Experimental and analytical investigations on bearing capacity of strip footing in reinforced sand backfills and flexible retaining wall

This study focuses on the experimental and analytical investigations of small-scale physical model tests. For this purpose, a set of tests were conducted with and without reinforcement on the top of the backfill. The specimens were different in terms of parameters like the number of geotextile layers, the vertical distance between layers and the strip footing distance from the wall. Soil failure in the bearing capacity step and the backfill shear zones was analysed using particle image velocimetry methods. Bearing capacity of the strip footings was studied using analytical procedures. The results indicate that a reinforcing top zone of the flexible retaining structures may be more appropriate than unreinforced case. The ultimate bearing capacity and wall deflection can be significantly improved by increasing the number of reinforcement layers. When the three layers of reinforcement are used, there is an optimum vertical spacing of the layers at which the bearing capacity is the greatest (h/H?=?0.12, d/H?=?0.33 and u?=?B). The study shows that the analytical solution and the results from the experimental models are in good agreement.     

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。     

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.     

非线性破坏准则与岩土材料地基承载力研究

根据线性Mohr-Coulomb破坏准则,前人运用极限平衡法、滑移线理论或极限分析等方法求解地基承载力问题,但实际上岩土在剪切破坏过程中破坏准则具有非线性。因此,为了研究非线性破坏准则对地基承载力的影响,基于上限定理,通过“切线法”引进变量,根据能量耗散情况,将承载力问题转变为非线性规划问题,运用“序列二次规划算法”求出地基承载力的最优解。数值计算结果表明,当非线性破坏准则转变为线性破坏准则时,非线性参数对地基承载力有重要影响。     

Bearing capacity of strip foundations reinforced with geogrid sheets by using upper bound finite‐element limit analysis

By using the upper bound finite‐elements limit analysis, with an inclusion of single and two horizontal layers of reinforcements, the ultimate bearing capacity has been computed for a rigid strip footing placed over (i) fully granular, (ii) cohesive‐frictional, and (iii) fully cohesive soils. It is assumed that (i) the reinforcements are structurally strong so that no axial tension failure can occur, (ii) the reinforcement sheets have negligible resistance to bending, and (iii) the shear failure can take place between the reinforcement and soil mass. It is expected that the different approximations on which the analysis has been based would generally remain applicable for reinforcements in the form of geogrid sheets. A method has been proposed to incorporate the effect of the reinforcement in the analysis. The efficiency factors, η_c and η_γ, to be multiplied with N_c and N_γ , for finding the bearing capacity of reinforced foundations, have been established. The results have been obtained (i) for different values of ? in case of fully granular and cohesive‐frictional soils, and (ii) for different rates at which the cohesion increases with depth for a fully cohesive soil. The optimum positions of the reinforcements' layers have also been determined. The effect of the reinforcements' length on the results has also been analyzed. As compared to cohesive soils, the granular soils, especially with higher values of ?, cause a much greater increase in the bearing capacity. The results compare reasonably well with the available theoretical and experimental data from literature. Copyright © 2013 John Wiley & Sons, Ltd.     

土工格栅加筋垫层加固软土地基模型试验分析

基于压缩、剪切、拉拔、抗拉试验结果分析了土工格栅加筋土的力学特性。通过室内模型试验研究了加土工格栅和不加土工格栅地基土的侧向位移和竖向位移随深度的变化规律、位移场、应力场、应变场。结果表明,土工格栅加筋垫层能有效地改变和阻止塑性区的形成和发展,对下卧软土地基起到扩散应力、均化应力的作用,控制软土地基塑性变形,增强地基延性和刚度,有效地控制地基竖向位移和侧向位移,增强地基的抗剪强度,增强整个地基的稳定性,提高了加筋土体的承载能力,为实际工程应用提供了可靠的理论依据。     

Effect of spatial correlation structure and transformation model on the design of shallow strip footing

Directional variability of spatial correlation is observed in natural soils due to their depositional characteristics and it influences the response of structures founded on these deposits. Nonetheless, the results presented in most of the available literature are based on the assumption of either isotropic spatial correlation or perfect spatial correlation of soil properties in horizontal and vertical directions. It is also observed from past studies that the effect of transformation model on the total uncertainty is quite significant. Hence, an effort has been made in this paper to study the effect of anisotropy of autocorrelation characteristics of cone tip resistance (q_c) and the transformation model on the bearing capacity of a shallow strip footing, founding on the surface of a spatially varying soil mass. The statistics in the vertical direction of the soil mass are taken from 8 Cone Penetration Test (CPT) records and statistics in the horizontal direction are assumed. For the case considered, it is observed that the transformation model significantly influences the degree of variability of design parameter. The results also show that isotropic correlation structure based on the vertical autocorrelation distance underestimates the variability of design parameter. On the other hand, perfect correlation in horizontal or vertical, or both directions, overestimates the variability of design parameters, and produces conservative estimates of allowable bearing capacity.     

