Abstract
Paper deals with designing spread foundation on inhomogeneous subsoil by PN- 81/B-03020, by Meyerhof and Hanna and by procedure often applied in Slovakia. The approach using substitute foundation, the approach using punching shear failure and the approach using shear surface are introduced. Three approaches were applied to design foundation on inhomogeneous subsoil for two cases: stronger soil underlain by weaker soil and vice versa. In case stronger soil underlain by weaker soil, PN-81/B-03020 prescribes to determine size of substitute foundation based on soil types (cohesive or cohesionless), distance from foundation base to the weaker soil top and also width of real foundation. Meyerhof and Hanna prescribe to determine adhesive force and passive force per unit length of the vertical planes crossing foundation sidewall. By Meyerhof and Hanna, adhesive force depends on cohesion of stronger layer and also on ratio between bearing capacity of stronger soil and weaker soil. Meyerhof and Hanna also introduce punching shear coefficient, depending on angle of internal friction of stronger soil and ratio between bearing capacity of stronger soil and weaker soil. In case weak layer underlain by strong layer, PN-81/B-03020 prescribes to ignore bearing capacity of strong layer, Meyerhof and Hanna prescribe to determine bearing capacity of both layers. Meyerhof and Hanna prescribe to take into account distance from foundation base to the stronger soil and also depth of failure surface beneath the foundation in the thick bed of the upper weaker soil layer. The procedure often applied in Slovakia does not differ above mentioned two cases and prescribes for both cases to found probably shear surface, to determine average values of subsoil shear strength parameters and unit weight, based on which a bearing capacity of inhomogeneous subsoil will be determined. The results show that in the case stronger soil underlain by weaker soil, foundation sizes obtained by three approaches are different. Results also show that in case weaker layer underlain by stronger layer, neglecting the stronger layer leads to uneconomical design.
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