Behavior of Shallow Strip Footing on Twin Voids

This paper numerically examines the bearing capacity and failure mechanism of a shallow strip foundation constructed above twin voids. The voids may refer to caves, caverns, underground aqueduct or tunnels due to water seepage, chemical reaction or deliberately excavated in soil deposit. The ability of numerical model to accurately predict the system behavior is evaluated by performing verification analyses on existing researches. Subsequently, a parametric study carried out to reveal the influence of size of footing/voids and their location (i.e. depth, spacing, eccentricity) on the bearing capacity of footing. To clarify the failure mechanism, the distribution of shear strain in the soil for different scenarios is assessed. The parametric study provided a new framework to determine the bearing capacity and the mode of failure for footings on voids. Based on the results, a criterion can be issued to avoid collapse of footing/voids regarding the shape, location and size of voids. The results can also be used to design construction of a footing on existing voids while the acquired failure mechanisms can be appointed to develop analytical solutions for this problem. Results demonstrated that a critical depth for voids and a critical distance between them exist where the influence on the ultimate bearing capacity of footing disappears.     

Collapse of a Model Strip Footing on Dense Sand Under Vertical Eccentric Loads

The paper focuses on the behaviour of a model strip footing, resting on a saturated dense sandy soil, subjected to centred or eccentric vertical loading. Experimental tests, carried out on a small-scale physical model, are able to reproduce effective stress levels equivalent to those prevailing in prototype problems, thanks to the maintenance of a downward steady-state seepage in the soil. The test program consists of three series of tests, each corresponding to an imposed value of hydraulic gradient, and each involving five load eccentricities; one series, in particular, is carried out with still water. Relevant points of load–settlement curves are related to the evolution of soil-footing collapse mechanism, evidenced by the distortion of some vertical coloured sand strips. The collapse mechanism is formed either by one or two sliding surfaces, depending on both load eccentricities and hydraulic gradient values. Significant differences are shown to occur between centred and eccentric loading footing response. Shear strength parameters obtained from back-analyses carried out on load values recorded at the appearance of each sliding surface on the free soil surface, in both hypotheses of associated and non-associated flow rule validity, are adopted to draw, for each test, a theoretical collapse mechanism consisting, in plane strain, of a log-spiral line with adjacent-end tangents; the obtained theoretical sliding surfaces, in turn, are compared to the experimental ones, showing that these latter are either stress characteristic or zero-extension lines depending mainly on cumulative footing displacements and current effective stress level in the soil.     

Experimental and Numerical Study of Strip Footing Supported on Stabilized Sand Slope

The paper presents the results of laboratory model tests and theoretical analysis on the behavior of a strip footing supported on sheet pile wall-stabilized sandy slope and loaded vertically to failure. The parameters varied in the study include the height, stiffness and location of the sheet pile wall, the location of the footing relative to the slope crest and the relative density of sand. Two-dimensional plane strain finite element analyses was used to analyze a prototype strip footing on sandy slope with same conditions. The results indicate that the inclusion of sheet pile wall has significant effect in improving the response of the strip footing and the slope itself. The theoretical results confirm the experimental results of the model footing tests and show reasonable agreement. Based on the numerical and experimental results, critical values of the sheet pile wall parameters for maximum stabilizing effect are established.     

采用分层地基系数的多支撑支护结构分析

以土层分层、支撑位置及抗弯刚度变化处等为结点对围护墙体进行单元划分，按地基系数法考虑开挖面下被动土体横向抗力．基于单元挠曲微分方程的初参数解．通过传递矩阵分析方法，结合围护墙两端及支撑轴力的边界条件．推导得多支撑支护结构各开挖工况下围护墙各结点的内力、变形及支撑轴力。编制了计算程序，并以上海太平洋广场一期工程基坑开挖为例进行分析计算。该法可用于求解上层分层、围护墙刚度随深度变化等情况．能更好地符合工程实际问题。     

Ultimate Bearing Capacity of a Footing Considering the Interference of an Existing Footing on Sand

