sinkholes treatment

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

S.E. Jammal1, M. ASCE, P.E., J.W. Casper2, M. ASCE, P.E. and A. M.

Sallam3, M. ASCE, Ph.D., P.E 1Nodarse & Associates, Inc., 1675 Lee Road, Winter Park, Florida, USA; PH

(407)740-6110; FAX (407)740-6112; e-mail: [email protected] 2Nodarse & Associates, Inc., 1675 Lee Road, Winter Park, Florida, USA; PH

(407)740-6110; FAX (407)740-6112; e-mail: [email protected] 3Nodarse & Associates, Inc., 1675 Lee Road, Winter Park, Florida, USA; PH

(407)740-6110; FAX (407)740-6112; e-mail: [email protected]

ABSTRACT 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.

681GeoFlorida 2010: Advances in Analysis, Modeling & Design(GSP 199) © 2010 ASCE

INTRODUCTION Beginning in the fall of 1999 when a regional distribution facility in North

Florida was nearing completion, several sinkhole events were experienced within the project stormwater retention basin and its immediate vicinity. These were relatively small, were readily mitigated and generally did not re-open. In June 2000 and July 2001 several additional spontaneous sinkholes opened in and near the stormwater retention pond after moderate storm events. Appropriate response guidelines were developed in concert with the owner and contractors to mitigate water regulators’ concerns for potential groundwater contamination. To make these guidelines acceptable for the regulators and interested public, it was first necessary to explain the cause and mechanism of the multiple sinkhole events. Project Background Sinkholes are a common occurrence and part of the landscape in most of Florida, including the region of North Florida where the distribution center is located. Prior to design and construction, the geotechnical conditions within the project site were explored and evaluated by others. Within building and paved areas, 90 borings were performed, while five only borings were made within the 10 hectare retention pond. The study also included a Ground Penetrating Radar (GPR) survey with transects taken mainly within building and paved areas. The original geotechnical report concluded that the site was well-suited for the proposed construction, and recommended use of shallow foundations after compaction of the building area with deep dynamic compaction to densify the soils in and above the vertical solution features in the limestone to further reduce the likelihood of soil erosion and sinkhole development. The building area was mostly filled. Most of the paved areas were also filled. The retention pond was cut 1 to 5 meters. At its highest level, the basin could hold as much as 3 meters of water.

Geologic Setting The site lies along the foothills of the Northern Highlands Marginal Zone and within the Western Valley. The Western Valley is a subdued limestone plane composed of the Ocala Group Crystal River Formation overlain by a thin and variable soil cover and occasional residual fills composed of sediment of the Hawthorn Group. Sinkholes and low hills composed of erosional remnants of the Northern Highlands provide much of the topographic relief. Overlying the limestone is a relatively thin layer of sand and soil cover including common residual chert boulders. Residual sediments frequently fill sinkholes and tend to mask the degree of irregularity of the limestone surface. Innumerable sinkholes divert all runoff underground and are the cause of the lack of surface streams. As a result of the geology described above, the limestone can be expected to have karst features including solution pipes in the limestone. Exposed limestone surfaces may be riddled with solution pipes. Figure 1 illustrates an example of the irregular limestone surface, from an exposed excavation not far from the subject site. Figures 2 and 3 illustrate solution pipes that are still filled with a soil plug (Figure 2) and where the plug has been washed out (Figure 3).


Figure 1. Illustration of irregular limestone surface.

Figure 2. Figure 3. Solution pipe with soil plug still in place Solution pipes with soil plugs washed out


Retention Basin Subsoil Conditions Figure 4 shows the locations of borings made within the retention basin and corresponding results. Based on the review of the geologic literature of the area and the results of the deeper borings made within the building area, Figure 4 was “enhanced” to characterize the hydrogeologic conditions below the basin. Figure 4 provides a reasonable characterization of the subsoil and hydrogeologic stratigraphy within and below the retention basin. The pond bottom was expected to be approximately 3 to 6 meters above the general limestone surface, and about 8 meters above the piezometric level within the limestone. Figure 4 also depicts the “Solution Pipes” that can be expected to dot the limestone below the site. The solution pipes may be filled or partially filled and are typically connected to a network of interconnected voids and fissures in the limestone. Due to changes in piezometric levels over the millennia, the surface of the limestone and the interconnected voids may extend above the piezometric surface, as is the case for the subject site. Prior to pond excavation, the average cover of sands, clayey sands and clays above the limestone was approximately 10 meters. Recharge to the limestone aquifer occurred mainly as a diffuse flow, concentrate to some degree in topographic lows which are typically influenced by long-term solution activity and slow raveling of soils through the solution pipes. Under natural conditions of diffuse flow, surface collapses would be a rare event as soils were slowly transported downward into solution pipes and the underlying system of cavities. In the higher topographic areas, the solution activity would be somewhat less due to the natural subsurface groundwater flow toward low areas. In the course of construction of the 10 hectare excavation, previous topographic highs and lows were masked, the soil cover over the limestone and its solution pipes was reduced, and the pond would allow concentration of water to a depth that would not have been experienced by the subsoils under a diffuse infiltration scenario. Sinkhole Events Construction of the 25 acre single cell storm water retention basin was nearing completion in September 1999, when an 8 to 10 cm rainfall depth storm event resulted in the occurrence of a cluster of few multiple and spontaneous sinkholes. The sinkholes which occurred would generally be considered “cover-collapse type sinkholes, as the actual solution pipes or “avens” were not visible. Initially and before the authors were involved, compaction grouting was used to stabilize the sinkholes. This is not an unusual approach, but if compaction grouting were used to repair every sinkhole in a normally dry pond, eventually the ability of the pond to drain and recover would be diminished. Early in 2000, as the basin partially filled from relatively small rain events, more sinkholes were experienced, which were repaired by sand filling. Late June 2000, a heavy storm filled 40 percent of design storage resulted in about two dozen spontaneous sinkholes that emptied the basin rapidly. Upon our review, evaluation and consultations, future multiple spontaneous sinkhole events were forecasted within other parts of the basin and a remedial response protocol for repairs was provided.


