Nonlinear analysis of reinforced concrete slab on partially softening ground : Ickelinjär analys av armerad betongplatta på delvis uppmjuknande grund

Abstract: Reinforced concrete slabs are a conventional type of foundation that is widely used in residential, commercial and industrial buildings. Most of the slab foundations are constructed directly on the ground without another structural medium in between and some of the slabs are therefore influenced by ground softening. A slab foundation is designed to transfer vertical loads and bridge imperfections in the ground, but excessive soil distortion may destroy the support conditions of the slab and through this influence the stability of the superstructures. The aim of this project is to study how ground softening and its further development influence a typical concrete slab. Furthermore, an approximate analytical method to evaluate the condition of a slab due to ground softening was studied. A practical case has been studied to capture the actual failure behaviours of a reinforced concrete slab, based on a previous project. For the case studied, possible future mining activities close to a high bay warehouse may cause damaging settlement in the underground. The fault that may appear softens the soils underneath the foundation, which may deteriorate and cause collapse of the concrete slab under the warehouse. Two types of failure scenarios were studied; subsoil softening and subsoil collapse. The Finite Element Method (FEM) was used to analyse the behaviour of the slab and the development of the failures in the subsoil. A commercial FEM software package, Abaqus, was used as the analytic tool, with a built-in Concrete Damaged Plasticity model (CDP) that for the concrete material model. Both linear and nonlinear material properties have been used in the analyses with same softening effect of the subsoil. A large number of models were analysed to simulate the development of the fault and capture the failure modes of the slab at different stages. Excessive tensile cracks and vertical deformations were found in both failure scenarios studied. A possible internal stability problem of the warehouse due to this was also found. The nonlinear behaviour of the concrete slab was captured through the failure scenarios before damage. It is shown that the serviceability of the reinforced concrete slab can be influenced by softening ground, and a more realistic description of the possible failures was here obtained based on the nonlinear model compared to previous elastic analyses. Keywords:     Reinforced concrete slab, Nonlinear, Ground softening, Finite Element Method (FEM), Abaqus, Concrete Damaged Plasticity (CDP).