The ultimate bearing capacity of a new strip footing placed on a cohesionless soil medium, in the presence of an existing strip footing, the load on which is assumed to be known, has been determined. Both the footings are assumed to be perfectly rigid and rough. The analysis is carried out by using an upper bound finite element limit analysis. For different clear spacing (S) between the footings, the values of the efficiency factor (ξ_γ) were determined; where ξ_γ is defined as the ratio of the failure load for an interfering new footing of a given width (B) to that for a single isolated footing having the same width. For ϕ < 30°, it is generally noted that the magnitude of ξ_γ increases continuously with a decrease in S/B. For ϕ > 30°, on the other hand if the applied load on the existing footing is approximately greater than half the failure load for a single isolated footing having the same width, the peak magnitude of ξ_γ was found to occur at around S/B ≈ 0.1 rather than at S/B = 0. The increase in ξ_γ becomes further significant with an increase in the magnitude of the load on the existing footing.     

层状地层全长粘结型锚杆锚固力学效应分析

考虑地层分层特征,根据剪切位移法基本原理,推导了拉拔荷载作用下层状地层全长粘结型锚杆位移、轴力及锚固体周边剪应力解析解,并结合FLAC模拟算例对比验证该方法的可行性;研究发现地层间界面处锚杆周边剪应力及轴力分布分别存在突变点和折点,其变化程度与地层物理力学性质有关,对此,提出了层状地层全长粘结型锚杆布置方案的建议。     

Prestressed Foundation Beams on Elastic Layered Soil

This paper develops a systematic procedure concerning the determination of the interaction of an arbitrary multistory frame with the elastic layered foundation soil by making use only of the soil surface deformational characteristics. The paper focuses on the response of the foundation beam through which the frame rests on the ground, showing the need for prestressing the beam in order to ensure better functionality. The investigation addresses the dual goal of excluding the undesirable cracked state of the beam and reducing its structural height together with the overall amount of reinforcement needed. A set of soil layered profiles is considered and it is investigated to what extent the required prestressing force depends not only on the soil deformational properties at hand, but also on the method of analysis used. The results are compared for both the elastic half-space and the Winkler’s method, taking into account either the whole structure stiffness, or the stiffness of the foundation beam only, as it is usually considered in practice.     

Seismic Bearing Capacity of Shallow Strip Footing with Coulomb Failure Mechanism using Limit Equilibrium Method

In this paper, an effort is made to evaluate the seismic bearing capacity of shallow strip footing resting on c–ф soil. The formulation is developed to get a single coefficient of bearing capacity for simultaneous resistance of weight, surcharge and cohesion. Limit equilibrium method in Pseudo-static approach with Coulomb mechanism is applied here to evaluate the seismic bearing capacity. The seismic bearing capacity of footing (q_uE) is expressed in terms of single coefficient N_γE. The effect of various parameters viz. angle of internal friction of soil (ф), angle of wall friction (δ), cohesion (c), ratio of depth to width of footing (d_f/B₀), seismic acceleration (k_h, k_v) are studied on the variation of seismic bearing capacity co-efficients.     

Rigid Foundations Resting on an Elastic Layered Soil

A rigid foundation of arbitrary shape resting on a stratified half space (soil), with specific geometrical and elastic properties for its horizontal layers, is examined under the action of a vertical load placed anywhere on the foundation area. On the basis of a purely analytical treatment of the deformability of the soil surface, i.e. without using a finite element discretization of the layered soil mass, a general method is developed in order to determine the contact soil pressures and the resulting settlements through an easily implemented numerical procedure. The possible presence of “inactive” foundation parts not being in contact with the soil surface is taken into account. Parametric studies are performed showing the influence of the variability of the soil layers, as well as of the vertical load location, on the contact pressures developed. In this context, for the cases of a rectangular and a circular foundation the area borders for the location of a vertical load are determined, either for the development of compressive soil pressures all over the footing (core), or for allowing the existence of some inactive part without contact with the soil (zero contact pressure), not exceeding the half of the total foundation surface (limit core).     