In July of 2001, a heavy rain event filled 60 percent of design capacity resulted in thirty sinkholes within forecasted areas, which were remediated in accordance with the response protocol. As reported in 1999 to 2001, when most of the sinkholes occurred, the pond had held just over 1 meter of water depth. However, considering that the pond can hold almost 3 meters of water depth, there remains some risk that more sinkholes can develop at greater depths of water not yet experienced. Of interest and as would be expected, the sinkholes are generally concentrated within or near closed topographic lows. Sinkhole Mechanisms Figure 5 illustrates sinkhole mechanisms interpreted for the site. In upland areas, the cover collapse sinkhole over a solution pipe would tend to be fairly small in diameter with roughly vertical sides, as there is no water to erode the sides to a flatter condition. This is illustrated on panel (a) of Figure 5. The collapse of the overlying sands into the pipe is typically “self-healing”, and the steep sides would eventually flatten over time if not filled. Also shown on panel (b) of Figure 5 is a typical scenario for sinkhole occurrence within a pond with conditions similar to the site. If sufficient sand cover remains above the limestone, the sinkhole can be- self-healing, with the water action washing sand in to fill the solution pipe. When several spontaneous or simultaneous sinkholes were experienced in the proximity of each other in July 2001 the basin was holding about 1.8 meters of water or a volume of about 75,000 cubic meters. We believe that the sinkhole events are related to the size of the basins and the volume of water being held just preceding the development of the first sinkhole. Over the years we have observed that small basins of say 0.5 hectare, develop one or two sinks and the number increases with the size of the basin as well as the corresponding water volume from the basin. We believe that when one sink develops, the “rush” of the large water volume spreads through the unsaturated interstices within the limestone. If the interstices interconnect with several solution pipes as illustrated on panel (c) of Figure 5, the rush or force of the water would have a tendency to erode or collapse the cover, filling the other pipes from the bottom-up, thus creating clusters of small sinks in the proximity of the original or initial sink. This explanation for the mechanism of spontaneous sinkholes is significant. It suggests that the draining water from the basin does not initially go directly into the saturated zone of the Floridan aquifer, but is mostly dissipated or lost within the unsaturated interstices or “voids” within the upper surface of the limestone. Hence, the movement of the water is not straight down but travels along a “tortuous” path in just about all directions and is initially very turbulent.


Repair Protocols For the actual events or conditions experienced at the project site, the most effective and most applicable repair method was to fill the sinks with native nearby sands. Filling the sinks with native sands as was recommended and practiced to date, is effective because it improves the quality of stormwater by filtration before it reaches the groundwater of the Floridan aquifer, being about 6 to 8 meters below the bottom of the retention basin. A sand fill also allows infiltration to continue to occur, which is important for recovery of storage volumes in a dry pond. The filling of the actual sinkholes with relatively clean native sands from the site also allows further infiltration to carry more sand into the various interstices and pore spaces in the limestone. This also diminishes the chance of sinkhole recurrence. Of significance for the project site is the fact that previously filled sinks have generally not reopened. This is consistent with what we have observed and learned over the years. A gravel plug is not appropriate because it would not provide sufficient filtration for groundwater quality protection. On the other hand, a clayey sand material may be appropriate as a final plug in instances where the pond is intended to remain wet and infiltration of stormwater is not necessary.

CONCLUSIONS The phenomenon of spontaneous sinkholes is due in large part to the availability of large volumes of water which may drain into interconnected interstices in primarily unsaturated zones in the limestone. Consideration should be given to dividing large stormwater ponds in karst areas into multiple compartments, to possibly minimize the amount of available water to flow into any sinkhole and reduce the “spread” of simultaneous sinkholes. The repair method practiced to date, where sinks are filled with native sands, is currently the most effective and most technically sound. Suggestions that such filling has caused or might cause contamination of the Floridan aquifer are unfounded if the sand backfill has a gradation appropriate for effective filtration. Because the native sands on site did not appear excessively coarse and permeable, we believe that past and near future sinkhole events did not and would not pose a pollution threat, and have not recommended further modifications of the protocol. Based on history of events, the current repair practices are proving to be successful, in that, while new sinkholes have developed since June, 2000, the new sinkholes are happening in other parts of the basin. To date we have not observed reoccurrence of previously filled sinkholes. The facility owner has also reported to us a diminishing frequency of sinkhole occurrence over time.

REFERENCES Williams, K. E., Nicol, D. and Randazzo, A. F. (1977). “Report of Investigations No.

85 - The Geology of the Western Part of Alachua County, Florida.” Bureau, of Geology, Department of Natural Resources, State of Florida, Tallahassee, Florida USA 98p.

Randazzo, A. F. and Jones, D. S., ed. (1997). “The Geology of Florida.” University Press of Florida, Gainesville, Florida USA 327p.


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