ABSTRACT

In this paper, a digital image technique is used to measure the displacements of sand particles at the soil-steel interface in friction tests. The friction tests are used to study the frictional characteristics between cohesionless soils and mild steel and are carried out using a shear box type apparatus. In order to observe the particle displacements, the front side of the shear box is made of a transparent glass plate and photos can thus be taken continuously by a CCD camera with a magnification of x32 through this observation window. These continuous photos are analyzed by image processing to track the movements and the rotations of soil particles. It is well known that the frictional behavior is influenced by the roughness of the surfaces. Therefore, three kinds of roughness for the steel surface are employed in the tests, i.e. very rough (R max ≥ 50 μm), medium (R max = 9.3 μm) and very smooth (R max = 2.4 μm). The normal stress is kept constant at 100 kPa during the tests. The soil specimen is Toyoura sand with a relative density of Dr ≒ 90%. The test results show that: (1) The digital image technique is effective in capturing the movements and rotations of soil particles near the steel surface. (2) In the case of a very rough steel surface, the soil particles move very actively, except those adjacent to the frictional boundary. (3) In the case of a very smooth steel surface, the rotation of particle near the interface is limited. (4) The volume change of the soil specimen depends on the rotation of soil particles and very little volume change occurs in the case of a very smooth steel surface.