A Practical LRFD Design Method for Deep Foundations Using Side Friction and End Bearing

One of the uncertainty components in Load and Resistance Factor Design (LRFD) stems from spatial variability in design parameters, such as local soil / rock strength or modulus. A solution is presented for uncertainty propagation of effective rock mass modulus through O’Neill’s (non-linear) equations for drilled shaft settlement in intermediate geomaterials. Associated tip resistance is combined with side friction resistance (fully mobilized) of one or more geological layers to form total ultimate shaft resistance with respective uncertainty. Based on this, a graphical iteration chart (existing for side friction in a single layer without end bearing) is generalized to find shaft length as a function of given reliability (probability of failure), factored design load and site conditions. Results are demonstrated by a practical example.