Rationalization of LRFD Method for Safe Bearing Capacity of Shallow Footings to Incorporate the Type of Shear Failure

Abstract

The geotechnical design philosophy is in the transformation stage to implement load and resistance factor design (LRFD). IS 6403 uses working stress method to calculate safe bearing capacity for shallow foundations, while Euro Code 7 and AASHTO use the LRFD method and provide guidelines for proportioning shallow foundations using partial safety factors. Hence, in order to adopt the LRFD design method, it has to be rationalized to have the convincing bearing capacity with respect to IS Code results to incorporate the shear failure criteria. In present work, safe bearing capacity for shallow footing is calculated as per IS Code, Euro Code 7 and AASHTO by taking the variations in shear failure criteria, i.e., with and without consideration of shear failure criteria (only for Euro Code design), soil parameters (cohesion, C and angle of internal friction, ϕ), size and depth of footing. Problems are then identified by checking the consistency of results obtained by Euro Code 7 and AASHTO with respect to IS Code results. Further, as a part of rationalization, the recommendation is given that in Euro Code 7 design, the partial safety factors must be applied to bearing capacity factors, instead of applying to soil properties (C, ϕ). Also, net safe load (kN) obtained from AASHTO is on lower side as compared to IS results because it considers higher average FOS than IS Code.