Soil-Structure Interaction — a Case Study

The effect of soil-structure interaction has often been neglected in seismic design and retrofit of structures in the past. For new construction, foundations are often not included as part of the primary lateral force resisting system. With pile supported structures, the lateral capacity is determined using an iterative procedure assuming a linear elastic response. Discounting effects of soil-structure interaction, such as increased energy dissipation and period shift, has lead to more massive foundations than may be necessary. For both new design and retrofit of existing structures, accounting for soil structure interaction can significantly reduce construction efforts and cost. With a greater knowledge of soil-structure interaction, the seismic behavior of structures can be more clearly understood. With a greater knowledge of the behavior of the combined structure and substructure systems, the design or retrofit may then be tailored to meet project specific seismic performance objectives. Accounting for soil structure interaction is important for design of new structures as well as providing seismic retrofit of existing structures. This paper describes a methodology used to analyze soil structure interaction for a seismic retrofit design. The retrofit uses calculated energy dissipation through soil structure interaction as an integral part of the seismic force resisting system. Seismicity, soil profile, and pile length vary along the entire length of the pipeline undergoing retrofit. To describe the methodology, pile capacities are established, soil springs developed and a combination of pushover, time history, and response spectrum analyses used to describe the expected behavior under anticipated seismic loading. The structure that serves as the basis of this paper is an existing large diameter above grade pipeline. Pile supported pipe bents are spaced at approximately 60 feet (18.3 meters) on center and massive pile supported temperature anchors are located at approximately 1000 foot (305 meter) intervals. An expansion joint is located midway between the temperature anchors. Design data was obtained from as-built drawings. Each pile group consists of at least four 16-inch square pre-cast concrete piles, with each pile driven on a 3 vertical to 1 horizontal batter, in directions 30 degrees from the perpendicular to the pipeline. In addition, there are 19 pile groups located at bends in the pipeline and road crossings and 44 pile groups located at temperature anchors. Each of these are supported on groups of 10 to 12 pre-stressed concrete piles. The pipeline is located in a moderate seismic zone. There are three elements to consider in the soil structure interaction study: the physical capacity of the pile as a structural element, the capacity limiting equilibrium of the surrounding soil, and the seismic input. All three components play an important part in establishing the overall structural behavior.