Distinct element simulation was performed for direct shear box (DSB) tests on a dense and a loose two-dimensional (2D) sample of 3259 cylinders. Special attention was devoted to the effect that the frictional force between the inside surface of the upper shear box and the sample had on the measured shear strength in the DSB test. Some ways of minimizing this interface frictional force were introduced in the paper. Given that the deformation approximates simple shear within the deforming zone across the sample centre (referred to as the shear zone), a method was proposed to evaluate the overall strains in the DSB test. The numerically simulated data were used to interpret, on a microscopic scale, the angle of internal friction and a 2D stress-dilatancy equation for the mobilized plane in granular material. It was found that the angle of internal friction in granular material is not directly related to the interparticle friction angle (φμ). Instead, it relates to the average interparticle contact angle (θ) on the mobilized plane and the ratio k/f0, representing the degree of the probability distribution of the interparticle contact forces that is biased toward the positive zone of the contact angle θ (along the shear direction), where k is the slope of the linear distribution of the average interparticle contact forces against the interparticle contact angle; and f0 is the average interparticle contact force.

This paper presents a numerical method to simulate the collapse behaviours of unsaturated soils by distinct element method (DEM). The influence of suction in unsaturated soil is reproduced in DEM simulation by introducing an interparticle adhesive force that acts perpendicular to the tangential plane at contact point. A biaxial compression test on an assembly of circular rigid particles is numerically simulated using DEM by incorporating interparticle adhesive forces. The collapse behaviours of unsaturated soils during isotropic compression and biaxial shear are simulated by means of releasing the interparticle adhesive forces from the initially incorporated values to zero at constant stress state. The numerically simulated results are then compared with the results of triaxial compression tests on unsaturated compacted clays and qualitative similarities are obtained. The influences of the mean stress, the stress ratio and the interparticle adhesive force on the collapse behaviours are numerically investigated. The fabric structure and the particle stability of the simulated specimen before and after collapse are also investigated and discussed.

建议了一个非线性弹性剪胀K-G模型，即通过变换应力法将SMP破坏准则简单地引入切线剪切模量Gt中，使其能反映中主应力σ2对土体变形强度的影响, 并通过在切线体积模量Kt中耦合一个反映剪切过程的系数的方法来考虑土的剪胀性。用该模型较好地预测了粘土、砂土的三轴压缩、三轴伸长与平面应变试验结果，并用其进行了挡墙土压力的有限元分析计算, 计算结果与模型试验结果基本吻合。

The main dam at the upper reservoir of Tianhuanping pumped storage power station is a rockfill dam with an asphalt concrete impervious lining on the upstream face. It is constructed on a non-homogeneous fully weathered rock foundation. In this paper, we present the case study on the treatment for this non-homogeneous fully weathered rock dam foundation. The treatment includes the partial excavation of the fully weathered rock in different depths with respect to the dam deformation, and the enlargement of the transition curvature at the junctions of the inclined dam face and the reservoir floor, as well as the reinforcement for the asphalt concrete impervious lining with a layer of polyester mesh at the parts where tensile strains of the lining are large. A three-dimensional (3D) finite element analysis is carried out for the main dam to provide the basis for the proposed compound treatment method. To account for the effect of the intermediate principal stressσ2 on the deformation and strength of soils, a simple constitutive soil model is proposed for the finite element (FE) analysis. So far, this project has operated well for more than 5 years without any maintenance for the main dam’s asphalt concrete impervious lining, illustrating the success of the treatment for the fully weathered rock dam foundation.

An in-situ direct shear test apparatus, in both small and large sizes, and its testing techniques have been developed. This test is performed simply by pulling horizontally a latticed shearing frame, embedded in the ground, with a flexible rope or chain under the application of a constant vertical load on the sample. In the new test, the real normal and shear stresses acting on the shear plane can be exactly measured. A number of the large-sized in-situ direct shear tests have been performed on eight kinds of coarse-grained granular materials including rockfills at different construction sites of embankments. The measured shear strengths approximate those of large-sized triaxial compression tests on samples with parallel grain-size distributions. The smaller version of this new test has been used successfully for testing sands and clays. For clays, artificially produced or in-situ, the measured undrained shear strengths are nearly equal to qu/2 by unconfined compression tests on the same clays. The extreme simplicity and high accuracy of this newly developed test are emphasized.

In unsaturated granular soil, capillary suction produces an adhesive force that acts perpendicular to the tangential plane at each contact point. In this paper, the collapse behavior of unsaturated granular soil during isotropic compression and biaxial shear are numerically simulated using the distinct element method (DEM) by incorporating this adhesive force initially at all contacts and then releasing it continuously to zero. The simulated results are compared with the results of triaxial compression tests on unsaturated compacted clay and qualitative similarities are obtained. Based on the simulation results, the yield stresses under different interparticle adhesive forces are determined and the collapse processes before and after yielding are investigated in detail. Moreover, the effect of initial void ratio and principal stress ratio on collapse is also investigated and discussed. The paper illustrates the capacity of simulating qualitatively the collapse behavior of unsaturated granular soils through the changes of the interparticle adhesive forces produced by the capillary suction.

用铝棒堆积体作为二维粒状体的模型材料进行单剪试验，然后对其用离散单元法进行数值模拟，对于宏观应力应变关系，两者结果基本吻合。基于数据模拟结果，对粒状体的剪切机理，例如滑动面上粒子接点角的分布及变化规律,进行了微观分析。

介绍了一种适用于大颗粒堆石材料的新型现场直剪试验法。该试验法用链条张拉剪切框使试样受剪，克服了剪切框内壁摩擦对剪切面上正应力的影响，从而能精确地测定出试样的抗剪强度。该试验法在日本应用于多个工程，江苏宜兴抽水蓄能电站上库工程中属于国内首次应用，取得了满意的结果。

Matsuoka (1974) proposed a two-dimensional stress-dilatancy equation of granular materials on the mobilized plane through the direct box shear tests on assemblies of aluminum and photoelastic rods, which was expressed as τ/σN = λ(-dεN/dγ)+μ. In this paper, the stress ratio τ/σN and the strain increment ratio -dεN/dγ are approximated to be tan x and tan θ, respectively, on the basis of the numerically simulated results for a simple shear test by DEM. Here, x and θ denote the average angle of the interparticle forces to the normal of the mobilized plane and the average interparticle contact angle on the mobilized plane, respectively. It was found that the difference between x and 0, denoted as δ, varies slightly during shear. The angle δ is related to the average interparticle contact force f0, the slope k of the straight line that characterizes the distribution of the average interparticle contact forces against the contact angle, and the average interparticle contact angles 0 on the mobilized plane. The intercept μ in the stress-dilatancy equation is interpreted as tan δ. The influences of interparticle surface friction, grain shape and confining pressure on the stress-dilatancy relation are examined. It was found that the angle δ is not very sensitive to the interparticle surface friction angle Φμ, except in the lower value of Φμ, and independent of the confining pressure. However, it is affected to some extent by grain shapes. Various results of the newly developed in-situ direct shear tests on various granular soils are presented to support the arguments in this paper.

用离散单元法模拟了粒状体的直剪试验。基于数值模拟结果，提出了修正剪胀试样与剪切盒内壁间的摩擦对剪切强度影响的方法，估算了直剪试验试样的整体应变，并探讨了剪切面上宏观应力-应变量与粒子的微观结构量之间的关系。