Use of Geophysics in Tracking and Verification of Low Mobility Grouting for Foundation Improvement in Karst Terrain: A Case Study

The use of the Resistivity Tomographic Imaging Method (RTIM) for the confirmation of low mobility grouting of subsurface anomalies in residuum and bedrock of the Knox Formation was presented at the 2005 Annual Conference of the Association of State Dam Safety Officials. The RTIM surveys of this 2001 test section, detected anomalies along a portion of a water storage pond dam for a large electric generating facility. This paper presents the results of additional RTIM surveys along portions of the ash pond dam for the same facility. These RTIM surveys consisted of pre-grout RTIM surveys and post-grouting RTIM surveys as a tool for evaluating the extent of grouting and the improvement in subsurface foundation conditions due to low mobility grouting. After the formation of a large sinkhole in the ash disposal pond in 2002, it was necessary to advance a plan for further evaluation of specific sections of the ash pond dike. The foundation conditions of the dikes consist of approximately 20 to 25 feet of residuum consisting of Knox overburden and solutioned limestone and dolomite. During the sinkhole occurrence, significant impacts to the potential integrity of several sections of the dikes occurred. Due to pre-1998 sinkhole activity within the ash pond, RTIM along the crest and downstream toe of the dikes were conducted in year 2000 to identify potential areas of concern relative to plant operations. These data were used as background information, in conjunction with the field conditions described above, to further delineate sections of the dikes to be grouted. After each area was grouted according to the pre-established grouting criteria, they were resurveyed using RTIM along the original lines and stations in order to evaluate any improvement of the foundation conditions due to the low mobility grouting. Drilling and grouting techniques that were used in this remediation program were established and tested in the 2001 trial program. This trial grouting program was implemented to determine the most effective geophysical, drilling and grouting techniques for use in foundation improvement in the specific karst features on this site. Evidence of foundation improvement was manifested by the grout quantities recorded. Additionally, results from the before and after grouting resistivity surveys showed convincing evidence that definite foundation improvement in the targeted areas had been accomplished by the low mobility grouting. The RTIM surveys were shown to be an effective tool for confirmation of low mobility grouting. (Proceedings includes full paper) 10 pp.