垃圾填埋场土工合成材料界面摩擦特性的拉拔试验研究

土工合成材料与填料的界面特性是决定垃圾填埋场中衬垫系统与土工合成材料受力特性的重要因素。选择3种不同种类的土工合成材料,用砂土和黏土为填料,通过拉拔试验研究土工合成材料的界面特性。试验结果表明:界面的峰值剪切强度与峰值位移随着法向应力的增加而增加;土工合成材料与黏土之间的摩擦角较大;填料为砂土时,无纺布与填料间的摩擦角最大,EPDM次之,HDPE最小;当HDPE上下都铺无纺布时,界面的摩擦系数最小。     

Laboratory and numerical modeling of strip footing on geotextile-reinforced sand with cement-treated interface

This paper presents the results of a laboratory and numerical study on the effects of cement treatment of the interface between geotextile and sand on the bearing capacity of a foundation built on geotextile-reinforced sand. The bearing capacity of a 25 cm × 7.5 cm strip footing on a 90 cm × 25 cm × 30 cm sand box reinforced using a single-layer reinforcement of different lengths including, 20, 30, 45, 60, 75 and 90 cm, was studied in a laboratory. A cement-treated zone was created on the geotextile to improve the friction and adhesion of the interface zone. Tests were also conducted on reinforced soil without a cement-treated zone and the results were compared. A finite element model was calibrated and used for further studies. The results of the laboratory tests indicated that cement treatment of the interface between the geotextile and sand increases the bearing capacity of the foundation by 6%–17%, depending on the length of the reinforcement. The effectiveness of the cement-treated interface on improving of the bearing capacity is more evident with shorter-length reinforcements. For a certain bearing capacity, the required length of the reinforcement was reduced by approximately 40% when the interface zone of the sand and reinforcement was cement-treated. The effect of the cement-treated zone on the bearing capacity was more evident in low settlement levels, and decreased as the length of the reinforcement increased.     

Experimental study on the behavior of eccentrically loaded circular footing model resting on reinforced sand

In this study, an experimental investigation has been conducted on a circular footing model subjected to eccentric load resting on the geonet-reinforced sand. To this end, five series of tests were carried out in order to evaluate the effect of reinforcement dimension and eccentricity on the bearing capacity, settlement, and rotation of the footing. Results show that the bearing capacity ratio (BCR) is in direct relationship with eccentricity and the impact of soil reinforcement at low settlements is much more significant in the case of eccentric loading. Additionally, the bearing capacity interaction diagram and variation in the position of rotation line at different load levels for reinforced and unreinforced conditions are presented.     

砂土地基承载力离心模型试验中的粒径效应研究

通过对原型的模拟结果进行总结,得出对于承载力试验只要离心试验中的模型基础宽度与重力场试验中的不同,离心模型试验结果总大于重力场试验结果,即离心试验中总是存在粒径效应问题。随着模型基础宽度与地基材料平均粒径比值的增大,离心试验结果逐渐接近重力场试验结果;当比值足够大时,离心试验结果趋于稳定且与重力场试验结果之差可忽略不计。因此,可以利用模型的模拟的方法,讨论离心模型试验中的粒径效应问题。在一系列砂土地基上条形基础地基承载力的离心模型试验中,使模型基础宽度与地基材料平均粒径的比值在较大范围内变化,即最大比值为最小比值的5倍,且其变化范围的上限达291。基于试验结果导入评价粒径效应对试验结果影响程度的指标,定量地讨论了离心模型试验中的粒径效应问题。本试验结果表明,当模型基础宽度与地基材料平均粒径的比大于233时,粒径效应对试验结果的影响小于1%,基本可忽略不计。     

Effects of Footing Size and Aspect Ratio on the Bearing Capacity of Sand Subjected to Eccentric Loading

