Case Study: Cement-Bentonite Pre-Trenching and Cutter Soil Mixing(CSM) for Temporary Shoring and Groundwater Cutoff

A multi-story office building in Seattle, Washington with four levels of underground parking was sited adjacent to the existing Alaska Way Viaduct in an area which had previously been occupied by turn of the century sawmills before being developed for commercial uses in more recent history. The soils at the site consist of approximately 8.5 m (29 ft) of sand and wood debris fill over estuary deposits and glacial soils. The ground water regime at the site consists of an upper and lower aquifer separated by an aquitard. The piezometric heads of the aquifers were 2.7 m (9 ft) and 2.4 m (8 ft), respectively, below existing grade. The proposed excavation was 13.1 m (43 ft) deep with a requirement that the shoring system extend an additional 7.6 m (15 ft) below the bottom of excavation to limit seepage from the upper aquifer into the excavation. Although initially tendered for construction with an owner designed secant wall or freeze wall system, the successful shoring contractor offered a design-build cutter soil mixed (CSM) shoring system which included pre-trenching the wall alignment under a cement-bentonite (C-B) slurry to remove the identified wood debris prior to installing the shoring system. This paper addresses the stability analysis and excavation of the C-B trench for removal of obstructions along with the design, construction, and performance of the CSM shoring system and associated depressurization of the deep aquifer.