Wetting of unsaturated soil reduces the suction in the soil, the pore pressure approaches the pore air pressure and the effective stress is reduced. Because of this, increasing moisture is associated with decreases in shear strength, stiffness and resistance to plastic deformation in all soils and aggregates and we can observe a decrease in bearing capacity and lower moduli of elasticity and increases in de — formability under the same applied loading.
This influence of moisture on the bearing capacity of soils can be easily observed in the simple CBR test. This test measures the resistance of the compacted soil to the penetration of a piston, to evaluate its bearing capacity. CBR values for three typical soils related to water content are presented in Fig. 10.8. The sensitivity to moisture variations is particularly important for the silty sand (a).
Decrease of bearing capacity and of unconfined compression strength of clay with increasing water content can also be observed in Fig. 10.9.
The CBR test gives a good indication of the moisture sensitivity of subgrade soils, but its results are only qualitative. The influence of moisture on the resilient modulus and resistance to permanent deformations of soils and unbound granular materials can be studied using repeated load triaxial tests.
