2005/03/23 Council Agenda Packet

8 the dam was estimated at the top of the dense silty sand under the loose and medium dense sand. The site specific response spectra at Portal 3 and the dam should be slightly different. However, given the proximity of the sites,and that the response spectra obtained at the dam is at the top of dense soils,URS will review and use the site specific response spectra developed for the dam. The objective of the seismic stability assessment of Portal 3 is to use the South Dam results to the maximum extent possible to evaluate the following two types of stability: • Bending and shear capacities of the Portal 3 structure and determine if it can safely resist the loads likely acting on it during a seismic event. This will help determine if the portal structure itself needs any seismic strengthening. • Overall sliding and overturning stability of the structure under seismic loads,such as tipping into the lake. This will help determine if the portal structure needs to be stabilized by holding it in place. The Phase 05A scope includes an Uncracked Structural Response Analysis(Gross Analysis)and Approximate Cracked Response Analysis(Approximate Analysis): • The Gross Analysis estimates the gross(uncracked)bending and shear capacities of the structure along its height before a possible occurrence of earthquake induced cracking. • The Approximate Analysis estimates the ultimate(cracked)bending and shear capacities based on post-cracking behavior.The analysis will be based on a published nomograph of post-cracking behavior of intake/outlet towers under seismic loading. Depending on the findings from the Phase 05A analyses,a Phase 05B scope consisting of a Detailed Cracked Response Analysis(Detailed Analysis)might not be necessary.Phase 05B will only be necessary if Phase 05A indicates marginal stability. Phase 05B will not be necessary if Phase 05A shows that the portal is either clearly stable or clearly unstable. Since Portal 3 is a link in a lifeline water-supply system,a leaking portal represents a failure. It is possible for the portal to crack and not leak. If the Gross and Approximate Analyses show that the structural capacity of the portal is marginal, a Detailed Analysis will be required.This analysis might show that the structure could crack,but not leak or collapse,thereby avoiding the need for seismic strengthening. Phase 05B would only be completed if the Phase 05A indicates a need for the Detailed Analysis. PHASE 05A—SEISMIC STABILITY ASSESSMENT The Phase 05A,Seismic Stability Assessment will be completed as the following tasks: • Task 05001: Seismic Criteria • Task 05002: Field Structural Audit • Task 05003: Review of Data • Task 05004: Seismic Response Analysis • Task 05005: Review Meeting • Task 05006: Preparation of Technical Memorandum The activities to be completed within each task are described in the following sections. 28