Slaking characteristics of unsaturated granite residual soils based on a modified slaking test method

Slaking is one of the distinct process involved in the structural breakdown that occurs when soils are suddenly immersed in, or placed in contact with, water. The process occurs because the tensile stress of soil skeleton cannot withstand the stresses caused by rapid water uptake. Some instability problems caused by slaking process were found on subway tunnels and engineered slopes excavated in granite residual soils (GRS) in Guangzhou, south China. A serious of experimental laboratory studies were carried out in order to get better understanding about the slaking characteristics of GRS. Unsaturated GRS samples with different initial moisture content and different degree of compaction were made for test using homemade apparatus. We proposed a modified slaking test mothod to obtain slaking curves so as to reflect the actual slaking process on the basis of experimental observation and mechanism analysis as much as possible. The method considerred air escape process during water uptaking which combined the two extremes involved in water uptaking with free escape of displaced air and with no air escape. Subsequently, a modified slaking velocity index based on the the slaking curve was calculated and utilized for further data processing and analysis. We discussed the relationship between two main control factors (fillable porosity of soil and initial matric suction of soil) and slaking velocity as well. The results reveal that it has exponential function relationship for fillable porosity of soil and logarithm function relationship for initial matric suction of soil.