The current practice of estimating bearing capacity usually employs the conventional bearing capacity formula originally developed for strip footings under vertical central loading. In order to account for the effect of footing shape and eccentricity and inclination of loads, correction factors are introduced in the formula, which are derived based on a number of small-scale model test observations.This paper describes research on the bearing capacity of rectangular footings on sand subjected to vertical eccentric loading. Two aspects, namely the effects of footing size and of footing shape on the bearing capacity and deformation characteristics, are focused on. A series of loading tests was conducted in a centrifuge on rectangular footings with aspect ratios from 1 to 5, at two different centrifugal accelerations. In addition, finite element analyses were performed in which factors influencing the angle of shear resistance including stress level dependency, anisotropy and coefficient of intermediate principal stress, were taken into account.It was found that the shape factor of footing apparently increased with increasing footing width. This indicates that the shape factor used in the current practice underestimates bearing capacity of footings. This was also the case for failure locus in the M/B-V (moment-vertical) load plane. Normalized failure locus for wider footings with a smaller aspect ratio is considerably larger than that reported in the literature. The stress level dependency of the angle of shear resistance appeared to be responsible for the scale effects of footings on the failure locus.     

Relative density effects on the bearing capacity of unsaturated sand

This study focuses on the impact of relative density on the bearing capacity of unsaturated sand using both theoretical predictions and measurements from physical modeling tests. The theoretical predictions incorporate the effective stress, quantified using the suction stress concept and friction angles obtained from direct shear tests on unsaturated sand specimens at different relative densities and degrees of saturation, into conventional bearing capacity equations. The suction stress values inferred from the failure envelopes were found to match well with values predicted from the soil-water retention curves for sands with different relative densities. Moreover, the bearing capacity values measured in physical modeling experiments involving loading of a circular footing atop unsaturated silty sand layers having different initial degrees of saturation matched well with the predicted bearing capacity values from an effective-stress based model. As expected, the bearing capacity was greater for soils with increasing relative density, but an interesting observation is that a transition from general to local shear failure occurred at a certain combination of relative density and degree of saturation. For the silty sand tested, this transition occurred at a relative density of 0% for degrees of saturation between 4 and 16% and at a relative density of 40% for degrees of saturation between 30 and 90%. General shear failure was always observed at relative densities of 70 and 90%.     

浅基础承载力离心模型试验研究

笔者利用容量为10gt的离心机对砂基上的浅基础进行了较为系统的试验,用以研究基础的尺寸、形状、埋深和砂土相对密度对浅基础承载力、承载力因数（Nr,Nq）、形状因数（ζr,ζq）及破坏型式的影响。笔者提出一种利用浅基础离心模拟试验资料确定浅基础的承载力因数和形状因数的方法。利用此方法确定的44T-4砂的承载力因数,形状因数与已有的各种理论解和试验解进行了对比。研究工作得出了一些有意义的结论。     

Effects of restraining conditions on the bearing capacity of footings near slopes

In evaluating the ultimate bearing capacity (q_u) of a strip footing adjacent to a slope, conventional correction formulas for the effect of load eccentricity may not be applicable because these formulas were developed exclusively for footings situated on horizontal grounds, where loads eccentric to opposite sides of the footing yield identical results for q_u. In this study, loading tests and analyses are conducted on a strip footing placed adjacent to a model slope with various slope angles. The experimental evidence shows that a load eccentric toward the heel of a footing leads to an increase in bearing capacity, whereas the analytical results based on conventional formulas show the opposite trend. To address this discrepancy, an approach is proposed that uses a bearing capacity correction formula for a footing with a setback from the crest of the slope. Results of a comparative study show that the experimental values for bearing capacity factor N_γ(test), with full corrections for load inclination, load eccentricity, and footing setback are comparable to the theoretical solutions. Furthermore, fully corrected values for N_γ(test) for the fixed footing approximately follow the line of the upper boundary; those for the free-rotating footing follow the lower boundary of the theoretical solutions reported in the literature. This discrepancy is due to the different failure mechanisms induced by the restraining conditions of the footing which have yet to be considered in engineering practice.     

