Magellan Pipeline Company retained Blackstone to conduct a slope stability evaluation of a streambank on the Verdigris River in Montgomery County, Kansas. Three underground pipelines and one cable-suspended overhead pipeline cross the river at this location. Cement grout matting was placed on both banks in 2001 to protect the underground lines and deter bank erosion. Over time, the grout mat buckled over on itself on one bank of the river due to soil movement beneath the grout mat. The project involved determining the root cause of the failure, evaluating the potential impact to the pipelines, and designing a long-term repair solution.
The buckled portion of the grout mat was removed and replaced with 24” diameter limestone boulders as a temporary protection measure. During the removal of the grout mat, it was determined the mat had torn away from its anchor trench and was rotating down the slope in a clockwise direction, causing it to buckle over on itself near water’s edge. Inclinometers were installed near the tops of failure scarps to monitor slope movement. The inclinometers were monitored for about one year and the data was used in conjunction with the geometry of the riverbank and other subsurface data to evaluate the depth of the failure plane and the potential for the slope movement to have an adverse impact on the pipelines. The data indicated an ongoing slide was not occurring, but that damage was occurring during periodic extreme high-water flows and recedences. The slope movement was largely surficial and not a threat to the integrity of the pipelines. The temporary bank protection was expanded into a permanent flexible armor in conjunction with the existing portion of the grout mat that was performing well.