Seismic Displacement Assessment of New Facilities for Post-Disaster Operation at a Wastewater Treatment Plant

New developments are planned at an existing wastewater treatment plant, which provides secondary treatment of the liquid stream for about one million residents and businesses. Existing facilities at the 45-year old plant were designed based on various versions of the building code in effect at the time of construction, and each expansion was designed to a different level of seismic performance. As part of the new expansion phase, additional gravity thickeners are proposed in an area of the site surrounded by several older facilities. The proposed gravity thickeners are designed for post-disaster operation after the design earthquake without adversely affecting the performance of the existing facilities. A suite of 15 spectrally matched ground motions were developed for the 2,475 year design return period, and nonlinear seismic deformation analyses were performed using the finite difference program FLAC and effective stress-based constitutive model UBCSAND. As per conventional local practice, time histories representing the average and upper bound estimates of the Arias intensity and significant duration were identified from the 15 earthquakes during preliminary design, and seismic-induced displacements of the facilities were evaluated. Subsequently, the performance of the structures was assessed under all 15 time histories. The analyses showed that the initial selection of time histories based on the average and upper bound Arias intensity and significant duration did not correspond to the average and upper bound displacements under all 15 time histories. It is evident that conventional selection of earthquake records may not result in the examination of the most critical displacement cases.