Analytical modeling of geogrid reinforced soil foundation

This paper aims at developing analytical solutions for estimating the ultimate bearing capacity of geogrid reinforced soil foundations (RSF) for both sand and silty clay soils. Failure mechanisms for reinforced soil foundations are proposed based on the literature review and the results of experimental study on model footing tests conducted by the authors. New bearing capacity formulas that incorporate the contribution of reinforcements to the increase in bearing capacity are then developed for both reinforced sand and silty clay soil foundations based on the proposed failure mechanisms. The predicted bearing capacity values are compared with the results of laboratory model tests on reinforced sand and silty clay soil. The proposed analytical solutions were also verified by the results of large-scale model tests conducted by the authors for reinforced silty clay and the data reported in the literature. The predicted bearing capacity values from analytical solutions are in good agreement with the test results.     

Modeling of the cyclic behavior of shallow foundations resting on geomesh and grid-anchor reinforced sand

Storage tank foundations with frequent discharges and filling or road embankments under repeated traffic loads are examples of foundations subjected to the cyclic loading with the amplitude well below their allowable bearing capacity. The concern exists for the amount of uniform and non-uniform settlement of such structures. The soil under such foundations may be reinforced with geosynthetics to improve their engineering properties.This paper deals with the effects of using the new generation of reinforcement, grid-anchor, for the purpose of reducing the permanent settlement of these foundations under the influence of proportion of the ultimate load. Unloading-reloading field tests were performed to investigate the behavior of a square footing on the sand reinforced with this system under such loads. The effects of footing size and reinforcement types on the cyclic behavior of the reinforced sand were studied experimentally and numerically by the aid of computer code. The large-scale results show that by using the grid-anchors, the amount of permanent settlement decreases to 30%, as compared with the unreinforced condition. Furthermore, the number of loading cycles reaching the constant dimensionless settlement value decreases to 31%, compared with the unreinforced condition. Another goal of this paper is to present the equations for reinforced soil under cyclic loading to prevent such complicated calculation involved in deformation analysis. According to these equations, calculation of the permanent settlement and the number of load cycles to reach this amount for each foundation with a given size on the geomesh and grid-anchor reinforced sand, without further need to carry out the large-scale test, is supposed to perform easily.     

Bearing Capacity of Shallow Foundations in a Low Gravity Environment

As a basic study for future lunar/planetary explorations and the in-situ resource utilization missions, bearing capacity characteristics of shallow footing systems in a low gravity environment were investigated. A series of model loading tests on a simulated lunar soil (lunar soil simulant) and Toyoura sand were conducted on an aircraft that flew in parabolic paths to generate partial gravity fields. As a result of the model tests, it became clear that bearing characteristics, including the coefficient of subgrade reaction and ultimate bearing capacity of the lunar soil simulant in a low gravity environment is hardly influenced by the gravity levels, while Toyoura sand exhibits a high dependence on gravity. From the observation of the failure mechanisms, it was found that the gravity dependence seems to correlate well with soil compressibility. To rationally explain the dependence of ultimate bearing capacity on gravity, theoretical evaluations were attempted in the framework of the upper bound method. The proposed calculation method not only makes it possible to correlate quantitatively the failure mode with dependence on gravity, but also may allow us to predict the ultimate bearing capacity in the lunar surface environment.     

无黏性土斜坡地基承载力模型试验研究

采用缩尺模型试验对砂土斜坡地基的土压力分布、变形机制、破坏模式进行探索,并研究了斜坡坡角、基础尺寸、相对密度、基础形状对斜坡地基破坏形态及极限承载力的影响。结果表明：斜坡地基的破坏模式与Choudhury提出的破坏模式相近,破坏区域由不对称楔体、辐射向剪切区、被动楔体组成。斜坡地基的破坏区域长度随斜坡坡角、基础尺寸的增大而增大,但不随相对密度的变化而变化;而斜坡地基的极限承载力随斜坡坡角的增大而减小,随基础宽度、相对密度的增大而增大。对相同尺寸的基础而言,方形基础下的地基极限承载力和破坏区域长度均大于圆形基础。试验研究成果对斜坡地基变形特征、破坏形态和斜坡地基承载力影响因素的探究具有一定理论参考价值。     

