- Lake Butte des Morts Pipeline
This project required the design and installation of two portable, temporary cofferdams in a lake to facilitate relocation of an existing natural gas pipeline. Cofferdams were dewatered and seepage water was controlled by maintenance pumping during excavation and installation by the pipeline company. Once backfill was completed, the cofferdams were flooded, disassembled and shipped back to the supplier for final disinfection.
- Maintain dewatered state
- Components demobilization
A highway expansion project by the Wisconsin Department of Transportation required construction of additional traffic lanes on a heavily congested four-lane U.S. route servng a large resort/vacation geographical area. An interstate 16-inch, high-pressure natural gas feeder pipeline, originally installed near the highway right-of-way, had to be moved. Relocation included intercepting an existing section of pipeline that crossed beneath a lake, already at a sufficient distance to be unaffected by the highway expansion. Two temporary cofferdams were required to create dry working areas to access the gas line from the north and south shores.
The north side 100-foot-wide cofferdam was designed and constructed to extend about 500 feet into Lake Butte des Morts from the shoreline to intersect the active gas line. A wetland extending inland from the shore about 1,000 feet to a staging area had to be matted and swept for a threatened species of turtle and secured with a turtle fence. Additionally, the entire area on both banks of the Fox River flowing between Lake Butte des Morts and Lake Winnebago had been a historical dwelling ground for Native Americans. The project schedule had to be coordinated with archaeologists and an Indian nation representative to map strata, artifacts and any burial grounds unearthed by excavations.
Once the portable cofferdam had been installed in the water and secured to the lake bed, a partial dewatering lowered water levels inside the cofferdam, establishing a hydrostatic differential to stabilize the dam. The Wisconsin Department of Natural Resources (WDNR) required a fish inventory and relocation program be executed by shocking the water in the enclosure by boat. As soon as the fish were captured and classification and relocation activities were completed, dewatering continued to expose the lakebed.
Maintenance pumping was established around the clock to allow excavation of the new pipeline trench. All dewatering was planned to minimize erosion scouring, turbulence and siltation of lake discharge areas. Once pipeline excavation and installation were complete, the trench was backfilled and the area within the cofferdam was re-contoured to pre-excavation conditions. Pumps were removed, followed by a controlled re-flooding of the cofferdam interior to normal lake levels. A turbidity barrier was installed around each cofferdam prior to removal to minimize impacts. Disassembly of cofferdam structures proceeded, including component cleaning, staging and preparation for shipment to the supplier's main facility for disinfection, in accordance with WDNR lake and wetland requirements.
A similar procedure was done simultaneously on the south shore, about a mile away. The south cofferdam extended about 200 feet into Lake Butte des Morts from the shoreline to complete the required active pipeline intersection. Complexities included access restrictions, property and easement lines, and the proximity to the U.S. Route 41 roadway structure, which reduced available working area inside the cofferdam. Both cofferdams were required to be available for excavation at the same time in order to keep the gas shutdown time to a minimum. Close coordination between all parties was key.
The project was successfully completed on schedule in less than seven weeks. All parties and the supervising regulatory agency developed strong positive relationships.
- Design and construct two temporary marine cofferdams
- Dewater the cofferdam structures and maintain cofferdams in a dewatered state while pipeline contractor excavated new gas line trench to intercept existing 16-inch gas main crossing underneath the lake
- Restore lake bottom to pre-existing conditions inside the temporary cofferdams after pipeline construction complete
- Dismantle, clean and ship cofferdam components back to supplier for final disinfection
- Multiple contractor coordination: pipeline construction company, pipeline owner inspectors and subcontractors
- Comply with Wisconsin Department of Natural Resources permit requirements, wetlands preservation and threatened wildlife protection
- Discharge pumped water through silt filtration structures and provide turbulence and erosion control throughout the project
- Coordinate with state archaeologists and Menominee Indian nation representative for mapping of strata and collecting of artifacts, since work area was in former Indian camp and burial grounds