Tehaleh, formerly known as Cascadia, is an innovative community development project encompassing more than 4,900 acres. It is being developed as an Employment Based Planned Community (EBPC). The community provides a full range of services for the residents including homes, schools, retail, and recreation.
In 2003 Terra Associates was hired to provide geotechnical engineering design, environmental and construction services to Tehaleh. The services involved designing creative and functional solutions to the construction of roads, houses,
schools, retail and commercial buildings, public works facilities, and community and recreational areas.
The project is expected to continue for at least 10 more years.
Bridge I-5 Seattle
The project site consists of 66 tax parcels totaling approximately 120.6 acres in Milton, Washington. Terra provided geotechnical engineering expertise to build an industrial complex of 5 buildings ranging from 108,750 to 1,000,000 square feet and associated access, parking, and utility improvements.
We were presented with the challenging topography that was consistent with an active mining site with various slopes, ponds, stockpiles, and access roadways throughout the property. The most significant slope was located in the northern portion of the site where the mining operations were active daily.
After intensive subsurface exploration and data collection, we performed analyses to develop geotechnical recommendations for project design and construction. Due to the significant topography, grading and development included 70 feet of fill on the south side of the site that was supported by the largest detention vault to date, and terraced Lock + Load walls. Permanent soil nail walls supported the excavations on the north, east and west sides of the site.
8th and Seneca
The project consisted of constructing two separate tower buildings over structured parking on a site in downtown Seattle, Washington. The north tower is a 12-story building and the south tower a 25-story tower. The parking structure extended eight levels below grade. The towers were separated at grade and connected with sky bridges.
Terra observed the drilling of four soil test borings ranging in depths of 60.5 feet to 77 feet. We conducted laboratory soils analysis on these test borings and developed geotechnical recommendations for project design and construction.
The excavation for the below grade parking structure exposed fully inundated advance outwash overlying the Seattle clay formation. We ensured shoring stability inclinometers were installed at the perimeter of the site to provide additional monitoring during excavation. The inclinometers showed all movement was within typical shoring tolerances.
North Bend Gasoline
In the summer of 2010, Terra became the engineer of record for the North Bend Gasoline remedial action. There had been a release of up to 8000 gallons of gasoline in 2004 from one of the underground storage tanks on site. The release impacted an adjacent property requiring the installation of a sub slab depressurization system. The eventual length of the plume of impacted groundwater by 2010 was about 1100 feet in length and involved two adjacent properties. Terra Associates, Inc. completed the site characterization effort, prepared a Remedial Investigation Report and Cleanup Action Plan for the site. The selected remedial technology was enhanced in situ bio degradation/natural attenuation. Two episodes of the injection of calcium peroxide were completed on the gasoline station property and along the plume. Remediation required coordination with the two adjacent property owners.
A site specific soil cleanup level was calculated and the existing model remedies established by the Washington State Department of Ecology were incorporated into the site remediation. In February of 2021 Terra Associates, Inc. requested No Further Action status for the site. The exposure pathways including vapor intrusion had all been addressed. No excavation and offsite disposal of soil was required to complete this cleanup. The gasoline station remained in operation through the entire remedial action. The Washington State Department of Ecology granted the unrestricted No Further Action status in November of 2021.