Permanent Excavation Support in Urban Areas Using Cutter Soil Mixing Technology: Elliott Avenue Case History, Seattle, Washington

In one of the first applications of Cutter Soil Mixing (CSM) technology in the United States, a 12 to 17.4 m (40 to 57 ft) permanent deep soil mixed wall was constructed to enable the excavation for an office building near downtown Seattle in difficult soil conditions with a high water table. Unlike conventional slurry walls and diaphragm walls that utilize concrete, soil mixing relies on mixing the soils in situ with a cement and bentonite slurry to create a soil-cement wall. Cutter Soil Mixing technology utilizes two sets of vertically mounted cutting wheels rotating about a horizontal axis to produce rectangular panels of treated soil. By overlapping the soil mix panels, a continuous rectangular wall is constructed, as opposed to circular columns created with conventional single-axis or multiple axes deep soil mixing systems. The Elliott Avenue project located just north of downtown Seattle at the base of Queen Anne Hill required permanent excavation support to depths of up to 17.4 m (57 ft) in mixed soil conditions consisting of loose sands and gravels, underlain by stiff plastic clays and very dense glacial till. Because the project is located on the water front, the groundwater table at the site was found at a depth of 0.6 m (2 ft) from construction grade. It was thus necessary to support the saturated loose sands and gravels with a robust earth retaining system. Cutter Soil Mixing (CSM) was selected as the method of choice for several reasons, namely: price and schedule relative to a concrete secant pile wall; the ability of CSM to construct a permanent, high quality cutoff wall in the dense gravels and stiff plastic clays; the capacity of the cutter mixing tool to key into the glacial till; and, the ability of CSM to produce a soil-cement material with a minimum strength of 1.38 MPa (200 psi) and a maximum permeability of 5 x 10^–6 cm/sec.