条形基础加筋砂土地基极限承载力计算

基于传统的极限平衡条分法,利用临界滑动场法计算了条形基础的加筋地基极限承载力。假定土体处于极限平衡状态时,土体与筋材间存在均匀的摩擦力,通过建立土体条块极限平衡方程,推导了地基承载力的递推关系式。首先,设定计算土体范围,并划分条块和离散状态点;其次,根据递推公式计算各个状态点的参数,并搜索临界滑面;最后,根据搜索出的滑面计算地基承载力。通过实例比较进一步验证了计算结果的可靠性,并分析了首层筋带埋深、铺设层数和长度对地基承载力和滑面位置的影响。研究结果表明：地基承载力随着筋带埋深的增加先增大后减小;随着层数和长度的增加先逐渐增大,最后趋于稳定;滑面位置的变化规律主要是垂直影响深度和水平影响范围增大或减小。该方法原理简单、易于编程,为条形基础加筋地基承载力的计算提供了一种新思路,是临界滑动场法在地基承载力计算中的推广应用。     

An analytical model of the strip footing reinforced with geogrid

The ultimate bearing capacity of a strip footing on soil reinforced with geogrids has been studied analytically. The reinforcing mechanism of the soil bearing on lateral surfaces and shearing over soils through the apertures of the grids was considered. The passive resistance against transverse ribs of geogrid was calculated using the Prandtl mechanism. The proposed equation incorporated the traditional Nγ factors, unit weight of soil, soil shear strength angle, footing width, and the geometry of the geogrid. The accuracy of theoretical predictions was verified by the experimental and analytical results from the existing literatures. The mean bias of the theoretical model was found to be about 10%.     

空洞上方浅基础地基破坏模式与极限承载力分析

利用极限分析上限法求解地基极限承载力问题的关键在于构造合适的破坏模式。当地基下方存在空洞时,地基的破坏模式变得相当复杂。通过分析空洞存在时地基的受力特点及破坏形态,将地基破坏范围划分成为不同的刚性区和过渡区,构造了空洞上方条形基础地基的破坏模式。利用上限法,建立与破坏模式对应的速度场,推导了破坏模式不同区域内的耗散功率和外力功率,得到地基极限承载力的目标函数,并采用数学优化方法进行求解,获得了极限承载力的上限解。通过算例分析,讨论了空洞顶板厚度、空洞大小与地基极限承载力的关系,并与无空洞条件下地基极限承载力进行对比分析。结果表明,随着空洞顶板厚度增加,地基极限承载力增加,破坏模式也由地基与空洞之间扩散到地基两侧;空洞顶板厚度存在临界值,当超过此临界值时,空洞对地基极限承载力的影响可忽略。     

条形基础极限承载力数值分析

根据塑性力学滑移线理论,推导了条形地基在极限平衡状态下的平衡微分方程,然后利用有限差分法推出地基土在极限状态时的有限差分公式,结合边界条件编制出地基承载力计算程序,该程序求解条形地基极限状态下的滑移线区域及相应的地基极限承载力值时可以考虑基础埋深、地基土重度、土的内摩擦角、基础与地基摩擦等参数。利用程序,得到了土的内摩擦角与地基承载力系数N? 的对应表,并全面讨论了基础埋深、地基土重度、内摩擦角及基础与地基摩擦角等参数对地基承载力和滑移线形状的影响,得到了一些有意义的结论。     

Upper bound solutions of bearing capacity of strip footing by pseudo-dynamic approach

This paper presents the pseudo-dynamic analysis of seismic bearing capacity of a strip footing using upper bound limit analysis. However, in the literature, the pseudo-static approach was frequently used by several researchers to compute the seismic bearing capacity factor theoretically, where the real dynamic nature of the earthquake accelerations cannot be considered. Under the seismic conditions, the values of the unit weight component of bearing capacity factor N _γE are determined for different magnitudes of soil friction angle, soil amplification and seismic acceleration coefficients both in the horizontal and vertical directions. The results obtained from the present study are shown both graphically as well as in the tabular form. It is observed that the bearing capacity factor N _γE decreases significantly with the increase in seismic accelerations and amplification. The results are thoroughly compared with the existing values in the literature and the significance of the present methodology for designing the shallow footing is discussed.     

