Distinct element simulations for the direct shear box tests on a dense and a loose 2D sample of a mixture of binary diameter cylinders were performed. Special attention is focused on the friction between the internal surface of the shear box and the sample. In the conventional direct shear test, where the up/downward movement of the upper shear box is restrained and the lower shear box moves horizontally, the frictional force that acts on the sample at the internal surface of the upper shear box is downward for the dense sample and upward for the loose sample, causing the real shear strength to be larger for the dense sample and smaller for the loose sample than the one calculated from the externally applied normal force. Two possible improvements for the conventional direct shear test to reduce the frictional force of the shear box are introduced: one is free of the upper shear box vertically by adding the low friction Teflon rods and a platen between the upper shear box and the bearing ring that measures the shear force; the other is to pull the upper shear box with a flexible rope or wire. The distinct element method simulation and the experimental results show that both these two improvements can reduce the influences of the interface frictional force on the shear strengths.

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。

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。

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.