Abstract:
During the construction of an emergency spillway for the Pipestem Dam in Jamestown, ND, a slide failure occurred in April of 2024. This failure impacted the terminal structure and the critical path of the overall project schedule. An accelerated procurement schedule began to design and install a temporary earth retention system (ERS) to allow for the excavation of the material suspected of causing the slope failure to shale bedrock at a depth of 39 ft and replace with engineered fill. A limited notice to proceed was issued 3 weeks after request for proposal at the end of May, with 60% design drawings submitted one week later.
An anchored soldier pile and lagging system was ultimately selected to allow for long-term seepage through the overburden. Both the soldier piles and two levels of tieback anchors were socketed into rock. Several components of the ERS were specifically chosen to minimize construction duration including CFA drill methods to install the soldier piles, elimination of a wale system for the tieback anchors, and optimizing the anchor loads in the two rows of anchors and the soldier pile section to eliminate a potential third row of anchors. An extensive slope stability analysis was also performed, accounting for the large surcharge loads from the drill rig required to excavate 10 ft rock sockets. The temporary grading plan consisted of two benches for the installation of the soldier piles with a downslope bench acting as a buttress.
The grading plan was refined well into the earthwork effort and instrumentation was added to monitor for slope movement and increases in porewater pressure from the large equipment surcharges. Installation of soldier piles began in early August, followed by anchor installation 4 weeks later. Despite challenging drilling conditions from obstructions and saturated sands, final excavation and replacement was completed by the end of 2024. Â