Mitigation of short term rutting by interlocking layer developed around a geogrid-sensitivity analysis

Abstract

This paper presents a numerical investigation of the mitigation of the rutting process developed in the early stages of the utilization of a flexible pavement structure. The analysis is supported by the results of experimental research available in the technical literature. The mitigation of rutting achieved by application of a geogrid is tackled in the framework of sensitivity theory. The fact that the installation of a geogrid results in a decrease of permanent deformations is associated with the interlocking action that develops around a geogrid. It creates the formation of a new, stiffer layer of composite material. The experimental results on the rutting process provide information in terms of accumulated permanent deformations caused by repetitive load of constant magnitude. The constitutive model adopted for analysis is a type of rigid, perfectly plastic material. It is described by a modified Hooke's model that incorporates a modulus of permanent deformation. The functional of permanent displacement is formulated with respect to an unreinforced pavement system with the aid of a suitable adjoint system. The changes of permanent displacements caused by the insertion of a geogrid at the midst of the base layer are described as changes (first variation) of modulus of permanent deformation of the composite layer developed around the known location of the geogrid. Sensitivity theory allows for an extension of the constitutive equation due to the postulate that material characteristics are also considered as variables. The simplifications of the constitutive relationships expressed in variational form result from the fact that variations of permanent displacement are imposed on primary structure in the presence of constant load. The determined final form of the sensitivity equation of permanent displacement due to the changes of modulus of permanent deformation of the composite layer forms the basis for the numerical investigations. They are performed by means of the finite element method (FEM) for each (N)-th load repetition. The results in terms of permanent modulus of deformation of the composite layer surrounding the geogrid for some discrete values of its thickness demonstrate a considerable increase in stiffness of the newly developed layer as compared with an unreinforced pavement.