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Development of sinkholes in a thickly covered karst terrane

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Title:
Development of sinkholes in a thickly covered karst terrane
Alternate Title:
NCKRI Symposium 2: Proceedings of the Thirteenth Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst
Creator:
Upchurch, Sam B.
Dobecki, Thomas L.
Scott, Thomas M.
Meiggs, Steven H.
Fratesi, Beth
Alfieri, Michael C.
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University of South Florida
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English

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Conference Proceeding
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pg(s) 273-277 A cluster of aquifer drawdown-induced sinkholes developed in eastern Hillsborough County, Florida (west-central Florida), during two major freeze events in 2010. The sinkholes resulted in millions of dollars in losses and caused us to revise our thinking about how sinkholes form in a terrain normally considered to have low sinkhole risk owing to thick, clay-rich cover. The cover material consists of the Miocene Hawthorn Group, which includes up to 120 m of interfingering expansive clay, sand and sandy clay, and carbonate strata. The lower Hawthorn Group Arcadia Formation is primarily carbonate and is up to 90 m thick. The upper Hawthorn Group Peace River Formation contains more clay and sand with minor amounts of carbonate and is up to 30 m thick. The Hawthorn Group constitutes an effective aquitard for the underlying upper Floridan aquifer (UFA), which is composed of karstic, Oligocene and Eocene limestone and dolostone. A rapid drawdown of up to 20 m in the potentiometric surface of the underlying UFA resulted in mobilization of water-saturated clays and clayey sands within the Hawthorn Group. Subsidence and possible clay consolidation resulting from dewatering and loss of support/buoyancy caused development of new sinkholes and reactivation of clay-filled sinkholes that had developed as early as the Miocene Epoch. Stable, clay-filled, relict sinkholes of apparent Miocene age discovered in an earlier investigation in the same area in 1998-1999 support the presence of clay-filled, relict sinkholes in the area. Combining information gathered from study of these modern and relict sinkholes presents evidence of sinkhole development mechanisms in the thickly covered karst of west-central Florida.
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k26.1269 ( USFLDC Handle )
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Description
pg(s) 273-277 A
cluster of aquifer drawdown-induced sinkholes developed in
eastern Hillsborough County, Florida (west-central Florida),
during two major freeze events in 2010. The sinkholes resulted
in millions of dollars in losses and caused us to revise our
thinking about how sinkholes form in a terrain normally
considered to have low sinkhole risk owing to thick, clay-rich
cover. The cover material consists of the Miocene Hawthorn
Group, which includes up to 120 m of interfingering expansive
clay, sand and sandy clay, and carbonate strata. The lower
Hawthorn Group Arcadia Formation is primarily carbonate and is
up to 90 m thick. The upper Hawthorn Group Peace River
Formation contains more clay and sand with minor amounts of
carbonate and is up to 30 m thick. The Hawthorn Group
constitutes an effective aquitard for the underlying upper
Floridan aquifer (UFA), which is composed of karstic, Oligocene
and Eocene limestone and dolostone. A rapid drawdown of up to
20 m in the potentiometric surface of the underlying UFA
resulted in mobilization of water-saturated clays and clayey
sands within the Hawthorn Group. Subsidence and possible clay
consolidation resulting from dewatering and loss of
support/buoyancy caused development of new sinkholes and
reactivation of clay-filled sinkholes that had developed as
early as the Miocene Epoch. Stable, clay-filled, relict
sinkholes of apparent Miocene age discovered in an earlier
investigation in the same area in 1998-1999 support the
presence of clay-filled, relict sinkholes in the area.
Combining information gathered from study of these modern and
relict sinkholes presents evidence of sinkhole development
mechanisms in the thickly covered karst of west-central
Florida.



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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 DEVELOPMENT OF SINKHOLES IN A THICKLY COVERED KARST TERRANE Sam B. Upchurch, Thomas L. Dobecki, Thomas M. Scott, Steven H. Meiggs, Sarah E. Fratesi, Michael C. Alfieri SDII Global Corporation, 4509 George Road, Tampa, Florida, 33634, USA, supchurch@sdii-global.com duration hydraulic head stresses can result in rapid sinkhole development. The three investigations relate to sinkhole activity in different stages of development, but with apparently similar origins. From these three investigations, a case can be made for (1) rapid dewatering and consolidation or form sinkholes. The Three Investigations The three investigations are discussed below in chronological order of occurrence. Tampa Bay Regional Reservoir Investigation In 1999, we completed site characterization for construction 1999; Dobecki and Upchurch 2010). The site is located on the Polk Upland Physiographic Province (White 1970) and is underlain by a thick (up to 120 m) sequence of clay, sand, and limestone and dolostone of the Peace River and (Scott 1988; Arthur et al. 2008). The Miocene strata form an effective aquitard for the underlying limestone of the upper Floridan aquifer (UFA). discovered as part of the reservoir investigation (Figure 2). The reservoir embankment footprint was altered to avoid one feature that was of concern because of loose sediments in the subsurface; the second feature sandy clay typical of the Miocene of Florida. This unconformity. Abstract in 2010. The sinkholes resulted in millions of dollars in losses and caused us to revise our thinking about how sinkholes form in a terrain normally considered to have expansive clay, sand and sandy clay, and carbonate primarily carbonate and is up to 90 m thick. The upper clay and sand with minor amounts of carbonate and is effective aquitard for the underlying upper Floridan aquifer (UFA), which is composed of karstic, Oligocene and Eocene limestone and dolostone. A rapid drawdown of up to 20 m in the potentiometric surface of the underlying UFA resulted in mobilization consolidation resulting from dewatering and loss had developed as early as the Miocene Epoch. Stable, discovered in an earlier investigation in the same area sinkholes in the area. Combining information gathered from study of these modern and relict sinkholes presents evidence of sinkhole development mechanisms in the Introduction This paper synthesizes evidence from three karst investigations suggesting that migration or consolidation 273

