Seattle Public Utilities (SPU) has four large, buried concrete reservoirs constructed between 2006 and 2013 and ranging in size from 5MG to 64MG (20 to 240 million litres). These reservoirs are essential for maintaining post-seismic event fire suppression and domestic water supplies for the City of Seattle, Washington. Three of the reservoirs are roughly square, with widths between 450ft (137m) and 670ft (204m), depths of 20ft to 30ft (6m to 9m), and 2ft (0.6m) soil cover. The fourth buried reservoir is roughly one-eighth the plan view size. Areas above the reservoirs provide public parks with sports fields and recreational facilities.

In response to a notice that these reservoirs might be seismically deficient, SPU decided to use leading edge three-dimensional (3D) nonlinear (NL) Soil-Fluid-Structure Interaction (SFSI) analysis methods to evaluate their performance. SPU elected SFSI analysis instead of traditional code-based methods because performance based design principles were key to addressing the seismic performance and retrofit of these reservoirs, and no off-the-shelf code-based or simplified methods were directly applicable or technically justified for buried structures of this type. This is the first time 3D nonlinear time domain SFSI analysis methods have been applied to structures of this type and size by SPU. The seismic performance assessments of these reservoirs show that such advanced analyses produced reductions in seismic base shears compared to equivalent static design methods, were essential in identifying the cause and extent of the vulnerability, and were necessary in evaluating and ultimately meeting the performance objectives of the retrofit.

The reservoirs were evaluated against performance criteria established for life safety and reliability after a 2,475-year return period Maximum Considered Earthquake (MCE) and for serviceability after a 100-year return period Operational Basis Earthquake (OBE). It was found that deficiencies in the reservoirs’ structural detailing could potentially lead to leakage exceeding the performance objectives for both levels of seismic hazard. These deficiencies resulted from: (1) the 3D nature of the seismic loading and the structural response; (2) the complex interactions between the tiled floor systems (designed to limit cracking from concrete shrinkage) and the supporting soil; and (3) the interaction between the walls and the surrounding soil. These interactions were aggravated by slippage below the wall foundations, intermittent gaps that developed between the soil and the walls, varying topography, and varying soil stratigraphy.

This paper addresses the utilization of performance-based design principles for the retrofit design. It discusses seismic hazard, 3D SFSI modeling, structural response and deficiencies, and mitigation measures. The paper also addresses applicability of the current United States (US) design codes and the shortcomings of the code-based methods for the design of buried water reservoirs in providing a more reliable prediction of performance with respect to water-tightness.

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