层状土壤上升毛管水运移特性试验研究

通过室内土柱模拟试验,进行了层状土与均质土的对比试验,对层状土壤毛管水运动特性进行了研究,分析了层状土壤毛管水上升高度、速率及地下水补给量与时间的关系,并研究了毛管水上升高度与地下水补给量之间的关系。结果表明,砂层对毛管水上升的高度、速率以及地下水补给量有明显影响,砂层对于水分有着明显的阻滞作用。     

Bearing Capacity of a Model Scale Footing on Crude Oil-Contaminated Sand

Onshore and offshore oil spills contaminate soil. In addition to environmental concerns for ground water pollution and other possible effects, the geotechnical properties of the contaminated soil such as the shear strength and the hydraulic conductivity are also altered. This note is a report of research in progress to evaluate the variation of the shear strength of a sand contaminated by a crude oil and thus the ultimate bearing capacity of shallow foundations. The limited results of the tests reported here relate to only one type of sand and one crude oil. The oil content was varied from zero to 4.2%. Results of direct shear tests for determining the soil friction angle are given. Along with these, laboratory model test results for the ultimate bearing capacity of a surface strip foundation supported by crude oil-contaminated sand are also presented. Based on these test results, the effect of oil contamination in drastically reducing the bearing capacity is discussed.     

夹硬层土中单桩贯入挤土效应的数值模拟

桩贯入土体产生的挤土效应问题较为复杂。利用ABAQUS软件建立了单桩贯入夹硬层土和均质土的二维轴对称有限元模型,经过分析比较,得出了单桩贯入夹硬层土体所特有的位移场及应力场的变化规律。分析表明：桩贯入夹硬层土过程中,软硬土层交界处土体水平位移变化剧烈;硬土层的存在,会使土体水平及竖向位移受到约束;夹硬层土的水平挤压应力要远大于均质土情况;与水平应力相比,竖向挤压应力在硬土层处明显偏小。     

斜阶跃波层状介质航空瞬变电磁响应数值计算

目前,航空瞬变电磁方法数据解释主要采用阶跃波均匀半空间模型计算视电导率值,而实际航空电磁系统发射波形的下降沿多为斜阶跃,导致解释时计算的视电导率值存在较大偏差。为此,笔者研究了航空瞬变电磁系统发射电流为斜阶跃时的电磁响应数值计算,将发射波形进行拉氏变换,利用G-S逆拉氏变换与241点汉克尔变换相结合的方法,实现斜阶跃波关断后的层状大地模型航空瞬变电磁响应计算;并对均匀半空间和层状大地模型下,不同关断时间和不同飞行高度对电磁响应的影响进行分析。得出结论:不同关断时间,关断后取样延时2 ms时,均匀半空间电磁响应的平均偏差为27.78%,三层模型的平均偏差为32.16%;当飞行高度从20 m增加到60 m时,均匀半空间和三层模型的感应电动势分别减小了43.6%和83.2%。     

层状地基静压桩贯入过程机理试验

通过在多层软粘土地基中静力压入单桩的室内模型试验,对模型桩在整个沉桩过程中压桩力、桩动端阻力及桩动侧阻力随桩贯入深度的变化情况进行了研究,获得了桩在贯入不同土层分界面时阻力的变化规律以及桩周土体应力的分布特征。并对开口管桩和闭口管桩贯入试验情况进行了比较分析,揭示了不同桩端形式桩在贯入过程中桩动侧摩阻力的发展规律,以及分层土体中开口管桩贯入过程中土塞的变化情况。试验结果表明：在粘性土中沉桩时,压桩阻力主要来自桩端向下穿越土体产生的端阻力,而侧摩阻力较小;由于桩侧水平应力的释放使得同一深度点上的动摩阻力随着桩的下沉表现出不同程度的降低,出现摩擦疲劳。     

