Building and maintaining vital infrastructure such as gas pipelines through miles of land with numerous karst features requires ingenuity and expertise to avoid pitfalls that affect construction and the long-term performance as well as the environment.
In 2015, Columbia Gas/TC Energy proposed to construct approximately 30 miles of various diameter pipeline, construct two new compressor stations, modify seven existing compressor stations, and increase the maximum allowable operation pressure on various segments in Virginia and West Virginia. This project, called The WB Xpress Project, included the “lift and replace” of segments of pipe that were more than 50 years old.
Karst is a landform type, characterized by the presence of sinkhole, caves, large springs and a highly irregular “pinnacled” bedrock/soil interface. It is a consequence of the presence of soluble bedrock, primarily limestone, dolomite and marble; but it also can develop in evaporite rocks including gypsum and halite (i.e. rock salt).
Terracon was commissioned to identify karst features along the proposed alignment (such as sinkholes, caves, springs, and stream crossings in karst-forming bedrock) due to the potential impact of pipeline repair and replacement activities on the karst aquifer and for the possible development of sinkholes or subsidence to potentially affect the integrity of the pipeline.
The geohazards presented by karst terrain greatly differ from other natural settings. Natural and human-induced subsidence and sinkholes, slope movements favored by karst conduits and caves, flash floods related to inability of the system to manage water from heavy rainstorms, and pollution caused directly or indirectly by human actions are the main types of geohazards typical of karst terrains. The karst geohazard can affect both structures and facilities, as well as human health and the environment.
Terracon has been leading the way in the development of a standardized method for karst survey, remediation, and management, which safeguards both the integrity of the installation and the conservation and protection of the subsurface habitat and karst aquifers. Over the last 11 years, Terracon has worked closely with the Federal Energy Regulatory Commission, U.S. Fish and Wildlife Service, the National Forest Service, and state departments of environmental protection to develop a protocol that has now become widely accepted as the industry standard for pipeline assessment in karst terrain. The elements of this method are:
The WB Xpress project was completed in 2019, and now transports approximately 1.3-billion cubic feet per day of natural gas to supply the increasing demand in northern Virginia and West Virginia.
Over 300 acres of access roads, temporary workspaces, and staging areas were assessed during the construction phase of the project, and these workspaces were returned to pre-construction condition after completion of the project. The project’s karst management and conservation efforts successfully prevented any known construction impacts to the karst resource.
Robert K. (“Bob”) Denton Jr., CPG, LPSS is senior geologist with Terracon’s DC Metro West office in Ashburn, Virginia. He is a Certified Professional Geologist (CPG), and a State of Virginia Licensed Professional Soil Scientist (LPSS) with more than 40 years of professional experience. His specialties are engineering geology, hydrogeology, and karst characterization, remediation, and management. He is a regional expert in stormwater management in karst terrains.