一种处理加筋体结构问题的有限元分析模型

在用有限元法分析混凝土中钢筋或峒室岩体中的锚杆变形问题时,经常会遇到线单元与体单元耦合变形的问题,现有的方法一般是将锚杆或钢筋简化成桁架或梁单元,作为嵌入单元将其贡献刚度叠加到体单元的刚度矩阵中。但无论哪种简化都没有反映复杂应力状态下加强构件变形处处不均匀的特性。在总结其他学者研究的基础上,将这类问题统称为加筋体问题,并提出一种新的的有限元分析模型。模型中利用连续介质力学基本理论将加强构件的轴向变形能、扭转变形能及弯曲变形能叠加到实体单元的势能泛函中,用最小势能原理导出了一种非线性的单元平衡方程。利用该模型编制了相应的程序,并与商用软件Abaqus的计算结果作了对比分析。     

土工格栅加筋膨胀土的三轴试验研究

土工格栅加筋膨胀土路堤或路堑、渠坡已在公路、铁路与水利工程中获得成功应用,但目前对加筋膨胀土的强度与变形特性的研究还较少。对不同方案加筋（水平1层加筋、水平3层加筋、竖向加筋）与不加筋的膨胀土样在不同围压下的三轴排水剪切试验结果的分析表明,（1）土工格栅加筋膨胀土形成排水通道,加速土体的排水固结,有利于膨胀土坡的稳定;（2）土工格栅加筋膨胀土一般呈应变硬化特征;（3）土工格栅的植入使剪切带的发展受到抑制,导致破坏模式变化;（4）加筋使膨胀土的内摩擦角变化不大,而黏聚力则有明显提高,其提高程度从小到大依次为水平1层加筋、水平3层加筋和竖向加筋;（5）采用加筋效果系数R评价,除了水平1层加筋方式的效果有所降低外,其余加筋方式都表现出加筋比不加筋的效果要好,其中以竖向加筋的效果最好     

土钉加固黏性土坡加载的离心模型试验研究

进行了不同坡度土钉加固边坡坡顶加载的离心模型试验,观测了土坡的破坏过程并测量土坡的位移场,研究了土钉加固黏性土坡的承载力、变形和破坏规律以及坡角对其破坏规律的影响。试验结果表明,坡顶荷载的增加引起土坡变形的增加,变形的集中产生和发展导致滑裂面的形成并使土坡发生破坏。土钉变形规律受加载阶段和加载底板的综合影响,坡顶荷载越大,接近坡顶的土钉弯曲挠度越大,钉土间的相互作用越强。土坡的坡角越大,承载力越低,土体呈现出更显著的向坡面位移的趋势。     

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.     

土工格栅加筋砂土地基大模型动载试验研究

为了研究土工格栅加筋砂土地基在动力荷载作用下的受力变形特性,利用自行设计的300 cm×160 cm×200 cm（长×宽×高）大比例地基模型试验装置,分别针对纯砂地基、土工格栅加筋地基进行了静动荷载破坏试验。分析地基承载力及基础沉降、地基土压力、动力加速度响应、土工格栅应变等参数变化规律,揭示了动力荷载作用下加筋砂土地基的承载力和变形特征,并对比分析静、动荷载对加筋地基承载性能的影响。试验结果表明,与纯砂地基相比,格栅单层加筋地基的承载力提高1.12倍,地基基础中轴线处沉降量减少24%,加筋土体的抗变形能力得到很大提高;加筋作用改变了地基的破坏模式,动载作用下纯砂地基为冲剪破坏而加筋地基为整体剪切破坏;筋材的存在对地基土压力及加速度峰值分别有明显的扩散作用和衰减作用,可有效降低在动力载荷下筋土的瞬态变形。     

条形基础下水平布置纤维增强聚合物筋材地基试验研究

纤维增强聚合物(FRP)具有耐腐蚀、耐水、耐高温、高强度、高弹性模量、质量轻等优点,将其用作加筋材料不仅能显著提高加筋地基的加筋效果,还能有效解决传统加筋材料的耐久性问题。分别对纯砂和5种水平加筋方式进行了加筋地基模型试验,测试了各级荷载下的地基沉降、FRP的应变及土压力数值,并初步探讨了FRP加筋的作用机制。试验结果表明,加筋能显著提高地基承载力和减小地基沉降,特别是双层水平加筋方式的加筋效果更加明显,而锚固件的设置对加筋效果的影响并不大,水平FRP筋材主要起到了土压力扩散作用。     

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.     

