The L-Structure is an inlet area retained on three sides by steel sheet piles that were driven into an underlying claystone layer 26 feet deep. City water is pumped from the inlet area through a series of steel pipes that run up into a small treatment building. The city needed to install new larger diameter inlet pipes at a deeper elevation.

The soil conditions consist of silty gravelly sand with cobbles over claystone. The existing steel sheet pile wall was embedded only to the top of the claystone and would be undermined with the planned excavation. Another key requirement was a temporary cofferdam to protect the excavation from the Yellowstone River.

SoilFreeze installed a series of 3-inch diameter steel freeze pipes to 40-feet around the entire planned excavation area. The freezing extended well into the claystone and directly against the existing building foundation to cutoff groundwater. Also, SoilFreeze created a frozen soil cofferdam inside of stacked trench boxes to keep the river out of the excavation.

The frozen soil wall was water tight and stable around the entire excavation. The frozen soil cofferdam along the river worked very well to allow the work inside the excavation to be completed “in the dry”.