Development of shear device for large deformation of sand

研制了一种新的砂土剪切试验装置，该装置可用于研究峰值强度前后和剩余强度状态下砂土颗粒的运动，试验装置内装有尺寸为5^<**>5^<**> 10 cm ^3的试样。将真空压力引入试样中以产生有效围压。这种措施使压力单元不必要，并使显微镜直接接近样品。通过用透明的亚克力板覆盖样品的一个侧面，使颗粒运动的显微镜观察更加容易。本研究所用的显微镜具有数字型CCD传感器，这使得通过计算机分析观察数据成为可能。首先，峰值强度之前的颗粒运动包括颗粒落入相邻的大空隙中。这种现象在样品中或多或少均匀地发生。相反，在峰值强度之后，晶粒的剪切运动集中在通常被称为剪切带的窄域中。剪切带的宽度约为晶粒直径的10倍。剪切带中的剪切运动和旋转运动都很明显。特别是，旋转运动被发现是不规则的，发生在正反两个方向。剪切带内的剪切运动比剪切带外的剪切运动更重要。

A new shear device for sand was developed which can study the granular movement of sand particles prior to and after the peak strength and during the state of residual strength.The shear device houses a specimen which measure 5^<**>5^<**>10cm^3in size. A vacuum pressure is introduced into the specimen in order to produce an effective confining pressure. This measure makes pressure cell unnecessary and enables a microscope to directly approach the specimen. The microscopic observation of granular movement is made further easier by covering one of the side faces of the specimen with a transparent acryl plate. The microscope employed in the present study has CCD transducers of digital type which make it possible to analyze the observed data by a computer.The present research revealed the following issues. Firstly, granular movement prior to the peak strength consists of falling of grains into adjacent large voids. This phenomenon occurs more or less uniformly in the specimen. In contrast, after the peak strength, shear movement of grains is concentrated in a narrow domain which is often called a shear band. The width of the shear band is approximately 10 times the grain's diameter. Both shear and rotational movements are significant in the shear band. In particular, the rotational movement was found to be irregular, occurring in both positive and negative directions. The dilatant motion was also found more important in the shear band than outside of it.