Development Of Simple Total-process Direct Shear Test Apparatus And Its Application Analysis

It is of great theoretical significance to study the soil total-process shear characteristics including the residual deformation state for the rational analysis of the mechanical behavior of the soil under large deformation.However,ordinary direct shear apparatus does not have the function of obtaining the soil total-process shear stress-displacement curve.Although the ring shear apparatus can achieve this function,its system operation is complicated,its cost is high,and its volume is large.In this work,a simple total-process direct shear apparatus for soil is developed,which is light in structure,simple in structure,low in cost and convenient in operation.The structural composition,working principle and operation points of this new apparatus are introduced;five soil sample tests including sandy and cohesive soils are performed,and the test results of this new apparatus are verified by comparing with the test results of the ordinary direct shear apparatus;based on the test results,a PFC3D particle flow numerical model was established to simulate and analyze the soil total-process shear characteristics of soil samples,and the whole process shear mechanism of soil particles was revealed from the perspective of micromechanics;in addition,the suitability of this apparatus was analyzed.The main contents and conclusions are summarized as follows:(1)The space occupied,net weight,and interior geometric dimensions of the upper and lower shear boxes of the new apparatus are 100×20×40 cm,30 kg,10×10×10 cm,and 50×10×11cm.The apparatus is mainly composed of four parts:vertical loading system,lateral force application system,shear box and measuring system.The main operation process of the apparatus includes six aspects:eliminating system friction,loading samples into boxes,vertically loading weights,zeroing the measuring system,performing shear tests and checking the state of samples.(2)The basic working principles of the new shear apparatus are as follows:Turn the hand crank to drive the thrust bearing to rotate to generate a horizontal thrust,and push the shearing upper box(with pressure load on the top surface)to slide on the upper surface of the lower box to generate shearing;the bottom surface of the upper box is completely located in the range of the lower box sample,and the test shear area,which is fixed during the shearing process,is the bottom area of the sample in the upper box,and the lower box has sufficient length.The soil sample can have a large enough shear displacement to obtain the displacement curve of the pre-peak stage and the residual stage.(3)The new apparatus can measure the shear stress-displacement curve of the sand and cohesive soil in the soil total-process,and the curve is in a single peak mode.During the shear process,when the shear stress reaches the peak shear strength,it gradually decreases until it becomes stable.When the clay content in the soil sample increased,the peak shear strength and residual shear strength both decreased,and the difference between them increased.The error of the peak internal friction angle measured by this new apparatus and the ordinary direct shear new apparatus is about 2%-7%,and the maximum error of the peak cohesion force is about 0.8 kPa.(4)The numerical simulation results of particle flow discrete element method show that during the whole shear process,the strong contact force chain is preferentially deflected from the left side of the upper box and the right side of the lower box,which means that the stress concentration effect here is significant.However,it lags behind at the central axis(uppser box)of the sample.The soil particles near the shear zone are directly affected by the shearing action,and the rearrangement of the soil particle structure occurs first,causing the evolution of the inter-partricle force chain.The evolution of the inter-particle force chain is mainly concentrated near the shear plane.The formation and evolution of the force chain in the shear zone with a certain thickness is the internal reason for the shear resistance and its development and change in the shearing process of soil samples.(5)The maximum particle size of the sample particles suitable for this new apparatus should not exceed 20mm,and generally should be less than 10mm.It is suitable for direct shear test of sandy soil and cohesive soil as well as fine gravels;it can generally implement a minimum shear rate of 0.6mm/min;it is suitable for fast shear and consolidation fast shear tests,but is generally not suitable for soils with large water content;it is suitable for studying the soil total-process shear characteristics under low stress level(below 135kPa),and is generally suitable for soil samples with a depth of less than 6m from the surface.The research results of this paper,including the simple total-process direct shear apparatus,soil total-process shear characteristics of soil samples and their meso-mechanical mechanism,can provide a powerful test tool and theoretical reference for engineering applications,which has important scientific significance and application value.