Abstract:
The Dakota-Federal 42X-36 well pad has been operating for approximately 13 years. Braun Intertec has monitored landslide activity impacting the pad for 3 years. While pad operations previously managed to avoid unstable areas in the past, the pending development of additional wells will expose impacted portion of the pad to 110, 000-pound truck loads, 158, 000-pound crane loads, 80, 000-pound material loads, and other surcharges.
The pad was built on sandstone, siltstone, claystone, and lignite of the Sentinel Butte Formation. Slope inclinometers originally installed outside the failure limits and close to the original 42X-36 showed no landslide impacts. A slope inclinometer later installed within the failure limits confirmed shear at depths up to 20 feet. Companion vibrating wire piezometers revealed elevated pore water pressure conditions within a deep lignite layer.
Braun Intertec designed an anchored soldier pile wall to isolate portions of the pad outside the failure limits and stabilize that portion of the pad impacted by the landslide. Ninety-five HP 14x73 and 14x89 piles were grouted into 2 1/2-foot diameter drilled shafts installed at 7 1/2-foot centers along the pad perimeter, each anchored 5 and 12 feet below the tops of the piles. The wall retains a 16-foot cut intended to unload and improve slope stability below the wall.
Heavy precipitation from late winter into early spring 2022 appeared to remobilize the landslide during drilled shaft installation, displacing three unanchored piles located within the failure limits. To limit damage to additional drilled shafts before anchors could be installed, a temporary stabilization plan was implemented that included unloading the landslide mass along the permanent retention alignment and reinforcing the landslide mass with additional H-piles driven downslope from the alignment.
Post-construction, the pad’s containment berm was rebuilt and the subgrade along the wall was reinforced with geogrid and concealed beneath a polyethylene liner.