Impact of convergence on slope stability analysis and design

"Failure to converge" during slope stability analysis for complicated slopes with external surcharge or soil nails using the "rigorous" methods are experienced by many engineers. The two main reasons for failure to converge are: (1) the fulfillment of the assumptions for a specific method of analysis; (2) the limitation of the numerical method in evaluating the factor of safety. It will be demonstrated that there are problems where convergence for the "rigorous" methods is not achieved by the conventional iteration analysis, but physically acceptable results may exist for some of these "non-converged" problems, if the failure to converge is due to the second reason as given above. With reference to some of the designs by the engineers in Hong Kong, the authors will also demonstrated that the minimum factors of safety obtained by the current design practice may be over-estimated due to the convergence problem in the stability analysis.