Combination of Ground Improvement Techniques for Support of Shallow Foundations in Karst

The subsurface conditions for a proposed six-story $94M medical facility in Knoxville, Tennessee, consisted of a thick clay layer overlying an irregular weathered limestone bedrock surface. Because the design loads were significant and because of the sinkhole risk associated with the underlying limestone bedrock, the initial design considered the use of drilled shafts bearing on rock. While drilled shafts represented a robust technical solution, construction of drilled shafts in karst can be expensive and time-consuming. In an effort to save both time and money, the authors and other members of the project design team developed a combination ground improvement program to allow support of the medical facility on conventional shallow foundations. The combination ground improvement program consisted of (1) cap grouting the rock surface to significantly reduce the sinkhole risk, and (2) construction of Geopier® elements to reinforce and stiffen the soils immediately below the planned shallow foundations. For development and refinement of the combination ground improvement program, extensive subsurface characterization was performed using cone penetration testing (CPT) at every column location. The CPT results were used to develop three dimensional models of the subsurface conditions including pertinent karst features. The cost savings provided by the combination ground improvement program is estimated to be in excess of $1M.