Study of Combined Pile Raft Foundations for Heavy Dynamic Equipment

A combined pile raft foundation (CPRF) is a type of foundation which has the advantages of both shallow foundations (e.g., mat foundations) and deep foundations (e.g., pile foundations). The CPRF includes piles, raft (mat), subsoil and, occasionally, a cushion. Unlike traditional foundations, where the load is supported either by the piles or rafts (mats), the CPRF considers the piles and raft to share the load along with the subsoil. The CPRF mostly requires piles to decrease the settlements of structures, therefore improve the performance of the soil in providing foundation support. A load-transfer cushion is occasionally included at the interface between the piles and the mat to help the rigid pile elements and the subsoil to work together. The CPRF system has been used throughout the world for several years to support tall buildings. The ideal soil profile for the CPRF is a combination of stiff clay and dense sand. The CPRF support system is studied for this paper for use in supporting heavy dynamic equipment. Such an installation imposes large dead load and dynamic load requirements on the subgrade soil and has stringent settlement requirements for daily operation. This installation is further complicated by the less-than-optimum sites which are common for industrial construction. Since the dynamic characteristics and shear capacity of pure pile installations are limited, the combination of a mat with piles should be considered in order to provide the needed load support and stiffness. In this study, the heavy dynamic equipment under consideration is a steam gas turbine (STG). An STG requires a large, thick concrete table-top, columns and a mat. The settlement and dynamic induced movements during operation are much more severe than typical high-rise buildings. Aside from the settlement improvement of CPRF typical for high-rise buildings, the heavy dynamic equipment performance can be substantially improved if the subsoil is considered in the design, as compared with a design which has only a pure pile cap foundation. The use of partial piling in the foundation provides soil improvement to increase dynamic load capacity and performance improvement. Modeling of the complex soil-structure interaction is required for a reliable design. In this paper, SAP2000 is used to model the dynamic performance of the foundation considering both operating static and design seismic loads. Soil conditions are included and modeled as properties of links in SAP2000 to search for the most favorable characteristics. Where appropriate, the results are checked for strain capability with the soil through a simple, iterative process. This paper also considers a soil improvement method which uses vertical reinforcement (such as drilled or driven piles with no connection to the foundation mat at the top) as rigid inclusions and transfer of mat loads to the deep foundation. The dynamic soil interaction analysis required for a CPRF foundation is often ignored by geotechnical consultants as a soil improvement option because it requires significant structural engineering input. The analysis presented here can help to understand the dynamic soil-structure interactions to determine a more realistic result.