Effects of free iron oxide on water retention behavior of lateritic clay

In order to obtain the soil-water characteristic curve (SWCC) of two types of undisturbed lateritic Guilin specimens with and without free iron oxide in full suction range, two measurement methods are used: the pressure plate method and the vapor equilibrium method with saturated salt solution, and the volumes of specimens are measured during the process. The mercury intrusion porosimetry (MIP) tests are used to investigate the pore-size distributions. The test results show that in the low suction range, the SWCC of undisturbed specimens where the free iron oxide is removed is gentle. Especially in the relationship of suction-saturation, it's still saturated when the suction is 1 MPa. But the SWCC of undisturbed specimens declines obviously. Compared with the undisturbed specimens, the undisturbed specimens where the free iron oxide is removed shrink obviously. The main reason is that the free iron oxide is distributed on the particle surface in the form of film enveloping and bridge, which strengthens the connection and the coat effect between particles, so as to enhance the capability of the soil skeleton to resist deformation. In the high suction range, the water retention and shrinkage characteristics of the two types of undisturbed specimens have little difference, indicating that the absorbed water in the soil is dominant. The undisturbed specimens with and without free iron oxide exhibit a unimodal pore-size distribution, and they are different from the traditional understanding of a double-porosity microstructure. There are mainly intergranular pores with a diameter of 40 nm in compacted samples.