Lake Stevens Wastewater Treatment Plant

Lake Stevens, Washington

HWA completed a geotechnical investigation for proposed improvements to the Lake Stevens Wastewater Treatment Plant. Improvements included rehabilitation of the access road to the plant, installation of a new waterline and structural improvements to the plant consisting of a canopy over the existing headworks building, remodeling the existing chlorine contact tank to convert it to a UV exposure basin, and constructing a vehicle ramp up to the tank.

HWA provided geotechnical recommendations that accounted for the soft soils present along the access road and around the plant, and the variable soil conditions and slope considerations along the waterline alignment. The recommendations included the use of steel plates on the road to distribute the load from heavy truck traffic, the use of horizontal directional drilling to probe the waterline alignment, the use of bottom ash to partially fill the existing chlorine contact tank so that the UV exposure basin could be placed within it without overloading the pile foundations, and the use of bottom ash contained between two rows of mechanically stabilized earth (MSE) walls attached by geogrid to build the ramp.

HWA also performed a Phase I Environmental Site Assessment of the treatment plant site, and facilitated characterization and remediation of petroleum contaminated soils encountered during construction of other wastewater facilities.