土工格栅加筋土挡墙试验研究

采用在墙面板背后安装土压力盒以及在土工格栅上安装柔性位移计的方法,对某高速公路加筋土挡墙水平土压力和土工格栅拉筋位移进行了系统测试。试验研究表明,施工期间土工格栅加筋土挡墙墙背土压力随填土高度的增加而增大,增长速率逐渐减小,其数值均小于理论计算结果,沿墙高分布形式与计算结果有较大差别;土工格栅拉筋在施工期应变变形较大,工后应变非常小,挡墙下部土工格栅拉筋端部应变随填土高度变化较大,在加筋体锚固区末端存在过渡区,其工程特性逐渐向非加筋体填土过渡。根据试验结果对土工格栅加筋土挡墙施工控制及关键技术提出了相关建议。     

ANN-based model for predicting the bearing capacity of strip footing on multi-layered cohesive soil

In reality, footings are most likely to be founded on multi-layered soils. The existing methods for predicting the bearing capacity of 4-layer up to 10-layer cohesive soil are inaccurate. This paper aims to develop a more accurate bearing capacity prediction method based on multiple regression methods and multi-layer perceptrons (MLPs), one type of artificial neural networks (ANNs). Predictions of bearing capacity from the developed multiple regression models and MLP in tractable equations form are obtained and compared with the value predicted using traditional methods. The results indicate ANNs are able to predict accurately the bearing capacity of strip footing and outperform the existing methods.     

Reliability analysis for bearing capacity of surface strip footing using fuzzy finite element method

ABSTRACT

This paper presents the reliability analysis on the basis of the foundation failure against bearing capacity using the concept of fuzzy set theory. A surface strip footing is considered for the analysis and the bearing capacity is estimated using the conventional Finite Element Method (FEM). The spatial variability of the variables is taken into consideration to capture the physical randomness of the soil parameters for an isotropic field. A variation of the probability of failure (P_f) against a varying limiting applied pressure (q) is presented for different Coefficient of Variation (COV) of the variables and different scale of fluctuation (θ). The results reveal that the friction angle of soil (?) is the most influencing parameter among the other variables. Further, the influence of the scale of fluctuation (θ) on the probability of failure (P_f) is also examined. It is observed that for a particular COV of ?, higher value of θ predicts higher P_f whereas, P_f increases as COV of ? increases for a particular θ value. Later, a comparison study is accomplished to verify the viability of the present method and it can be noticed that the present method compares well with the other reliability method (First Order Reliability Method) to a reasonably good extent.     

Reliability analysis of seismic bearing capacity of strip footing by stochastic slip lines method

In this research, the reliability analysis of seismic ultimate bearing capacity of strip footing is assessed with implementing slip lines method coupled with random field theory. The probability density functions of seismic and static bearing capacities which are log-normal and nearly normal distribution respectively are compared to each other. The predicted Probability Density Function (PDF) of the seismic bearing capacity by slip line method is verified, with those of the Terzaghi equation and Monte Carlo simulation (MCs). For uncertainties analysis by Terzaghi equation the N_c, N_q and N_γ are assessed stochastically.     

Bearing capacity of a strip footing on brittle rock

Summary Proposed method provides at least two advantages in its implementation in spite of the limitations. It needs a few material properties data and is therefore simpler for parametric analyses than a more sophisticated finite or distinct element method. Also, it accounts for shear strength activation level at each discontinuity and therefore it provides more realistic results for brittle materials than the limit equilibrium methods, which consider only average level of activation of peak or residual shear stength along sliding surface alone.     

