Interaction between road structure and subsoil

The mechanical behavior of the road pavements under traffic loading depends on the properties of the subgrade. However, these properties and thus the response of the soil are equally dependent on the load itself. Due to this interaction, the mechanical behavior of the road pavements and that of the soil cannot be decoupled from each other. Since the soil behavior is strongly nonlinear and inelastic, the deformations of the subsoil can add up to non-negligible amounts over longer periods. This accumulation depends on the cyclic change of the state variables. These include the effective stresses, which change not only as a result of the traffic load but also due to fluctuations in the pore water pressures. The soil structure, the number of pores, the degree of saturation and the temperature are

further parameters that influence the behavior of the subgrade. Consequently, the stiffness and the strength of the soil are not material parameters, but state-dependent quantities that can only be realistically recorded with the aid of advanced material models. However, the influence of the relatively low stress level prevailing under the road pavement on the soil behavior has hardly been researched so far. Subproject A07 will investigate the condition-dependent stiffness of the subgrade and the accumulation processes of the irreversible deformation due to traffic loading and climatic influences. The objective is a permanent evaluation of the soil properties, in particular the state- and direction-dependent stiffness. This is determined with the aid of a suitable constitutive model and the measured state variables of the subgrade.