BPA, Liquefaction Hazards Assessment
Preliminary studies completed for BPA indicated a high seismic vulnerability of their critical transmission towers at four major Columbia and Willamette river crossings near the Portland, metropolitan area and Longview, Washington. GRI completed a phased liquefaction and lateral spreading assessment for the towers at all four crossings. The first phase of the project involved completion of geotechnical explorations, including borings, cone penetration tests, and shear wave velocity testing at each tower location. The drilling required extensive communication with BPA, various property owners, permitting agencies, and local levee districts to address safety and access considerations. GRI staffed and mobilized up to four drilling crews to complete the field work on time. The exploration work included four island explorations only accessible with a barge and track-mounted drill rigs. The liquefaction and lateral spreading analyses were completed for at least five different probabilistic and deterministic scenarios to incorporate into the overall structural fragility model that was completed by BPA. GRI worked closely with the structural design team to evaluate acceptable total and differential settlement and lateral movement for the structures, which are supported on a combination of shallow and deep foundations within liquefiable soils. The scope of work included development of plans and specifications for ground improvement or other mitigation at each of the crossings to meet the deformation criteria. Site-specific cyclic testing of the low- to medium-plasticity silt soils that mantle much of the alignment was completed to better estimate the strength and deformation behavior of these materials, together with two-dimensional, non-linear effective stress FLAC modeling to supplement more simplified, Newmark-type displacement estimates and develop a series of mitigation concepts for each site.
Location: Various locations throughout Oregon and Washington
Owner: Bonneville Power Adminstration (BPA)