Bearing capacity of rectangular footing on reinforced soil

In the present paper, a method of analysis for calculating the pressure intensity corresponding to a given settlement for a rectangular footing resting on reinforced soil foundation has been presented. The process has been simplified by presenting non-dimensional charts for the various terms used in the analysis, which can be directly used by practicing engineers. An empirical method has been suggested to find out the ultimate bearing capacity of footing on reinforced soil. The results have been validated with large-scale model tests also. The procedure has been made clear by giving an illustrative example. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of spatial variability of cross‐correlated soil properties on bearing capacity of strip footing

Geotechnical engineering problems are characterized by many sources of uncertainty. Some of these sources are connected to the uncertainties of soil properties involved in the analysis. In this paper, a numerical procedure for a probabilistic analysis that considers the spatial variability of cross‐correlated soil properties is presented and applied to study the bearing capacity of spatially random soil with different autocorrelation distances in the vertical and horizontal directions. The approach integrates a commercial finite difference method and random field theory into the framework of a probabilistic analysis. Two‐dimensional cross‐correlated non‐Gaussian random fields are generated based on a Karhunen–Loève expansion in a manner consistent with a specified marginal distribution function, an autocorrelation function, and cross‐correlation coefficients. A Monte Carlo simulation is then used to determine the statistical response based on the random fields. A series of analyses was performed to study the effects of uncertainty due to the spatial heterogeneity on the bearing capacity of a rough strip footing. The simulations provide insight into the application of uncertainty treatment to geotechnical problems and show the importance of the spatial variability of soil properties with regard to the outcome of a probabilistic assessment. Copyright © 2009 John Wiley & Sons, Ltd.     

Behaviour of square footing on sand reinforced with coir geocell

The use of geosynthetics as a ground improvement technique offers the advantages such as space saving, environmental sensitivity, material availability, technical superiority, higher cost savings and less construction time. Coir geotextiles can be considered as an efficient replacement to its synthetic counterparts due to its economy and excellent engineering properties. The present study aims at exploring the possibilities of utilising coir geocells as a potential reinforcement material for shallow foundations and thereby increasing the load carrying capacity of soil. Geocells were fabricated from coir geotextiles with the aim of providing an additional confinement to the soil. An enumerated parametric study was conducted by varying the relative density, depth of the first layer, width and height of coir geocell. The surface displacement profiles of the non-reinforced and coir geocell-reinforced soil indicate that the footing rotation and heave are considerably reduced with the provision of geocell. The results of the relative density study indicate that bearing capacity characteristics increase with denseness of the soil sample. It was also observed that geocell arrangement and configuration play a pivotal role in the performance characteristics of reinforced soil.     

动载下土工格栅加筋桥台挡墙承载性能分析

为研究加筋土桥台结构在顶部条基动载作用下的动力响应问题,通过MTS伺服加载系统施加循环动载,开展室内加筋桥台挡墙动载破坏试验,对比分析3种格栅长度和3类格栅型式的加筋土挡墙沉降及面板水平位移、土压力、筋材应变等参数的分布规律,揭示加筋桥台挡墙的动力承载性能。试验结果表明：在循环动载下不同格栅长度及型式的加筋桥台挡墙破坏模式存在差异,M、A、B型格栅加筋长度 1.0H（H为挡墙高）的挡墙破坏模式均为冲切剪切破坏,A、B型格栅 0.7H和 0.4H的挡墙破坏模式为局部剪切破坏。加筋桥台挡墙面板侧移随筋材长度增加依次减小,A型格栅加筋土挡墙侧移系数总体上相比B型小。桥台挡墙因加筋格栅长度及型式不同导致动土压力衰减规律差异明显,当 1.0H时M型及A型筋材竖向动土压力衰减系数沿墙高呈抛物线函数模型,当 0.7H时,A型和B型筋材竖向动土压力衰减系数沿墙高皆呈指数函数模型。     

砂土加筋挡墙筋条临界长度的神经网络预测

对于加筋支挡结构的设计,神经网络模型不同于基于凝聚力基础之上的半经验公式,它不需要主观的人为假设,而是模拟人脑思维,通过数据样本的学习来获得预测结果.BP神经网络是对非线性可微分函数进行权值训练的分层网络.文中采用BP网络对给定极限荷载下砂土挡墙筋条的临界长度进行预测,试验点几乎分布在预测曲线附近,说明网络学习是成功的.对不同筋材的极限荷载与临界长度的关系进行了对比分析,表明筋材的弹性模量及筋材与填料之间的摩擦系数对加筋性能有着重要的影响.     

