AN ESTIMATE OF THE INFLUENCE OF SOIL WEIGHT ON BEARING CAPACITY USING LIMIT ANALYSIS

ABSTRACT An estimate of bearing capacity coefficient Nγ for a strip footing is made using the kinematical approach of limit analysis. This approach leads to an upper bound on the true limit load when the calculations of the three terms in the bearing capacity formula are consistent with one collapse mechanism. However, as accepted in other proposals, the estimate of the influence of the soil weight is calculated here separately from the terms dependent on the cohesion and the footing depth (overburden). It is conservative to take the minimum of each term in the bearing capacity formula, rather than minimum of the sum of the three terms. Coefficient Nγ from such calculations becomes increasingly conservative (underestimated) with the increase in the soil cohesion and the footing depth. In view of using the upper-bound approach, such “all-minimum” procedure is prudent and appropriate for design. Results are given for both rough and smooth footings. A substantial influence of the dilatancy angle on Nγ was found, and this is a reason for concern, since the non-associativity of plastic deformation of soils is usually not accounted for in design. Although calculations are numerical in nature, with optimization of the failure mechanism to obtain the best estimate, closed-form approximations are suggested for practical purposes.