On the Dynamics of Porous Media - Application to Road and Railway Structures

Doktorsavhandling, 2010

Load-bearing road and railway structures normally consist of layers of particulate porous materials, often with bitumen-based binders. The layered system is subjected to dynamic loading from traffic, which inevitably causes material deterioration that enhances permanent surface deformation. For example, internal erosion of fines from the solid skeleton is brought about by high seepage rates in the open pore system. The interplay between deformation, pore-water flow and material degradation is a highly complex problem that poses significant challenges from both the modeling and computational viewpoints. The thesis is, therefore, concerned also with means of reducing the computational cost without jeopardizing the predictive capability. The so-called Porous Media Theory (a mixture theory with phase volume fractions) is adopted as the modeling paradigm. In order to model the coupling between deformation and pore-water flow in the presence of internal erosion, a triphasic erosion system is considered in terms of the solid skeleton and an ideal mixture of pore-water and eroded fines (the abrasive phase). Small strain kinematics is assumed, and the pertinent model is obtained from consistent linearization of finite strain kinematics. Linear elastic response is assumed in the absence of erosion (for fixed porosity); however, strong nonlinearity in stiffness and permeability arise due to the finite change of porosity from erosion. The developed simulation tool is used to analyze a simplified road structure subjected to dynamic loading. As part of the effort to reduce the model complexity and computational cost, different simplifying approximations for the relative fluid acceleration are assessed. From the numerical examples, it is concluded that the convective part of the relative fluid acceleration may be neglected without significant loss of accuracy; however, simply using the ”added mass” approximation is normally unacceptable. A novel space-variational format with reduced number of global fields was developed based on the time-discretized balance equations. For the adopted triphasic system, three global fields (displacement, pore-pressure and abrasive volume fraction) and two local fields (seepage velocity and total porosity) constitute the coupled problem. The local fields can be eliminated similarly to internal variables in constitutive models. A comparison with the ”classical” format carried out for a biphasic model (without erosion) showed that the novel format competes well in terms of convergence behavior.