The bearing capacity of surface strip foundations on geogrid-reinforced sand and clay — a comparative study

Summary Laboratory model test results for the ultimate bearing capacity of a strip foundation supported by geogrid-reinforced sand and saturated clay are presented. One type of geogrid only was used for all the tests. On the basis of the model test results, the optimum depth and width of reinforcing layers and the optimum depth of the location of the first layer of the geogrid in sand and saturated clay were determined and compared.     

基于二阶锥规划有限元增量加载法的条形浅基础极限承载力分析

地基极限承载力分析是土力学研究中的一个经典课题。基于Hellinger-Reissner混合变分原理和有限元方法,将岩土体弹塑性问题构造成基于有限元框架的二阶锥规划（second-order cone programming,SOCP）问题,进而提出一种基于二阶锥规划理论的增量有限元法,即FEM-SOCP法。将岩土体弹塑性问题构造成二阶锥规划的数学优化问题,可以避免采用传统弹塑性计算中复杂的应力点积分等算法和屈服面棱角的平滑处理。此外,对于二阶锥规划问题,可以采用具有原始?对偶内点求解法的标准数学规划求解器MOSEK进行求解。将增量加载FEM-SOCP法应用于经典的基底粗糙的条形浅基础地基极限承载力分析中,分别考虑了关联和非关联塑性条件下的Mohr-Coulomb屈服准则。数值结果表明：所提出的增量加载FEM-SOCP法获得的地基承载力系数及地基承载力与传统FEM计算结果基本一致,而与常规有限元计算结果相比,基于增量加载的FEM-SOCP法所获得的屈服区更加平滑。     

Failure modes and bearing capacity of strip footings on soft ground reinforced by floating stone columns

This study evaluates the failure modes and the bearing capacity of soft ground reinforced by a group of floating stone columns. A finite difference method was adopted to analyze the performance of reinforced ground under strip footings subjected to a vertical load. The investigation was carried out by varying the aspect ratio of the reinforced zone, the area replacement ratio, and the surface surcharge. General shear failure of the reinforced ground was investigated numerically without the surcharge. The results show the existence of an effective length of the columns for the bearing capacity factors N _c and N _γ. When certain surcharge was applied, the failure mode of the reinforced ground changed from the general shear failure to the block failure. The aspect ratio of the reinforced zone and the area replacement ratio also contributed to this failure mode transition. A counterintuitive trend of the bearing capacity factor N _q can be justified with a shift in the critical failure mode. An upper-bound limit method based on the general shear failure mode was presented, and the results agree well with those of the previous studies of reinforced ground. Equivalent properties based on the area-weighted average of the stone columns and clay parameters were used to convert the individual column model to an equivalent area model. The numerical model produced reasonable equivalent properties. Finally, a theoretical method based on the comparison of the analytical equations for different failure modes was developed for engineering design. Good agreement was found between the theoretical and numerical results for the critical failure mode and its corresponding bearing capacity factors.     

Bearing capacity of strip foundation on geogrid-reinforced sand

Results of small-scale laboratory model tests to determine the ultimate bearing capacity of a strip foundation supported by sand with multiple layers of geogrid reinforcement are presented. Tests were conducted with only one type of geogrid and a sand compacted to one relative density. The embedment ratio of the foundation was varied from zero to 0.6. It is found that, for the given reinforcement-depth ratio, the bearing capacity ratio with respect to ultimate load increases with embedment. The relationship between the bearing capacity ratio at ultimate load and at limited levels of settlement (less than or equal to 5% of foundation width) is also presented. The bearing capacity ratio at limited levels of settlement is smaller than the value at ultimate load.