Seismic design of a gargantuan underground pump station

Abstract

This paper presents a practical analytical technique that was used for seismic resistant design of a major underground pump station structure. The Swan Island Pump Station, located in the City of Portland, Oregon, USA, is a critical component of the West Side Combined Sewer Overflow (CSO) Tunnel project which will, at its design capacity, pump 833 million liters per day (220 mgd) of effluence uphill through three force mains into existing gravity sewers. The 41.15m(135ft) diameter by 49.38 m (162 ft) deep reinforced concrete underground structure houses an array of pumps and appurtenances and also serves as the foundation for the reinforced concrete Operation and Maintenance Building, which is located directly above at the ground level of the pump station. The structure is located in close proximity to the Willamette River and in seismic zone 3 of the seismic zone map of the United States with a peak ground acceleration of 0.34 g. The structure comprises of a shaft structure, interior walls, five levels of floor slabs and one mezzanine. The perimeter of the shaft structure consists of a 1.22 m (4 ft) thick slurry wall and a cast-in-place interior liner of varying thickness from 0.30 m (1 ft) to 0.91 m (3 ft), which are constructed in stages to provide support for the slurry wall as the excavation of soil inside the slurry wall progresses. The behavior of the pump station structure under seismic ground motions was analyzed using 3-D soil-structure interaction computer model. The analysis was performed using GTSTRUDL finite element computer program. The soil layers were modeled as solid elements and the shaft wall and other reinforced concrete members were either modeled as shell elements or solid elements. The computer model encompasses a square ground area of about nine times of the shaft diameter. The seismic load applied on the structure in the analysis was the seismically induced free-field ground displacement. The inertia forces of the Operation and Maintenance Building are applied as shear forces distributed to the floor slab at the ground level. Analyses for other loads such as hydrostatic pressure and loads during construction are also elucidated in the paper. The FEA-based technique presented permitted the determination of the load effects in the shaft structure due to earth and hydrostatic pressure for each construction stage. And to study the behavior of any underground structure, such as tunnel and the pump station in this case, subjected to earthquake ground motions, the right approach is to treat the issue as a ground displacement problem rather than an inertial problem as for above ground structure. A FEA-based soil-structure interaction analysis can be employed for that purpose as explicated in the paper. This technique permits the determination of the structural deformations as well as the stresses in the structure. This technique was demonstrated with a typical personal computer to illustrate that a rigorous 3D soil-structure interaction analysis can be easily and practically performed in any design office. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.