Behaviour of unsaturated cohesive soils subjected to cyclic loads

Abstract

Cyclic or alternating load impact provoked e.g. by wind or traffic loads is one of the most important types of soils strains. Beside this mechanical impact, unsaturated soils are sometimes subjected to cyclic loads as a result of changing pore water pressures by infiltration or evapotranspiration. Therefore, it is necessary to distinguish between the mechanical and the hydraulic cyclic load impact on unsaturated soils. In contrast to saturated cohesive soils, the behaviour of unsaturated cohesive soils subjected to mechanical induced cyclic loads is similar to the one of non-cohesive soils. In some cases, the result of the ongoing alternating load effect is not an increase, or more generally said, a significant change of the pore water pressure resp. the matric suction, but rather an increase in the stiffness of the soils skeleton below a critical stress state depending on the number of load cycles. This effect is typically known as the cyclic shakedown in sands, i.e. the decrease of plastic strain increments when approaching the elastic state and is also called the ’ratcheting-effect’, especially for metallic materials. Besides the mentioned number of load cycles, the plastic deformation of cohesive soils depends on the stress state as well as the initial void ratio, the initial degree of saturation, the load frequency and the cyclic load amplitude. Most importantly, the impact of matric suction concerning the ’ratcheting-effect’ is not yet clearly clarified for unsaturated soils. Systematic cyclic triaxial tests and oedometric compression tests are carried out on a remoulded cohesive soil to evaluate the influences on matric suction and the degree of saturation on the plastic deformations. The results of these investigations yield to a constitutive relation based on an elastoplastic two-surface model developed by Li [7].