A Case Study of Risk Assessment and Mitigation during Design and Construction in Karstic Geology

Subsurface conditions for a combined cycle combustion plant in Warren County, Virginia were dominated by karst geology. Active sinkholes, numerous soil-, water-, and air-filled voids in the limestone bedrock, and highly varying rock surfaces associated with karst activity were present. To better characterize the site conditions, an extensive subsurface investigation with multiple phases was performed. Specific design and construction approaches were developed and utilized based on the site conditions to ensure the foundations would properly support the structures. A foundation type selection matrix was developed based on the results of the geotechnical investigation and structure-specific characteristics to consistently evaluate and quantify the level of risk associated with the foundation type selected for each piece of equipment. The use of the foundation type selection matrix aided in selecting a combination of micropile supported foundations and shallow foundations based on the risk of failure associated with each structure. A load testing program and full-time construction observation were employed to confirm micropile design and construction.