Tuesday, February 24, 2009: 1:00 p.m.
N.S. Nettles & Associates, Inc. (NSN) has incorporated the use of Multi-Electrode Electrical Resistivity (MER) and the seismic method of Multi-Channel Analysis of Surface Waves (MASW) into two specific geotechnical investigations for the purposes of identifying the nature and extent of karst activity and assisting in mediatory efforts. MER and MASW provide rapid, cost-effective methods with which to examine subsurface stratigraphic and density variability, the location of karst features within the sediments, and the general hydrogeologic characteristics of a particular environment. The first project is located in Palm Harbor, FL, where a 100,000 ft2 church building was proposed to be constructed. Both MER and MASW mapping were used to provide a detailed survey across the site. This testing identified a large (100 ft2) buried karst fracture feature that had reactivated. Excellent correlation was observed between CPT and SPT drilling data and geophysical results. These data ultimately provided a comprehensive understanding of the aerial extent and depth of the karst activity. The results of the mapping enabled the geotechnical engineer to design a remediation program using deep pilings to span and support the structure across the fracture. The second project was an existing shopping center, in Lake City , FL, where extensive subsidence resulted in damage to the buildings. MER and MASW mapping around the perimeter of the buildings and MASW mapping inside the structures resulted in an accurate description of the sinkhole conditions that were causing the subsidence. The MER and MASW data were correlated to CPT and SPT borings. Excellent correlation was observed between the drilling data and geophysical results. These data ultimately provided a comprehensive understanding of the aerial extent and depth of the karst activity and determined that remediation of the sinkhole conditions was not a viable economic solution for many of the existing structures.