Experimental And Numerical Simulation Study On The Influence Of Sand Shape On Shear Characteristics

The shape of different types of natural sands often varies greatly,and the question about how the particle shape affects the shear instability properties of sands is still not well resolved.Since the shape analysis of particles needs to be handled by image processing,which is easily ignored in engineering practice.Thus,it is the key to solve the above problem by proposing quantitative characterizations of particle shape and establishing its connection with shear properties.In this paper,3 different shapes of quartz sand(Ottawa sand,Fujian sand and White Quartz sand)and calcareous sand are selected as test materials,and the shear instability characteristics of different shapes of sands are investigated by means of microscopic image acquisition and analysis,triaxial test and discrete element numerical simulation,etc.The main conclusions are as follows:(1)For the same quartz sand,the distribution interval of shape parameter roundness R is larger than that of aspect ratio AR and sphericity S,indicating that the shape of quartz sand is mainly controlled by roundness R,which shows the difference of angularity;the median of shape parameters(R,AR and S)all show the pattern of White Quartz sand < Fujian sand <Ottawa sand,indicating that the shape of Ottawa sand is rounded and regular compared with Fujian sand and White Quartz sand.(2)In terms of shear properties,the irregularly shaped sand(White Quartz sand)exhibits strain hardening,while the regularly shaped sand(Ottawa sand)exhibits strain softening;the critical state friction angle increases with decreasing regularity of shape;in addition,the stress path of the regularly shaped sand(Ottawa sand)exhibited a "sawtooth" pattern,and the increase of excess pore pressure and axial strain had a "stepwise" manner.Yet,the curve of sand samples of irregular shape(White Quartz sand)exhibited a smooth trend without sudden change.(3)The numerical simulation results show that irregularly shaped particles have a higher coordination number and contact sliding rate,but a smaller cumulative rotation angle and percentage of strong chain compared to round spherical particles,indicating stronger resistance to rotation.The difference in shear and destabilization characteristics between different shapes may be due to factors such as contact mode and form of motion.(4)The shapes of calcareous sand are very different from quartz sand.A new particle shape descriptor(deviation factor α)was introduced for calcareous sand,which can be used as an effective method to distinguish calcareous sands of different particle shape;the addition of stripshaped particles increases the strain hardening tendency and shear strength of calcareous sand specimens,while the addition of disc particles shows the opposite tendency;the undrained shear strength decreases with the addition of disc-shape calcareous sands(α increases),whereas it increases with the addition of strip-shape calcareous sands(α decreases).Therefore,the influence of particle shape should be properly considered in engineering construction.