Tacoma Dome Station

Tacoma, Washington

HWA performed a preacquisition environmental site assessment to determine the presence and levels of contamination over a two city block area. HWA represented a regional transportation authority in negotiations with Washington State Department of Ecology for a Prospective Purchaser Agreement (PPA). A PPA is a recently legislated type of consent decree in which a party is protected from future environmental liability prior to purchase and development of a property. HWA drafted consent decree documents, developed a remedial action plan, assisted in the public participation process, provided support during property acquisition negotiations, and assisted with obtaining State grant funds for remediation.

HWA prepared bid plans and specifications for excavation and remediation, assisted with the bidding and contracting process, and oversaw the excavation and remediation of 19,000 tons of petroleum contaminated soil by on-site thermal desorption. Limited amounts of metals contaminated soils and soils meeting dangerous waste criteria were also characterized and disposed off-site. HWA sampled and analyzed over 700 samples to accurately define the extents of contamination, delineate soil for remediation, and assure compliance with the Consent Decree. HWA's thorough and efficient sampling approach, along with statistical evaluation of confirmation data, insured that the volume of soil treated was kept to a minimum, while complying with regulatory standards and achieving site cleanup.

HWA conducted a preliminary geotechnical investigation in support of the schematic design phase. Work efforts included field, laboratory and office work necessary to provide engineering recommendations for project foundation and retaining wall alternatives. Following the preliminary investigation, HWA performed design-level and construction services for the project. Services included foundation design parameters for the difficult site conditions, which spanned from dense glacial soils to soft alluvium and fill over the length of the structure. A dual foundation system was developed, consisting of driven pipe piles in the portion of the structure underlain by poor soils, and spread footings beneath the remainder. HWA performed site seismicity studies, and developed geotechnical parameters for use in seismic design. HWA also monitored pile driving, foundation excavations and subsurface drain installations during construction.