Pore Structure Evolution Of Nanjing Sand In Continuous Dehydration Condition

The pore structure is an intrinsic index that reflects the change of soil physical and mechanical properties.This article uses a micro-CT with a resolution of 6.5?m to scan the Nanjing sand sample under continuous dehydration condition and control the moisture content in a graded manner.We selected dry sand sample and samples with mass moisture content of 30%,25%,20%,15%,10%,0%,respectively,to obtain micro-CT scan images of soil samples under different water content conditions.We propose a dynamic threshold algorithm based on gray level clustering,on the basis of which we automatically obtain a binary image of the scanning slice sequence.We extract the internal typical slice as the research object and analyze the formation reason and development process of the closed bubble and annular crack.The typical REV is selected as the research object to generate the pore network model and analyze the pore structure parameters.We obtain the pore structure evolution trend and characteristics of Nanjing silty-fine sand under continuous dehydration,and the following conclusions are drawn:(1)Adding water will reshape the pore structure while the impact of losing water is relatively small.(2)Under the effect of matrix suction,adding water will form closed bubbles inside the sand body.When the moisture content dropped from 30%to 10%,the closed bubbles inside the sample are connected,forming a ring-shaped crack of 8mm.(3)Unsaturated Nanjing sandy soil samples have three kinds of morphological pores.The first one is a fissure-sized annular fracture with the largest scale,about 8mm in length and about 150 ?m in width.The second type is a circular bubble with a diameter of about 500 ?m,and the last one is an ordinary pore with a width of 70 ?m which lies between particles.(4)In the process of adding water,the porosity and coordination number of the sample decrease,the pore volume decreases,the pore radius decreases,the number of pores increases,the shape factor increases,and the fractal dimension does not change significantly;in the process of continuous dehydration,the porosity and coordination number of the sample gradually increase,the pore volume and pore radius increase gradually,the number of pores decreases,the shape factor decreases,and the fractal dimension fluctuates slightly.(5)When the mass moisture content drops to about 10%,the pore structure of the sand samples tends to be stable,and after the water content decreases,the pore structure remains basically stable.