Soil–water retention curves and microstructures of undisturbed and compacted Guilin lateritic clay

To investigate the soil–water retention curves of undisturbed and compacted lateritic clay specimens in the full suction range, suction is imposed or measured by using the pressure plate method, filter paper and vapor equilibrium technique with a saturated salt solution. The mercury intrusion porosimetry (MIP) tests were also performed to observe the pore size distribution (PSD) of undisturbed and compacted samples. The test results show that the undisturbed samples have lower gravimetric water content and degree of saturation than the compacted samples at the same density in the suction ranging from 0 kPa to 10 MPa. According to the study on microstructures, the observed phenomenon can be explained by internal cracks of undisturbed samples continuing to develop along with an increase in suction. When the suction is higher than 10 MPa, the soil–water retention curves (SWRCs) coincide with each other for both undisturbed and compacted samples. Undisturbed samples exhibit a unimodal PSD, while unsaturated, compacted samples have a distinct bimodal PSD. The structural degradation of the clay aggregate due to the moisture change leads to the change of the inter-aggregate pores (the intra-aggregate pores remain almost unchanged). For unsaturated, compacted samples with different void ratios, the intra-aggregate PSDs are almost the same, while the inter-aggregate PSD varies with the void ratio. This can be adopted to explain that the wetting branches of the SWRCs are independent of the dry density in the high suction range when the SWRC is expressed by the relation between suction and gravimetric water content. The results of MIP tests on a saturated sample are used to predict the SWRC of the same soil in the main drying.