This paper introduces a new alternative for soil-pile-structure interaction analysis, based on established soil-pile interaction theory, which accommodates a wide range of foundation conditions. A description of the theoretical formulation and its implementation into the prevalent SASSI software framework via SC-SASSI (SC Solutions, 2018) is provided in this paper, including examples and usage recommendations. A companion paper (Rangelow et al, 2018) discusses benchmark results (impedance functions) of this implementation derived for various pile foundations.

A key component to solve the soil-structure interaction problems in the frequency domain (such as in the SASSI framework) is the compliance matrix. The traditional formulation for the compliance matrix implemented in SASSI (Lysmer et al., 1981) has proven to be effective for the simulation of shallow foundations, but it is found inadequate for simulation of pile foundations. An alternative formulation for the simulation of pile foundations builds the compliance matrix for the portion of the soil where piles are embedded using Green’s function formulations for ring and disk loads, which are semi-analytical formulations to calculate displacements both outside and inside of a cylindrical core. The Green’s functions are evaluated using the equivalent radius of the pile, considering a disk load distribution at the interaction node located at the pile tip, while ring load distributions are considered at the remaining interaction nodes along the pile shaft.

The Green’s functions approach for computation of the compliance matrix for pile foundations is implemented in SC-SASSI (SC Solutions, 2018), together with pile finite elements (similar to mechanical formulation of beam elements, but accounting for the effect of the excavated soil) to permit the simulation of 3D soil-pile-structure interaction. The implementation has been generalized to supplement (rather than replace) functionality of the traditional SASSI framework via interaction node groups, where the compliance matrix can be calculated using Green’s functions or traditional formulation for different regions of a foundation model corresponding to deep (pile) or shallow foundations, respectively. This implementation allows for analysis of deep foundations, shallow foundations, or a combination thereof in a single model, in addition to modeling single piles, pile groups, and pile configurations with varying pile diameters. Verification and sensitivity analyses using the SC-SASSI pile element implementation have been performed to demonstrate the technical basis, modeling flexibility, best practices, and recommendations for usage.