡基础最小置放距离偏于危险,设计时仍需考虑边坡对承载力的减损作用;在土体强度较低、坡角较小时,规范规定值偏于保守。美国AASHTO规范对边坡地基极限承载力的取值在砂土边坡时较为可靠,但其仅适用于坡面破坏模式的情况;饱和黏土边坡的承载力曲线有悖于理论解,对临界距离的规定同样存在低估。     

Performance-based analysis and design for internal stability of MSE walls

ABSTRACT

Mechanically stabilized earth (MSE) walls must be designed with adequate margins of safety against internal stability failure described by tensile strength and pullout limit states. Probabilities of failure (or reliability index) will vary strongly with the accuracy of the underlying models that appear in limit state performance functions. In this paper, prior work by the authors and co-workers on this topic is reviewed. Relative performance is explored in the context of the reliability (or probability of failure) of steel and polymeric reinforcing elements in MSE walls using limit state performance functions with load and resistance models having different accuracy.     

Estimation of ultimate bond strength for soil nails in clayey soils using maximum likelihood method

This paper presents a maximum likelihood estimation of the ultimate bond strength for soil nails in clays. Both uncensored and censored ultimate bond strength data for soil nails are collected from the literature. Based on the concept of maximum likelihood, a log-likelihood function is constructed for estimating the mean and coefficient of variation (COV) of the ultimate bond strength jointly using the two types of data. The mean and COV are determined as the pair that maximises the log-likelihood function. Two distribution models (normal and lognormal) are used for the estimation. A comparison of the relative competence between the two candidate distribution models that are adopted for describing the collected uncensored and censored data is performed using the Bayesian Information Criterion. Example designs of soil nail walls against internal pullout limit state of nails and overall stability limit state are provided to demonstrate the benefit of taking censored data into account for estimation of the ultimate bond strength of soil nails.     

基坑开挖与土钉支护的数值模拟

运用FLAC程序对深基坑土钉墙支护设计进行了数值模拟,分析了边壁土体的变形大小和土钉应力分布,并与现场实测结果对比,得出基坑顶的水平位移值与实测值相差不大,并对差值进行了分析。在应力方面,改善了土体内应力集中的现象,从而制约了土体的变形。剪应力在支护结构和坡角处有集中现象,在坡角处较为严重。     

均质边坡稳定性极限曲线法

基于滑移线场理论,按边坡坡面变形量评价其稳定性,提出均质边坡极限曲线法。该法是求有重边坡极限荷载的逆过程,也是强度折减法的对偶过程。以特征线法差分方程组（SCM）和试验方程近似公式（CCM）求得的极限坡面曲线与坡面线相交为变形破坏准则,定义了安全度（DOS）和破坏度（DOF）2个评价指标。该方法不必假设和搜索临界滑动面。经典考题和典型算例的验算表明,随着节点的增加SCM法计算精度增加,边界步长不变时,3次样条插值求得的变形破坏准则判断值不变,说明SCM算法稳定。典型算例的计算数据和图例表明,边坡角变大时边坡稳定性降低,极限坡面曲线与坡面由无交点变为有交点,证明了变形破坏准则的正确性。由2个例题计算结果对比可知,安全系数较大时,SCM法、CCM法计算结果与其具有可比性,相对于原边界条件增加了外荷载;安全系数变小时,SCM法、CCM法偏于保守。34个样本计算正确率为安全系数法67.7%,应力状态法73.5%,CCM法79.4%,SCM法70.6%,表明SCM法和CCM法正确率较高,计算结果可靠,SCM法、CCM法因素敏感性分析结论与安全系数法完全一致。在露天矿边坡稳定性和最终边坡角的分析与计算中,SCM法、CCM法结论与原报告相同,当参数变小时CCM法更有利于实践,具有一定的工程应用价值。     

斜坡地基刚性桩水平承载力上限分析

为探讨斜坡地基刚性桩水平极限承载力的计算方法，介绍柔性桩的等效刚性桩有效嵌入深度并引入极限水平地基反力分布形式。根据荷载指向坡外及坡内两种情况，提出适用于斜坡地基桩前土体的两种极限破坏模式。然后，基于极限分析上限定理，推导出两种荷载方向下的刚性桩极限承载力，并引入多组现场试验，验证了理论方法的合理性。探讨了边坡坡角、内摩擦角、黏聚力及荷载方向对极限承载力的影响，得出了一些规律性结论，并基于以上分析结果，提出斜坡地基刚性桩水平极限承载力随坡角变化的拟合公式。这些分析为斜坡地基上基桩设计提供了一定的参考，具有理论及工程应用价值。     

MCS-based probabilistic design of embedded sheet pile walls

A probabilistic approach to the design of embedded sheet pile walls is developed in this paper. The approach is based on Monte Carlo simulation (MCS), and it is used to investigate the performance of the partial factors and different design approaches in Eurocode 7 in achieving the target degrees of reliability. The approach is illustrated through an embedded sheet pile wall design example that has been used in literature for the evaluation of Eurocode 7. The approach deals rationally with the correlated load and resistance, and it bypasses a difficult but frequently asked question in Eurocode 7 (i.e. should the passive earth pressure be considered as a load (i.e. action) or a resistance?). The probabilistic design approach (DA) is also used to explore the effects of the soil unit weight variability and uncertainties in over-digging depth and wall friction. The effects of uncertainties in over-digging depth and wall friction are found to be significant. It is also found that, although the soil unit weight variability is generally minor, its effect on the design of embedded sheet pile walls is significant and should be properly accounted for in the design. The MCS-based probabilistic DA proposed in this study provides a straightforward way for proper consideration of such variability with relative ease.     

Stability of anchored sheet wall in cohesive‐frictional soils by FE limit analysis

This study extends the limit analysis techniques used for the computation of strict bounds of the load factors in solids to stability problems with interfaces, anchors and joints. The cases considered include the pull‐out capacity of multi‐belled anchors and the stability of retaining walls for multiple conditions at the anchor/soil and wall/soil interfaces. Three types of wall supports are examined: free standing wall, simply supported wall and anchored wall. The results obtained are compared against available experimental and numerical data. The conclusion drawn confirms the validity of numerical limit analysis for the computation of accurate bounds on limit loads and capturing failure modes of structures with multiple inclusions of complex interfaces and support conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Serviceability limit state design for uplift of helical anchors in clay

Presently, no displacement-based design methodology exists for helical anchors subjected to tensile or uplift loading. This study investigates the statistical and probabilistic aspects of the load-displacement uncertainty associated with a database of thirty-seven uplift loading tests of helical anchors founded within cohesive soils. Initially, an ultimate resistance model is identified, and the semi-empirical uplift breakout factor statistically characterized. A relationship between ultimate resistance and slope tangent capacity is established, and used to form the basis for normalizing the load-displacement response. Hyperbolic and power law models are statistically evaluated for use in serving as a reference load-displacement model; the hyperbolic curve was selected based on goodness-of-fit statistics. Monte Carlo reliability simulations are used to establish an equivalent-deterministic load factor that associates the selected load factor with a probability of exceeding a pre-determined allowable uplift displacement, given uncertainty in the undrained shear strength, ultimate resistance model, transformation uncertainty, uncertainty in the allowable displacement, and variability in uplift loading. A practical example is provided to show the intended use of this probabilistic helical anchor displacement model.