Uplift Capacity of Horizontal Strip Anchors in Cohesionless Soil

This paper presents the details of the theoretical analysis of net uplift capacity of horizontal strip anchor in cohesionless soil using Kötter’s equation. A plane failure surface inclined at a characteristic angle with the ground surface is assumed. Results obtained using the proposed method are compared with the available experimental results of 30 cases for dense to loose cohesionless soil, with the maximum embedment ratio of 8. It is observed that the proposed method leads to the predictions of net uplift capacity of horizontal strip anchor that are very close to the experimental results in 93% cases. The comparison of results with available theoretical solutions shows that, proposed method makes better predictions for anchor embedment ratio less than 8 in dense cohesionless soils.     

基于MAPGIS的概率性地震危险性分析

对工程场地的地震危险性分析是地震安全性评价的主要方法, 并且为工程师提供抗震参数.在分析国内概率性地震危险性分析(PSHA) 方法基础上, 提出了基于GIS的概率性地震危险性分析的可行性方案.通过MAPGIS二次开发编写了地震危险性分析程序, 并以三峡坝区某工程场地为例对程序进行了测试.基于MAPGIS的概率性地震危险性分析程序提供友好的人机交互界面, 提高了地震危险性分析的可操作性, 更重要的是帮助用户从空间数据中挖掘更多的信息.      

Bearing Capacity of Embedded Strip Footings on Cohesionless Soil Under Vertical and Horizontal Loads

In the last decades a few attention was given to the evaluation of the bearing capacity of embedded footing under inclined loads on a frictional soil. This paper focuses on a numerical study using the finite-difference code Fast Lagrangian Analysis of Continua (FLAC), to evaluate the bearing capacity of embedded strip footing on a frictional soil. The soil is modeled by an elasto-plastic model with a Mohr–Coulomb yield criterion and associative flow rule; the effect of non-associativity of the soil on the bearing capacity is also investigated. The effect of the embedment is estimated though a depth factor, defined as a ratio of the bearing capacity of a strip footing at a depth D to that of a strip footing at the ground surface. The inclination effect is estimated by inclination factors, defined as the ratio of the limit vertical load for a footing under inclined loading to that of the vertically loaded footing. Both swipe and probe analyses were carried out to identify the vertical force–horizontal force (V–H) failure envelope. The results have been compared with those available in the literature.     

Measured and Back Analysed Soil Structure Interaction Effects in a Layered Stratigraphy During Tunnel Boring

The paper compares the soil structure interaction effects predicted using a finite element (FE) program with those observed during a bored tunnel project conducted in Perth, Western Australia. Both the measured data and geotechnical characteristics of the subsoils are described before presenting comparisons between measurements and analyses. The measured settlement data for two buildings are seen to be dependent on the structural stiffness. It is shown that, in keeping with observations by others, FE predictions of tunnelling induced movement patterns are not predicted to a satisfactory level of accuracy. However, it is also shown that FE analyses of the type described may be employed to provide qualitative information on soil-structure interaction effects.     

Dynamic Response of Machine Foundation on Layered Soil: Cone Model Versus Experiments

This paper presents the experimental validation of analytical solution based on cone model for machine foundation vibration analysis on layered soil. Impedance functions for a rigid massless circular foundation resting on a two layered soil system subjected to vertical harmonic excitation are found using cone model. Linear hysteretic material damping is introduced using correspondence principle. The frequency-amplitude response of a massive foundation is then computed using impedance functions. To verify the solution field experiments are conducted in two different layered soil systems such as gravel layer over in situ soil and gravel layer over concrete slab (rigid base). A total 72 numbers of vertical vibration tests on square model footing were conducted using Lazan type mechanical oscillator, varying the influencing parameters such as depth of top layer, static weight of foundation and dynamic force level. The frequency-amplitude response in general and in particular the resonant frequencies and resonant amplitudes predicted by cone model is compared with the results of experimental investigation, which shows a close agreement. Thus the cone model is reliable in its application to machine foundation vibration on layered soil.