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Cushman No. 1 Spillway – Core Wall Extension
Location: Hoodsport, WA

Project Partner: Tacoma Public Utilities-Tacoma Power

Challenge: In the vicinity of Spillway 1 at Lake Cushman, it was observed that water from within the lake was seeping through the earth dams surrounding it. Further sub-surface investigations within the earth dams confirmed that there was some loss of material within due to this leakage. The subsurface investigations indicated that the overburden was mainly of recessional outwash composed of GM, SP-SM and SP type material followed by a recessional lacustrine deposit layer of mainly ML material down to the lodgment till. To prevent further loss of material and water from within the lake, it was decided to extend the existing core-wall using the soil mixing technique. The soil mixed panels were to be installed down to 5 feet within the glacial till. The design requirement for the soil mixed material was a permeability of 1 x 10-6 cm/sec and strength in the range of 50 psi – 100 psi at 14-days.

Overview: Condon-Johnson was contracted to perform a horizontal extension of the soil-cement core wall by 273 feet horizontally, 2.8 feet thick and up to 75 feet in depth.

Results: Condon-Johnson carried out the core-wall extension using cutter soil mixed panels of dimensions 2.8 feet x 9.2 feet with a 1 foot overlap. The contractor developed mix design consisted of bentonite slurry and cement grout to provide a soil mixed material capable of meeting the design requirements and successfully cut off the seepage that had been occurring.