Chapter 18 STABILIZATION OF A NATURAL GAS PIPELINE UTILIZING SOIL NAILS AND HIGH TENSILE STEEL WIRE MESH

A landslide investigation and mitigation was conducted for a pipeline in southeastern Ohio. The landslide, with a lateral extent of 185 feet and a length more than 300 feet had encroached on the pipeline right-of-way and impacted an existing 26-inch natural gas pipeline. The subsurface conditions were observed in test pits advanced parallel to the pipeline. The design parameters used in the stability evaluations were based on limit equilibrium and a back-analysis approach. A range of soil parameters were evaluated along with published literature and our prior experience working with similar landslides in the region to establish the likely failure conditions. Based on the field observations, the water level in the slope was modeled to ensure the assumed slide plane is below the water table.

It was determined that the most appropriate stabilization system for this application was a combination of soil nails and high tensile strength steel mesh on the ground surface. The soil nail stabilization elements were designed with their spacing and lengths adjusted to ensure a minimum global Factor of Safety of 1.5 was achieved. The wire mesh facing was designed to retain smaller wedges of soil between the nails and was installed with turf reinforcement mat to promote the growth of vegetation on the slope face. The wire mesh was connected to the soil nails by use of plates and nuts and tensioned to provide confining pressure on the landslide mass. The benefits of this system included:

• The soil nails and wire mesh provide long term stabilization of the slide with no maintenance.

• The soil nails were installed using a crawler mounted drill rig, creating minimal disturbance to the slope following the installation of the new pipeline.

• Should future maintenance of the pipeline be required, the mesh can be detached from the soil nails and rolled back allowing for excavation/backfill prior to replacement.

• The mesh allows for vegetation to grow on the surface, reducing the likelihood of soil erosion, and is aesthetically pleasing.

Both verification and proof testing of the soil nails was completed to ensure the nail capacities assumed during the design process were achieved in the construction of the soil nails. Design and construction of the slope stabilization system was completed over a three-month period in 2015. The design and installation of this system allowed the owner to reopen the pipeline on schedule and will allow for future maintenance and operation of the line.