Hydro-Mechanical Analysis of Crack Initiation in Expansive Soils

Pavements constructed on expansive soil subgrades experience longitudinal cracks due to the desiccation of the subgrade soil from the environmental loads. During the soil desiccation, cracks initiate as a consequence of the soil shrinkage. These cracks can propagate to the overlaying pavement layers. Further, cracks decrease the soil strength and increase its hydraulic conductivity and compressibility. This paper presents a laboratory testing protocol and a simple analytical approach in identifying the crack initiation criteria. Four different laboratory tests have been undertaken to investigate the shrinkage cracks in expansive soils. These tests are suction measurements, free shrinkage test, constrained shrinkage test, and splitting tensile strength test (STT). The constrained shrinkage test was conducted using the restrained ring test (RRT). The soil water characteristic curve (SWCC) in terms of the degree of saturation was predicted by utilizing the gravimetric water content measurements and the soil shrinkage characteristic curve. The suction measurements and the tensile stress measurements from the RRT were tracked from the beginning of the shrinkage test until the crack initiation stage. The results confirm that a crack first initiates when the suction becomes close to the air entry value (AEV). A qualitative comparison between the tensile stress from the RRT and the tensile strength from the STT was carried out. It was found that the tensile stress increases with the soil shrinkage while the tensile strength starts to decrease at a suction stage close to the AEV.