Bearing capacity of shallow impervious footing in soil under sub-vertical seepage

Seepage in soils changes the effective stresses in the soil masses due to seepage forces. Bearing capacity of shallow foundations in soils with strictly vertical seepage can be determined applying the classic bearing capacity equation using a unit weight taking into account the seepage forces. This strictly vertical seepage can only be possible if the footing is pervious. Impervious foundations change the flow net and therefore the seepage forces. The present paper evaluates the influence of this change on the bearing capacity of shallow foundations. A finite element code with the capability of performing seepage and upper-bound plastic calculations was used. A parametric analysis of geometric (soil height to footing width ratio), hydraulic (average gradient) and mechanical (soil friction angle) parameters is performed and their influence on the bearing capacity and on the failure mechanisms is presented and analyzed. Results show that: (1) the effect on the bearing capacity is greater for soils with lower friction angles; (2) this effect is nonlinear and therefore cannot be described by a change in the soil unit weight only; and (3) for the same average hydraulic gradient, the geometric parameter has little influence on bearing capacity.