Soil failure in shallow covers above flexible conduits

Abstract

Field experience has shown that shallow soil covers above large flexible conduits may fail under concentrated live loads. To avoid such failure, an empirical minimum depth of cover of one-sixth of the span of the conduit is required by the design codes, irrespective of the shape of the conduit.The present paper examines the stability of the soil cover using a finite-element analysis in which the soil stresses are calculated in the upper zone of the mesh. Failure in a soil element is determined using Mohr–Coulomb criteria. A stress transfer technique is used to distribute the stresses exceeding the failure limit between neighbouring elements. Failure propagation is detected when the live load reaches a critical limit. Analytical results are in good agreement with laboratory test results.A parametric study shows that the stability of soil cover is governed by height of cover, conduit shape and size, and the eccentricity of the live axle loads. It also shows that the present code requirements are not adequate for horizontal ellipses, and are too conservative for vertical ellipses.