Wet Mass Soil Stabilization for Excavation Support-Paper

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At Minneapolis–St. Paul International Airport, construction of a new 19-ft retaining wall required excavation beside an active roadway, utilities, and an existing storm sewer. Loose, variable granular fill and native soils could have become unstable during excavation, threatening nearby infrastructure and causing settlement.<br /><br />The project specifications permitted permeation grouting or jet grouting. The contractor proposed shallow mass wet mixing as a more efficient alternative: a rotary mixing tool blended cement slurry directly into the in-situ soil to create a stabilized soil-cement mass. Design checks addressed overturning, sliding, shear, and bending, using a target treated-soil strength of 200 psi. The treatment was laid out in contiguous panels across an area approximately 200 ft long, with depths of 8–12 ft. Ready-mixed cement grout reduced the need for on-site storage and batching equipment.<br /><br />Construction was monitored in real time for grout flow and properties, binder content, tool speed, and other operating data. Once the treated material reached the required strength, excavation proceeded so that continued curing would not make removal more difficult.<br /><br />The stabilization enabled the planned retaining-wall construction and was judged cost-effective and beneficial to the schedule. Two unforeseen conditions arose: an overhead sign foundation conflicted with the wall footing and was removed, and excavation exposed a short, untreated zone below the bedrock surface that required remedial stabilization. Despite these issues, the work performed as intended. The case demonstrates the value of considering alternative geotechnical methods when conventional approaches may be inefficient or poorly suited to site constraints.

Keywords

Minneapolis–St. Paul International Airport

retaining wall

excavation stabilization

shallow mass wet mixing

soil-cement

cement slurry

in-situ soil treatment

grout monitoring

geotechnical construction

foundation remediation