AN EVALUATION OF THE EFFECT OF GEOFABRICS ON STRESSES AND DISPLACEMENTS IN BURIED CULVERTS

Abstract

The results of a study to evaluate the behavior of a flexible culvert with the inclusion of a goefabric above the culvert are presented. In the soil-fabric-culvert system the geofabric becomes an interactive stress-carrying component of the system. Insight into the phenomenon of the stress-attenuation due to the inclusion of a semimechanical reinforcement, like geofabric, is also obtained from this research. The numerical technique of the finite element method is used for the analysis of the soil-fabric-culvert system. In the finite element model, two-dimensional triangular and quadrilateral elements having nonlinear, stress-dependent material properties are used for representing the soil. Beam elements are used to model the culvert, no-compression bar elements are used for the fabric, and two-dimensional interface elements for the contact surfaces between the soil and fabric. Incremental construction sequence and approximate nonlinear geometry are adapted in the analysis. Because of the relatively recent usage of fabrics for engineering purposes, no information regarding their employment as an inclusion in a soil-culvert system is available in the open literature. However, a review of the literature is conducted to bring out the current state of understanding of the behavior of soil-culvert systems without the presence of a fabric. The classical design concepts which reflect the development of the design methodology for flexible culverts are reviewed. The phenomenological concepts of soil-culvert interaction, particularly those of arching in the soil above the culvert and buckling of the soil-surrounded culvert wall are discussed. The burial depths of D/2 and D are considered for the horizontal configuration of the fabric when D = diameter of culvert. A burial depth of 2D is considered for the inclined configuration of the fabric. The following surface concentrated loading conditions are considered for the horizontal configuration of the fabric. Loads of 10 kip are placed symmetrically at S/D = 1/4, 1/2, 1, or 2 and 20 kip load at S/D = 0 (S = distance from vertical centerline of culvert). For the inclined configuration of the fabric above culvert, 50 kip surface concentrated loads are placed at S/D = 0, 1/4, and 1/2 simultaneously. This study shows that the fabric alters the stresses in the soil-fabric-culvert system by two mechanisms: the fabric can carry part of the load in tension and/or it can distribute the load more uniformly over a wider area. Under most practical conditions, the inclusion of fabric causes a significant reduction in the magnitudes of the following culvert design parameters: maximum axial force, maximum moment, vertical crown deflection, and horizontal springline deflection. The presence of fabric is more effective in attenuating the culvert design parameters for the location of surface loads within the horizontal projection of the culvert and for shallow depths of soil cover.