Determination of the Adequate Thickness of Granular Subbase Beneath Foundations


Where the native soils have poor structural qualities or are expansive, the soilinvestigation report may recommend importation of soils better suited to providing asubbase for structures. This requires considering two soil layers in bearing capacitycalculations.Calculation of the ultimate bearing capacity of shallow footing on a two layered systemof soil depends on the pattern of the failure surface that develops below the footing. For aweak clay layer overlaid by a top dense sand layer, previous studies assumed that thefailure surface is a punching shear failure through the upper sand layer and Prandtl'sfailure mode in the bottom weak clay layer.In this paper, the bearing capacity of subbase layer underneath by a soft clay layer isinvestigated. The properties of the subbase material are measured in the laboratory.Design charts were obtained which can be used to select the suitable thickness of thesubbase layer for a design allowable bearing capacity.