A new approach for estimating the axial capacity of driven piles in sand up to true soil failure

Models and procedures predicting the ultimate capacity of axially loaded piles embedded in sand are numerous. Nevertheless, they lack to fulfill the true definition of “capacity”, which according to them, is tied to a “preset” value of pile settlement. This dissertation introduces a new procedure through which pile top load-displacement response can be evaluated up to “true” soil failure. This procedure takes into account a new model for pile tip behavior in sand. It then employs the load transfer function technique (t-z), after evaluating Ramberg-Osgood fitting parameters, to compute top load-displacement response.; The model for pile tip load-displacement response presented, yields values that compare favorably with Vesic's cavity expansion capacity corresponding to very large displacements (greater than 10–25% of the pile tip diameter) and, simultaneously, with FHWA's lower capacity corresponding to the Nordlund type of failure criteria. The model relies on an envisioned lower bound developing failure mechanism in which there are different zones of soil at different pressure states (Mohr circles) such that the secant drained friction angle varies throughout the developing failure mass. Pile tip displacement is related to the mobilized stress and the associated strain in the soil immediately below the pile tip as per Schmertmann's triangular strain profile for footings in sand. A so-called “modified hyperbolic” stress-strain relationship is used to assess the modulus and, hence, the strain for the stress hardening response (increasing confining pressure) of the sand below the pile tip. From the characterization above, the Ramberg-Osgood load (and, thereby, the unload-reload) fitting parameters for the tip load-displacement response can be assessed. Such fitting parameters are good over the entire pile tip displacement range (≤0.0001 inch to ≥6 inches) and, therefore, can be used in the evaluation of seismic pile foundation stiffness as well as the load-displacement curve carried to ultimate load.; Due to the intense nature of the computations involved, three computer programs were developed. “P-tip” was written employing the model presented for pile tip. A semi-programmable spreadsheet was developed to compute Ramberg-Osgood fitting parameters for the load transfer function. Finally, “TZCURVE” was written to combine outputs for the previous two programs to evaluate pile top load-displacement response.