Air entrapment and differential swelling as factors in the mellowing of molded soil during rapid wetting

The two principal mechanisms thought to be responsible for the mellowing (or weakening) of moulded soils during rapid wetting are air entrapment and differential swelling. These were investigated in order to obtain a quantitative estimate of the effect each mechanism has on the tensile strength of soils. Rapid wetting experiments were conducted on dry soil discs which had been prepared from moulded moist soil. The discs were wetted on sintered-glass funnels using two different wetting fluids at three different ambient air pressures. Discs were dried again and crushed between flat parallel plates to determine their tensile strengths. For the two soils used in this study, it was found that air entrapment on its own did not generate sufficient stress to induce mellowing, and furthermore that differential swelling on its own also had a limited effect. When the two processes acted simultaneously, however, a marked reduction in the tensile strength of between 35% and 47% resulted.