Client: City of Charleston, SC
Consulting Engineer: Davis & Floyd/URS
Construction Manager: Black & Veatch
Contractor’s Consulting Engineer: ARUP
Contract Amount: $16,700,000
Scope: 4,000 LF of 108” monolithic concrete in tunnel.
Access Shaft: One 20 foot diameter access shaft, 140 feet deep installed by sunken caisson method. 54”
Drop Shafts: Three 54” diameter drilled drop shafts.
Overflow structure and pipe: 54” overflow sewer connecting to a precast overflow outlet structure.
Tunnel Pump Station Connection: Tunnel connection to the Concord Street live wet well and pump station.
Triad Midwest Mole JV, a joint venture of Triad and Midwest Mole, Indianapolis, Indiana recently completed the Market Street Drainage Tunnel in downtown Charleston, SC. Triad was the managing partner of the JV.
This high profile project included a number of unique challenges:
- Two tunnels were to be installed from one access shaft to one dead end and one live pump station. There was no outlet for the tunnel excavation equipment. The tunneling machine (TBM) was required to be removed from the initial access or starting shaft.
- The original downstream tunnel outlet was to be a connection into an existing live tunnel.
- The tunnel was to be excavated by Sequential Excavation Methods, TBM with precast segments or by contractor design (by contractor’s consulting engineer) approved by the owner. Triad Midwest Mole submitted its own two stage, tunnel design which included TBM excavation using ribs and boards followed by the installation of a monolithic concrete lining.
- Three drilled drop shaft connections were to be installed in the Charleston Market area (on Market Street). The drill drops were in very tight, congested, tourist filled areas either adjacent to or between two market buildings.
- The access shaft site was very small and immediately adjacent to Charleston’s cruise ship dock.
- The Concord Street leg of the tunnel system was to be excavated downhill at a very steep grade(2.6%) to the live tunnel connection.
- Ground Freezing was required in the zone of the live tunnel connection.
Large problem discovered
The project required initial vertical probe drilling along the Concord Tunnel alignment to verify that the new tunnel would not encounter the old abandoned Charleston Water System sanitary sewer tunnel. The old tunnel was full of water and subject to the hydraulic head produced by the Cooper River/Atlantic Ocean. The probe drilling revealed that a zone of the original CWS tunnel crown had failed and expanded outward directly into the new tunnel zone.
Triad Midwest Mole engineers in coordination with their outside consultant ARUP redesigned the project. The following changes and solutions were made:
- The tunnel depth at the access shaft was lowered to 140 feet from its original 83 feet. That change lowered both tunnel runs to a safe depth below the abandoned CWS tunnels. This eliminated any risk of intrusion from the old tunnels. The resultant tunnel grades were at a much flatter gradient, eliminating the original steep downhill grade and providing much safer working conditions.
- The tunnel outlet was moved from the original connection point on the East Bay Calhoun tunnel to a direct connection with the wet well of the Concord Street Pump Station. This was accomplished by the rerouting of the Concord tunnel from a straight line into an S curve which ended at the wet well.
The access shaft was installed by the sunken caisson method.