Keeping pace with development and improving wastewater system reliability, Gwinnett County’s Department of Water Resources constructed a new tunnel to help manage wastewater flows from the No Business Creek and Jacks Creek drainage basins. The 12-ft diameter, 3-mile long tunnel enables steady pumping even during peak usage. Schnabel performed geotechnical investigations and construction engineering for the project.

The tunnel was excavated with a tunnel-boring machine (TBM) through rock at depths ranging from 60–225 feet below grade. Five shafts, ranging from 15–40 feet in diameter, were constructed along the alignment. Working for the contractor, we led a geotechnical investigation effort to further characterize the saprolite zones that were discovered during the earlier design phase of the project. Our work included a surface geophysical investigation followed by a phased targeted drilling program. Our results formed the basis of the contractor’s value engineering proposal, and recommended lowering the tunnel alignment by 24 feet to improve tunneling conditions and reduce associated risks.

Following the investigation, we developed the initial support designs for the conventionally mined starter and tail tunnels and for the portions of the running tunnel where saprolite would be encountered. We designed the initial soil and rock support for each of the five shafts and the shaft break-ins/break-outs. Three of the shafts used secant pile support in the soil; one used traditional ribs and liner plate; and the fifth was excavated and supported using the sunken caisson method. We also designed concrete shaft covers for four of the five shafts, as well as the excavation support system and structural cover for a grinder vault adjacent to one of the shafts.

During its excavation, we provided geologic mapping services for the conventionally mined portions of the tunnel and assisted the contractor in selecting the appropriate ground support.