Development Mechanism and Remediation of Multiple Spontaneous Sinkholes: A Case History

In September 1999 a Regional Distribution Center in North Florida that included a 10 hectare single cell storm water retention basin was nearing completion. Runoff to the basin from a three inch rainfall event resulted in the occurrence of a cluster of multiple and spontaneous sinkholes within the basin. Compaction grouting was used to stabilize the sinkholes. In early 2000, as the basin partially filled from relatively small rain events and more sinkholes were experienced, which were repaired by sand filling. In late June 2000, a heavy storm filled 40% of design storage and resulted in about two dozen spontaneous sinkholes that emptied the basin rapidly. Upon review, evaluation and consultations, future multiple spontaneous sinkhole events were forecasted within other parts of the basin, and a Remedial Response Protocol was provided. In July of 2001, a heavy rain event filling about 60% of design capacity resulted in thirty sinkholes within forecasted areas, which were remediated in accordance with the response protocol. The magnitude and frequency and repeated occurrence of the sinkholes became a media event with considerable public concerns. The concerns were addressed to the satisfaction of the regulatory agencies and reasonable assurances were given that the sinkhole events have matured and did not result in environmental degradation of the ground water. Since then, even though the basin has experienced about 60% filling, no new multiple sinkholes of consequence have taken place. While multiple sinkholes in retention basins are frequently experienced, to our knowledge, no prior explanation has been made as to the reasons for Multiple Spontaneous Sinkholes. This paper provides a brief history of the project, the area hydrogeologic setting, and an explanation of the mechanism of Multiple Spontaneous Sinkholes. Remedial measures are presented to reduce or mitigate sinkhole events and the potential for contamination of groundwater.