Soil Stabilization of the Valley Creek Trunk Sewer Relief Tunnel

Compaction grouting is the placement of very stiff, low-mobility grout under relatively high pressures in strategic locations so as to consolidate soils and fill voids. Normally a hole is predrilled and grout is either pumped top down or bottom up through a tight-fitting steel casing. The Valley Creek Sewer Relief Tunnel in Bessemer, AL was to be constructed with a tunnel boring machine (TBM) designed for rock, but poor ground conditions consisting of karst limestone formations, which produced tremendous amounts of loose soils, mud, boulders and water prevented adequate advancement. Compaction grouting was employed to improve poor soils along the tunnel alignment ahead of boring operations for about 3,500 feet. The treatment zone was designed to improve a soil block in excess of 20 feet wide from a depth of 30 feet to 65 feet below grade. Grouting was also performed around drop shafts to prevent water inflow. This paper will discuss the design and implementation of a full-scale production-grouting program that began as a test-grouting program. A program, which spanned three construction seasons and saw the placement of approximately 35,000 yards of compaction grout in about 1,000-drilled holes. As a caveat to the grouting program, the tunnel came in close proximity to several sensitive structures, which required underpinning with micropiles and/or conventional shoring. These contractor-designed solutions will also be examined.