筋土界面变形破坏模式细观试验研究

利用自制的模型试验设备,在平潭标准砂中对玻纤网布和土工格栅进行了一系列拉拔试验,应用数字照相变形量测技术,从细观角度研究土工合成材料(片状的和格栅状的)接触界面的变形模式,得出接触面的形式和厚度,还研究了土工格栅横肋作用下土体的破坏模式。接触界面区域在传统意义上并不被认为是剪切带,本文模型试验的结果表明二者在本质上是一致的。在土工格栅拉拔试验中,土工格栅的位移逐步从前部向后部发挥,出现上下两条接触面区域,其厚度在密砂和松砂拉拔试验中相当于5倍和7.5倍平均颗粒直径;在玻纤网布拉拔试验中,只出现一条接触面,其厚度为7.5倍平均颗粒直径;随着位移的增加,格栅横肋在土中的最大剪应变集中区域呈“x”形,但并不对称。本文揭示了加筋土的宏细观力学机理,研究内容、方法和主要结论可为类似的研究提供参考,并从细观机理上为接触界面的研究提供新的认识和理解。     

Dynamic friction and the seismic performance of geosynthetic interfaces

The stability of geotechnical structures which contain geosynthetic interfaces is closely linked to the shear strength between the geosynthetics themselves, both in static and dynamic conditions. Static friction is the maximum interface shear strength mobilised before displacement, whereas dynamic friction is related to the kinematics of the displacement itself. In polymer materials, dynamic friction may be widely variable, depending on the type, geometry and integrity of the surfaces in contact, as well as on the intensity and time-history of the seismic signal. This means that predicting interface shear strength is not simple. This paper focuses on the evaluation of dynamic interface shear strength between geosynthetics, using the results of both inclined plane tests and shaking table tests; this latter test also provided a means to analyse interface behaviour under the conditions of real seismic records. To this purpose, two common geosynthetic interfaces, which exhibit different behaviour under dynamic loading, were tested. One interface was a smooth HDPE geomembrane in contact with a nonwoven polypropylene geotextile, while the second was a textured HDPE geomembrane in contact with a different type of nonwoven polypropylene geotextile.The test results shows that dynamic friction mobilised during seismic events depends on the relative speed according to the same law outlined by the free sliding tests and by the shaking table tests carried out with sinusoidal base motions. Moreover, for the two different types of studied interfaces dynamic friction may be greater, lesser or equal to the static friction and the assumption of a constant value of dynamic friction does not lead to an accurate prediction of the seismic displacements under various earthquakes.     

Laboratory investigation of bearing capacity behaviour of strip footing on reinforced flyash slope

The paper presents the results of laboratory model tests on bearing capacity behaviour of a strip footing resting on the top of a geogrid reinforced flyash slope. A series of model footing tests covering a wide range of boundary conditions, including unreinforced cases were conducted by varying parameters such as location and depth of embedment of single geogrid layer, number of geogrid layers, location of footing relative to the slope crest, slope angles and width of footing. The results of the investigation indicate that both the pressure–settlement behaviour and the ultimate bearing capacity of footing resting on the top of a flyash slope can be enhanced by the presence of reinforcing layers. However the efficiency of flyash geogrid system increases with the increasing number of geogrid layers and edge distance of footing from the slope. Based on experimental results critical values of geogrid parameters for maximum reinforcing effects are established. Experimental results obtained from a series of model tests have been presented and discussed in the paper.     

砂土地基上圆形浅基础三维破坏包络面的理论研究

对砂土地基上圆形浅基础在竖向荷载V 、水平荷载 H 及力矩 M 复合加载条件下的承载力进行了系统的三维有限元分析。在分析中,砂土假定为纯摩擦材料,遵循基于 Mohr-Coulomb 破坏准则的理想弹塑性本构关系。首先,对圆形浅基础的竖向承载力进行了有限元计算,并与滑移线解法进行了对比,两种方法所得结果比较吻合。进而探讨了砂土内摩擦角对于基础在 V-H 、V-M荷载平面与V-H-M 三维荷载空间内的破坏包络轨迹的影响。计算结果表明,与不排水情况下软黏土地基上基础破坏包络面相比,砂土地基上圆形浅基础的破坏包络面形状有较大差异,但V-H 和V-M 平面内的破坏包络面形状仍具有较好的归一化特性。基于有限元计算结果,建立了圆形浅基础在V-H-M 三维荷载空间内的破坏包络面方程,该方程可用来合理评价复合加载条件下砂土地基上圆形浅基础的整体稳定性。     

