HVSR Measurements to Investigate Sinkholes and Treatment Efforts along a Roadway

The evaluation of sinkholes continues to present challenges when managing highway infrastructure given current geotechnical subsurface investigation techniques. Typical drilling efforts combined with standard penetration testing (SPT) and cone penetration testing (CPT) provide useful information, but only at a limited number of points throughout the site. Such efforts are also not routinely utilized after the implementation of sinkhole treatment efforts as a means of quality control due to logistical challenges and costs. The horizontal-to-vertical spectral ratio (HVSR) method, which records ambient seismic noise using a single, broad-band three-component seismometer, can be rapidly deployed throughout a site and provides information regarding the stiffness of the subsurface materials. Repeated HVSR measurements at the same locations can potentially identify problematic areas present after the implementation of sinkhole treatment efforts. In this paper, HVSR measurements were collected along a roadway where known sinkholes were present. The HVSR testing locations were collocated with boreholes to allow for a comparison with the inferred subsurface profile from geotechnical investigations. The roadway was then subjected to an extensive grouting program to treat the active sinkholes. HVSR measurements were subsequently repeated at the same pre-grouting locations to investigate changes in the subsurface stiffness. The results exhibited an increase in HVSR amplitude toward lower frequencies when comparing the pre- and post-grouting ambient noise measurements, possibly indicating a stiffening of the soil post-grouting. Despite rejection of time windows that exhibited evidence of near-field anthropogenic waveforms, there was still significant variance in the results attributed to poor coupling of the seismometer (i.e., pavement surface and gravelly fill on the roadway shoulder), adverse weather conditions (i.e., wind and rain), and the complexity of the site conditions (i.e., the extent and spatial variability with which grout flowed into voids and interaction of grout with nearby buried utilities). Use of HVSR as a quality control tool for sinkhole treatment efforts should be further investigated with consideration of the complexities that can affect such variance in the results.