Digging Deeper; Soil Mixing in the Pacific Northwest-Paper

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The former Time Oil Bulk Terminal in Seattle contained petroleum hydrocarbons, metals, solvents, and light non-aqueous-phase liquid (LNAPL) in source-area soils. The redevelopment project required in-situ solidification and stabilization (ISS) to depths of up to 40 feet.<br /><br />The project team considered drill-rig auger mixing and excavator bucket mixing. Although augers could reach the required depth, cemented sand lenses, high soil resistance, cost, schedule constraints, and difficult site grades made that option too risky. The team selected a large conventional excavator for its digging force, flexibility, and production rate, despite its limited reach and reduced efficiency at depth.<br /><br />An engineered soldier-pile shoring wall beside an operating rail line enabled a two-stage approach. Crews first mixed and removed the upper 15 feet to create a lower working platform. They then treated the deeper soil in small 8-by-10-foot cells, sequencing cells in a “piano key” pattern beside the wall to protect its supports. Surveyors monitored wall stability weekly.<br /><br />Quality checks included treatment depth, visual mixing inspections, hydraulic conductivity, and 28-day unconfined compressive strength (UCS). Test results met project requirements: hydraulic conductivity ranged from 5.9 × 10⁻⁶ to 4.7 × 10⁻⁸ cm/s, and UCS from 35 to 768 psi.<br /><br />Construction challenges included an unexpectedly higher silt key-in layer, supply-chain disruptions, and equipment damage from deep mixing in cemented sands. The team adjusted the treatment boundaries, planned materials and spare parts in advance, and changed mixing methods to reduce equipment stress.<br /><br />Through collaboration among the contractor, engineer, and owner, the team treated 30,000 cubic yards of impacted soil on schedule and within budget, enabling redevelopment. The project illustrates how phased excavator mixing and careful shoring management can extend conventional equipment’s use at depth, while highlighting the importance of thorough site investigation and equipment planning.

Keywords

Time Oil Bulk Terminal

Seattle redevelopment

in-situ solidification and stabilization

petroleum hydrocarbons

LNAPL

excavator bucket mixing

soldier-pile shoring wall

unconfined compressive strength

hydraulic conductivity

impacted soil remediation