Sand displacement and nonuniform swelling in sand–bentonite mixture by image processing

This paper presents a method for obtaining the displacement of sand particles in a sand–bentonite mixture (SBM) when saturated with water, based on particle tracking velocimetry (PTV). The raw photographs were first converted into binary images. The sand particles were then detected, and the displacement of the sand particles was obtained by comparing their positions in adjacent images. The swelling strain induced by saturation was also obtained using the proposed PTV method. This method was validated by comparing the result with those obtained using a displacement transducer. Subsequently, a comparative analysis of sand particle displacements was conducted for specimens with varying bentonite content (BC), initial thickness, and water infiltration directions. The experimental results obtained were as follows: (1) For specimens with different BCs, local swelling displacement of sand particles at the top part of the specimen increased with higher BCs; (2) For specimens with various heights (hsp), larger local swelling displacement was generated at lower hsp at the initial state; (3) Local swelling characteristics differed in different water infiltration directions. Top-side infiltration showed a significant downward movement of particles during the first several hours of swelling. An estimation method for the dry density distribution of the specimen was proposed based on PTV data and then verified by slicing dry density and water content measurement results.