Monitoring upstream sinkhole development by detailed sonar profiling

A number of large (10-20m diameter) sinkholes are known to exist upstream from a ageing earth dam. The dam is founded on deep sands and gravels, and the sinkholes have developed due to improper construction of the original upstream impervious blanket. Soundings conducted over the years have indicated that these sinkholes redevelop after being infilled. Detailed sonar profiling was recently developed in order to enhance the soundings, and has achieved accuracy and repeatability in the range of a 100mm. Precision of the system is such that small areas can be re-surveyed and differences can be defined with confidence. Overhangs and ‘pipes’ in sinkholes can be surveyed in great detail. This is possible by lowering the sonar head to immediately above the reservoir bottom, correcting the data for any underwater pitch, roll and yaw of the head, and positioning the system via total station surveying techniques. The system enabled the detection of a previously unknown small sinkhole at the immediate toe of the structure. Regular profiling of the sinkhole was conducted, that confirmed on-going development of an angled ‘pipe’ at the bottom of the sinkhole, with a diameter of about 1m and a minimum length of 15-m. Risk-reduction and remedial works schemes were initiated. The detailed sonar profiling was used to gauge the effectiveness of an infilling program, Variations in density between the infill material and the dam fill can be detected by manipulation of gain and frequency. Results suggest the development of a new sinkhole that has not yet broken through to the face of the dam. The sonar system continues to be an integral part of the monitoring of this structure while an accelerated program of remediation is implemented. The system has proven to be accurate, repeatable, and cost effective. It provides a new tool for the assessment of reservoirs and upstream slopes of earth dams. 11 pp., 8 figures.