Study On Shear Mechanical Properties And Particle Breakage Effect Of Coral Sand-Steel Plate Contact Surface

With the extensive application of coral sand in island engineering construction,the interaction between coral sand and structure contact surface also exists in practical engineering.Under complex loading conditions in the Marine environment,discontinuous phenomena such as slippage,mismovement,detachment,and tension-closing may occur at the contact surface of coral sand and structures.Therefore,the mechanical characteristics of the contact surface involve nonlinear,large deformation and local discontinuity.In addition,the phenomenon of stress concentration is easy to occur near the contact surface of coral sand and structures,resulting in serious particle breakage,which changes the interaction between pile wall,pile end and coral sand,resulting in pile bearing capacity reduction,building instability and other phenomena.Therefore,a series of ring shear tests were carried out in this paper to study the mechanical properties,particle breakage and particle morphology changes of the coral sand-steel plate contact surface,and the main conclusions were drawn as follows:(1)By controlling the vertical pressure and particle size of coral sand,a ring shear test was conducted on the coral sand steel plate contact surface.It was found that as the vertical pressure increased,the shear displacement corresponding to the peak shear stress of the contact surface increased,the softening characteristics of the contact surface weakened,and the shear shrinkage of the sample increased;When the vertical pressure is low(50-100 k Pa),the peak shear stress of uniformly graded coral sand is relatively small due to the lubrication between particles.When the vertical pressure is high(200-400 k Pa),the peak shear stress of uniformly graded coral sand is relatively high;As the particle size of coral sand increases,the contact surface cohesion at peak strength increases,and the change in contact surface friction angle is relatively small.(2)The ring shear test of coral sand-steel plate contact surface by controlling the roughness of steel plate found that,different from the mechanical properties of the contact surface between coral sand-smooth steel plate,the influence mechanism of mechanical properties of the contact surface of rough steel plate is more complex.The peak shear stress of the contact surface is mainly composed of the binding force between the coral sand and the steel plate surface,as well as the occlusal force and friction force between the coral sand particles near the contact surface.The strain softening phenomenon occurs in the shear stressshear displacement curve and satisfies the exponential hyperbolic model.The vertical displacement-shear displacement curve appears dilatancy under a small vertical pressure(50-200 k Pa),and the cohesion and friction Angle of the contact surface are obviously greater than that of the smooth steel plate.(3)Based on the sieving results of coral sand particles after ring shear test,particle size distribution was obtained,and particle breakage was analyzed with the help of fractal theory.The results show that the coralline sand exhibits obvious self-similarity with the increase of vertical pressure and the change of steel plate roughness during the shearing process,and the PSD curve gradually tends to fractal distribution,and the larger the grain size,the more obvious fractal behavior.The critical vertical pressure of 0.5-1mm and 1-2mm particle size coral sand is 300 k Pa and 200 k Pa respectively under the condition of each rough steel plate,which makes the fractal dimension reach the maximum.With the increase of roughness,the main crushing form of coral sand particles changed from particle grinding to the overall crushing and angular breaking.(4)The Part An 3D observation system was used to scan the coral sand and quartz sand particles with 1-2mm particle size before and after the test,and the grain contour and edge Angle were analyzed by the aspect ratio,flatness and fractal dimension.It was found that after the test,the Angle of grain edge decreased and the shape tended to be round.When the vertical pressure is 400 k Pa,the length-width ratio and flatness of the particles decrease with the increase of roughness,and the particles tend to be more circular.The fractal dimension of morphology decreases first and then increases with the increase of roughness,and the concavity of particle surface is the strongest under the condition of smooth steel plate.