Case Study: Cleanscapes CNG Plant

Client: Cleanscapes, Inc. Location: Seattle, WA Contact for geotechnical projects: John Peterson

Project Background & Challenges

Cleanscapes, Inc., a relatively young, local company, won the solid waste collection contract for half of Seattle starting in April 2009. To fulfill the contract, they needed to develop capacity, including obtaining a fleet of trucks. The trucks run on efficient compressed natural gas (CNG), so a fueling facility was also needed.

Cleanscapes secured a property for the CNG facility in south Seattle near the Duwamish River, an area with weak soil and liquefaction hazards. Cleanscapes retained Aspect to complete a geotechnical study in support of the design of the CNG facility. The equipment for the facility is sensitive to settlement. To mitigate liquefaction hazards, Aspect recommended options to densify and provide drainage for the liquefaction-prone soils to safely support the equipment in an earthquake.

Solution

Aspect specified and designed an earthquake drain system to reduce total and differential settlement to an acceptable tolerance. Nilex Construction installed the drains in February 2009. The drains were installed vertically on a grid throughout the part of the facility where liquefaction risks are greatest. The depths of the drains alternate between 50 feet and 18 feet in consideration of the lower permeability soils at shallower depths. The installation process densified the site, causing about a foot of settlement in the treated area. After the drains were installed, a stone reservoir layer was placed to provide storage of water ejected by the drains in an earthquake. Conventional crushed rock base and foundation pads were then placed on the stone reservoir, providing stable support of the facility.