Numerical studies of bearing-capacity factor N-gamma

Plane-strain, finite-element, and finite-difference computations of the bearing-capacity factor, N-gamma, have been made for smooth and rough footings, for friction angles, phi, in the 30 degrees-45 degrees range, and dilation angles, psi, in the 0 less than or equal to psi less than or equal to phi range. The finite-element analyses were based on a tangent stiffness approach. The commercially available code, fast Lagrangian analysis of continua (FLAC), was used for the finite-difference analyses. It was found that these methods may be used to obtain reliable N-gamma values provided they are used cautiously. To obtain high-quality solutions, however, it is necessary to review the results critically, and, if necessary, alter the control parameters (e.g., mesh dimensions, number of calculation increments) being used. Computational difficulties were found to increase with increasing phi, and with increasing difference between phi and psi, and the significant of these difficulties with regard to the use of the two computational techniques is discussed here. Curves relating N-gamma to phi are presented, and compared to previously published relations. It is shown that commonly used values of N-gamma, which have generally been based on associated plasticity calculations,are unconservative for real soils, for which psi<phi.