In situ rainfall infiltration studies at a hillside in Hubei Province, China

Field infiltration tests were conducted at a hillside near the ship lock of the Three Gorges Dam in Hubei Province, China. The test site consists of residual soil and decomposed granite. The infiltration rate is estimated from the in situ tests to be 1.465–2.778×10−6 m s−1, depending on the initial water content. The rate at which the infiltration front moves down through the soil matrix within 2 m of the ground surface is estimated to be ca. 0.26 m day−1 on average. At the end of the in situ tests, the matric suction profiles show that the soil below a depth of 80 cm remained unsaturated, while the zone above was almost fully saturated. This finding was unexpected. The site was excavated after the test to examine the abnormal behaviour of the matric suction profiles in the depth. A relic joint was identified at a depth of 78 cm at an attitude almost parallel to the slope surface. It is surmised that the joint transmitted water laterally and limited further penetration of the wetting front. The water in the zone above the joint appeared to be ‘perched’. This experiment indicates that, to describe thoroughly the infiltration process within a weathered jointed granite profile for slope engineering design purposes, a model based on the assumption of a uniform porous media is inadequate. The model should include the discontinuities. This is challenging since it requires field studies to identify the pattern and distribution of the joints. The implications of the experimental results on slope stability are discussed. The in situ tests provide important information for further studying groundwater seepage under rainfall conditions and a dewatering system design for the slope above the ship lock of the Three Gorges Dam in China.