土工格室于砂土之承载能力及动态特性试验研究

在沙漠地带中,在路基材料缺乏的状况下,利用土工格室与现场砂土可有效提高沙漠道路的承载力,并能达到经济效益与路基的设计要求。为了解土工格室于砂土路基的加筋行为,分别进行了静态承载板载重试验与动态载重试验,探讨土工格室加筋后的承载能力与动态特性。与未加筋砂土比较,土工格室发挥良好之加筋效果,其能提高砂土的承载能力;于静态承载试验中,承载能力随着土工格室高度增加而提升,并有效减少沉降量之产生。在动态试验指出,土工格室加筋于砂土受动态反复作用下,增加土工格室高度,并不相对能提升加筋的承载成效。以动态载重试验反应公路的反复载重条件,对于沙漠公路设计使用土工格室加固路基时,得以选择具有经济效益的优化方案。     

进入持力层不同深度对单桩承载力的影响

本文讨论了桩端进入持力层深度对单桩极限承载力的影响,提出了持力层的临界深度和临界厚度的参考数据。这是根据桩端进入粉砂持力层的两根桩（按进入深度变化分11个阶段进行试验）及进入硬粘土持力层的两根桩（按进入深度分12个阶段进行试验）的静载荷试验资料。用静力触探法估算了相应的单桩极限承载力及其侧摩阻力和端阻力。     

低超载下条带式带齿加筋界面特性

在提出立体加筋的基础上,对条带式带齿加筋砂土进行了低超载下大量的拉拔试验来研究筋土的界面特性,分析了带齿筋的条带式加筋对拉拔性能的影响,探讨了不同上层覆压下筋条的拉拔力与水平位移的关系以及似摩擦系数f*的变化规律。在试验成果分析的基础上,分析了条带式带齿加筋与砂土的相互作用机理,建立了条带式带齿加筋砂土的拉拔力模型。并将试验结果与理论值比较,二者基本吻合。     

Load-settlement response of shallow square footings on geogrid-reinforced sand under cyclic loading

To study the settlement and dynamic response characteristics of shallow square footings on geogrid-reinforced sand under cyclic loading, 7 sets of large scale laboratory tests are performed on a 0.5?m wide square footing resting on unreinforced and geogrid reinforced sand contained in a 3?m?×?1.6?m?×?2?m (length?×?width?×?height) steel tank. Different reinforcing schemes are considered in the tests: one layer of reinforcement at the depth of 0.3B, 0.6B and 0.9B, where B is the width of the footing; two and three layers of reinforcement at the depth and spacing both at 0.3B. In one of the two double layered reinforcing systems, the reinforcements are wrapped around at the ends. The footings are loaded to 160?kPa under static loading before applying cyclic loading. The cyclic loadings are applied at 40?kPa amplitude increments. Each loading stage lasts for 10?min at the frequency of 2?Hz, or until failure, whichever occurs first. The settlement of the footing, strain in the reinforcement and acceleration rate in the soil have been monitored during the tests. The results showed that the ultimate bearing capacity of the footings was affected by the number and layout of the reinforcements, and the increment of bearing capacity does not always increase with the number of reinforcement layers. The layout of the reinforcement layers affected the failure mechanisms of the footings. Including more layers of reinforcement could greatly reduce the dynamic response of the foundations under cyclic loading. In terms of bearing capacity improvement, including one layer of reinforcement at the depth of 0.6B was the optimum based on the test results. It is found that fracture of geogrid could occur under cyclic loading if the reinforcement is too shallow, i.e. for the cases with the first layer of reinforcement at 0.3B depth.