- ANR Pipeline Cofferdams
Work included the design and installation of two portable, temporary cofferdams for use in a lake to facilitate relocation of an existing natural gas pipeline.
- Maintain dewatered state
- Components demobilization
A Wisconsin Department of Transportation highway expansion project required extra traffic lanes on a heavily congested four-lane route serving a large resort/vacation area. An interstate 16-inch high-pressure natural gas feeder pipeline, originally installed near the highway right-of-way, needed to be moved to make way for the highway expansion.
Relocation included intercepting an existing section of pipeline underneath a lake, already at a sufficient distance to be unaffected by the highway expansion project. Two temporary cofferdams were necessary to create dry working areas for accessing the gas line from the north and south shores.
The north, 100-foot-wide cofferdam was designed and constructed to extend about 500 feet into Lake Butte des Morts from the shoreline. A wetland extended inland from the shore about 1,000 feet to a staging area, which had to be matted and swept for a threatened species of turtle and secured with a turtle fence. 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. Construction required coordination with archaeologists and an Indian nation representative who mapped strata, artifacts and burial grounds unearthed by excavations for the new pipeline.
Once the portable cofferdam had been installed in the water and secured to the lake bed, a partial dewatering took place to lower water levels inside the cofferdam, establishing a hydrostatic differential for further stabilization. The WDNR required a fish inventory and relocation program be executed by shocking the water in the enclosure by boat. When fish capture, classification and relocation were completed, dewatering continued to expose the enclosed lakebed.
Maintenance pumping was established on a 24-hour basis so that excavation of the new pipeline trench could begin. All dewatering was conducted 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 recontoured to pre-excavation conditions. Pumps were removed, followed by a controlled flooding of the cofferdam interior to normal lake levels. A turbidity barrier around each cofferdam minimized impacts to the surrounding lake. Cofferdam structures were deconstructed, and components were cleaned, staged and prepared for shipment back 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. Complicating factors included access restrictions, property and easement lines, and the proximity to U.S. Route 41, which effectively reduced available working area inside the cofferdam.
Both cofferdams were available for excavation at the same time to keep the gas shutdown time at a minimum. The project was completed on schedule in less than seven weeks.
- Design and construct two temporary marine cofferdams
- Dewater the cofferdam structures and maintain cofferdams in a dewatered state while the pipeline contractor excavated new gas line trench to intercept the existing 16-inch gas main crossing underneath the lake
- Restore bottom of the lake to pre-existing conditions inside the temporary cofferdams after pipeline construction is complete
- Dismantle, clean and ship cofferdam components back to supplier for final disinfection
- Multiple contractor coordination: pipeline construction company, pipeline owner inspectors and subcontractors performing work elements throughout the course of the project
- Comply with Wisconsin Department of Natural Resources (WDNR) 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 camping and burial grounds