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE The latter two features were geotechnically stable and levels at the reservoir are regularly monitored and it is clear that these relict sinkholes are isolated and stable. The latter two features are deemed to be safe for impounding water within the reservoir because they do not react to sudden stresses caused by drawdown in the underlying Floridan aquifer or to changes in reservoir stage. This is not the case for the two case studies that are described below. The Plant City Sinkhole Cluster Investigation In January 2010 a hard freeze with overnight temperatures UFA for irrigation to protect crops from freezing. The potentiometric surface of the UFA declined up to 18 m, with up to 9 m daily excursions in potentials. As a result, Testing of many of these sinkholes by the authors revealed a persistent pattern: there was a thick (up to 274 Figure 1. The Tampa Bay area of west-central Florida. Locations of the three investigations are shown in red, and known sinkholes that have formed since 1984 are shown in violet. Figure 2. North-south cross section through the reservoir to illustrate the locations and stratigraphic context of the Polk Upland sinkholes. Source: Dobecki and Upchurch 2010.

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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 moisture contents that were at or near the liquid limits of the clays. Under standard penetration (SPT) testing, surface sediments, there was little evidence of suffosion; resulting in sinkhole depths of up to 5 m and diameters The Southeast Hillsborough Landfill Investigation In December 2010 there was a second, but less severe, episode of rapid drawdown of the UFA as a result of pumping for freeze protection. Shortly after this event a 45 m wide and nearly 60 m deep sinkhole (Figure 5) km south of the reservoir site. sand, clay and limestone strata (Figure 6). The unique feature of this sinkhole is that there is no evidence of a bottom of the aven. void space where the materials had been removed from the stratigraphic column to form the aven and The sediments in this larger void space had also been previously removed. While the waste mass or the shallow limestone bed (Figure 6) may have bridged the void, a mechanism was required for the removal of the missing siliciclastic sediment. Refusal strength (N > 50 275 Table 1. Sample results from a standard penetration test boring adjacent to a sinkhole in Plant City. Lithology N Values* clayey sand seams were approx. 1.5 m thick clay for 5.5 m) chert Figure 3. A large sinkhole that developed by vertical movement near Frostproof, Florida. Note the intact trees in the down-thrown sediment plug. Figure 4. Sinkhole that developed in a suburb of Plant City. This was one of seven on the street. Sand has been placed in the foreground to reduce risk of additional damage to the street. Figure 5. The landfill sinkhole in southeastern Hillsborough County, Florida. Note the slump features developed within the waste mass.

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE clay, sand, and carbonate units (Figures 2 and 6). the underlying UFA. When extremely heavy withdrawals of groundwater from the UFA occur, lack of concomitant leakage from overlying water surface. 2. Karst features, including sinkholes, began unconformity; Scott 1988). Some sinkholes features thought to have formed this way at sediments that appear unaffected by modern hydraulic stresses. Elsewhere, some of these features appear to have been well consolidated, resulting in localized surrounds most of the void. As such, the walls of the the perplexing question is: where did the collapse material go? It appears that the sediments that occupied the void space had been washed out of the space at some earlier time, perhaps during the freeze event in January 2010 or even earlier. This would explain the absence of breakdown debris at the bottom of the void. The void, therefore, was either bridged over by the upper limestone, which was not detected as rubble at the base of the void, or perhaps the waste mass itself. 276 Figure 6. North-south cross section of the landfill sinkhole. (Note that depths are presented in feet and that the cross section was developed from standard penetration testing after initial stabilization of the sinkhole and backfilling to allow safe access for testing. CLSM is cementitious Controlled Low Strength Material used to cap the aven and stabilize the site) Source: on-going investigation by the authors. Epilogue Based on comparisons of the results from the three investigation areas, we suggest that the Polk Upland sinkholes developed as follows (Figure 7): 1. The Polk Uplands are underlain by the Miocene

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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 that links these sinkholes genetically with the Southeast The Polk Uplands remain a low sinkhole probability area, but when sinkholes develop they can be locally common, large, and catastrophic. The ones we investigated hydraulic gradients caused by groundwater withdrawals Florida Water Management District, has taken steps to water caution area and adopting additional regulations related to water use and sources. References Arthur JD, Fischler C, Kromhout C, Clayton JM, Kelley, the Southwest Florida Water Management District. 66, 102 p. 59 plates. to site characterization C.W. Bill Young Upchurch SB, Dobecki TL, Daigle DM. 1999. investigation. In: Law Engineering Services and White WA. 1970. The geomorphology of the Florida pockets of soft clay with natural moisture contents near or exceeding their liquid limits. 4. caused by groundwater withdrawals occur in the potentiometric surface of the UFA, some of these may fail, either by simple dewatering and rapid consolidation or by migration of the clay and associated sandy sediments into the voids of the As shown in Figure 7, our current concept as to how these sinkholes form is by migration of clay and clayey sand or the underlying Oligocene and Eocene limestone of the and clayey sands allow them to migrate farther laterally than might be otherwise expected into void space not directly beneath the sinkhole. This migration explains why there was no evidence of breakdown or collapse Clay and clayey sand remain under the sinkholes we tested near Plant City. In this case, either the clay and clayey sand were simply dewatered and consolidated or migration into nearby void space was incomplete. While this clay and clayey sand sediment is poorly sudden migration of water out of the clay mass would be hindered by the low intrinsic permeability of the material. Therefore, partial physical migration of the 277 Figure 7. Potential mode of sinkhole formation within the clay-rich strata of the Hawthorn Group on the Polk Uplands of Florida.

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 278