RED RIVER VALLEY | TRENCHLESS
Left, figure 5: Construction of the temporary sheet pile cofferdam in the Missouri River
slurry separation plant, the construction footprint around the launch shaft is smaller.
DESIGN AND CONSTRUCTION PROCESS Intake System Intake System Contracts 1 and 2 have been completed after detailed design and construction. The location and sizing of the Intake required detailed hydraulic modeling of the intake system as well as its impact on the Missouri River. As such, it is situated on an erosional bend in the river where water depth and velocity are advantageous, and the potential for sediment deposition and silting in of the structure is lower. This location is west of a USACE armored shoreline that regulates the main channel and prevents further erosion of the riverbank. Due to in-water construction and presence of
Threatened & Endangered (T&E) species, coordination among multiple agencies and firms was instrumental to ensure compliance with several USACE permits. USACE NWP12 required monitoring and compliance
measures for T&E species. Various monitoring periods between April to July and September to October required the MRI construction and 24/7 tunneling schedule be developed to reduce the potential for T&E species-related work delays. Compliance measures included daily surveys for Piping Plover and Whooping Crane and suspension of work if spotted within the jobsite radius. In-water work was prohibited for Pallid Sturgeon in April to June and clearing and grubbing were prohibited from June to July for the Northern Long- Eared Bat. Preferred construction means and methods were specified; 3 intermediate jacking stations (IJSs) were
specified and installed to ensure adequate management of jacking forces during mining and any unanticipated stoppages. A top hat seal system was required at both launch and reception shafts, consisting of two seals and adjustable fingers to prevent groundwater and soil infiltration. To construct the intake, an MTBM was launched from the MRIPS wet well/shaft on land to install a 1,600-ft long 72-inch ID Permalok steel pipeline. A temporary steel sheet pile cofferdam (Figure 5) was constructed in the Missouri River to serve as both the foundation for the MRI and a reception shaft for the MTBM. The conveyance tunnel was excavated in soft ground at a 0% slope and installed using a one-pass method with the jacking pipe serving as the carrier pipe connecting the MRI to the MRIPS. Despite careful planning and coordination, this
project encountered several challenges. Executing a schedule around T&E black-out periods led to a winter construction for the temporary access bridge and cofferdam. Sheet pile driving operations dealt with 50+ mph wind speeds, sub -40°F windchill temperatures, and 2ft-thick river ice. Split sheet pile interlocks and material migration into the cofferdam caused considerable time spent by divers welding steel plates over the gaps and re-excavation before placing the concrete plug. Midway through the 24/7 mined tunnel drive, the
launch portal seal was tearing. The repair, which completely replaced the launch window and rubber seal, halted mining operations for ~48 hours to ensure a total failure wouldn’t flood the tunnel or shaft. After 26 days of near 24/7 operations, the MTBM mined into a flooded cofferdam and was successfully retrieved.
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