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No Business Creek Tunnel

Gwinnett County's goal is to continually improve the reliability of the wastewater system that its residents depend upon. To help meet that goal, the County’s Department of Water Resources recently undertook construction of a 12 ft diameter, 16,000 ft long tunnel to manage the wastewater flows from the No Business Creek and Jacks Creek drainage basins. The TBM driven tunnel was excavated in rock at depths ranging from 60 to 225 ft below grade; and five shafts ranging from 15 to 40 ft in diameter were constructed along the alignment.

Working for the contractor, Lachel led a geotechnical investigation effort to further characterize the saprolite zones that were discovered during the earlier design phase of the project. The work included a surface geophysical investigation followed by a phased targeted drilling program. The results of the effort formed the basis of the contractor’s Value Engineering proposal, and recommended lowering the tunnel alignment by 24 ft to improve tunneling conditions and reduce associated tunneling risks.

Following the investigation, Lachel developed the initial support designs for the conventionally mined starter and tail tunnels and for the portions of the running tunnel where saprolite was to be encountered. Lachel designed the initial soil and rock support for each of the five shafts and the shaft break-ins/break-outs. Three of the shafts utilized secant pile support in the soil; one used traditional ribs and liner plate; and the fifth shaft was excavated and supported using the sunken caisson method. The rock portions of each shaft were designed to be supported using shotcrete, rockbolts and welded wire fabric. Lachel 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 tunnel excavation, Lachel provided geologic mapping services for the conventionally mined portions of the tunnel and assisted the contractor in selecting the appropriate ground support types.