Probabilistic analyses of a strip footing on horizontally stratified sandy deposit using advanced constitutive model

An advanced hypoplastic constitutive model is used in probabilistic analyses of a typical geotechnical problem, strip footing. Spatial variability of soil parameters, rather than state variables, is considered in the study. The model, including horizontal and vertical correlation lengths, was calibrated using a comprehensive set of experimental data on sand from horizontally stratified deposit. Some parameters followed normal, whereas other followed lognormal distributions. Monte-Carlo simulations revealed that the foundation displacement u_y for a given load followed closely the lognormal distribution, even though some model parameters were distributed normally. Correlation length in the vertical direction θ_v was varied in the simulation. The case of infinite correlation length was used for evaluation of different approximate probabilistic methods (first order second moment method and several point estimate methods). In the random field Monte-Carlo analyses with finite θ_v, the vertical correlation length was found to have minor effect on the mean value of u_y, but significant effect on its standard deviation. As expected, it decreased with decreasing θ_v due to spatial averaging of soil properties.     

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.     

砂土-格栅筋土界面特性的本构模型研究

基于砂土与土工格栅、土工织物界面的室内大型直剪试验,分析了两种不同界面的力学性能。试验结果表明,峰值、残余剪切应力与界面剪胀性曲线存在一定的联系,即峰值强度通常会发生在界面相对减缩过程结束和残余剪切强度发生在界面的相对剪胀过程结束。加载过程中由于土工合成材料的逐步磨损、褶皱或者断裂导致了筋土界面强度出现较为明显的强度软化现象,在界面抗剪强度的研究中不可忽略。在试验基础上提出一种能够描述筋土界面力学性能的组合本构模型,该模型包含4个关系式：(1) 峰值、残余强度包络线;(2) 强度峰值前的双曲线模型;(3) 强度峰值后的位移软化模型;(4) 反映剪胀特性的界面剪胀模型。该组合模型的预测结果与直剪试验结果吻合较好,表明所用模型是合理的。     

Vertical vibration of a rigid strip footing on viscoelastic half-space

This paper presents an analytical method for modeling the dynamic response of a rigid strip footing subjected to vertical-only loads. The footing is assumed to rest on the surface of a viscoelastic half-space; therefore, effects of hysteretic soil damping on the impedance of the foundation and the generated ground vibrations are considered in the solution. To solve the mixed boundary value problem, we use the Fourier transform to cast a pair of dual integral equations providing contact stresses, which are solved by means of Jacobi orthogonal polynomials. The resulting soil and footing displacements and stresses are obtained by means of the Fourier inverse transform. The solution provides more realistic estimates of footing impedance, compared to existing solutions for elastic soil, as well as of the attenuation of ground vibrations with distance of the footing. The latter is important for the estimation of machine vibration effects on nearby structures and installations.     

水泥土桩联合土工格栅复合地基的离心模型试验研究

在软土地基上修筑路堤需要兼顾承载力和变形两方面的要求。针对山区沟谷软基的特点,采用了夯实水泥土桩与土工格栅联合加固的方法,是一种创新,选择合适的试验手段来研究该种型式的复合地基对于工程设计和施工具有重要意义。通过离心模型试验,模拟夯实水泥土桩联合土工格栅复合地基上路堤的填筑进程,水泥土桩采用石膏柱模拟,土工格栅采用预拉伸加筋格网模拟,试验得出了该种复合地基的工程特性。夯实水泥土桩具有较好的加固基础作用,格栅的主要作用是均衡上部荷载和减小差异沉降,采用夯实水泥土桩联合土工格栅的复合地基形式可以满足沟谷软基的沉降控制要求,其最终结论可以为设计和施工提供参考。     

饱和地基上条形弹性基础的摇摆振动

基于Biot动力方程,研究了饱和均质弹性半空间上弹性条形基础的摇摆振动问题。通过Fourier积分变换求解了饱和土的动力控制方程,然后结合基础底部为混合边界的条件得到了弹性条形基础的摇摆振动对偶积分方程,利用正交多项式将对偶积分方程转化为求解一组线性代数方程组,同时利用复合Simpson法则,得到了动力柔度系数的表达式,通过算例得出了不同参数时地基动力柔度系数随无量纲频率的关系曲线。