pg(s) 79-88 Induced
sinkholes are a known geologic hazard and may be associated
with construction activities that cause alteration of ground
water flow patterns or induce rapid loads and/or vibrations on
karst-affected soils and rocks. This study describes the
geophysical and geotechnical investigation of a site in
northern Hillsborough County, Florida, where a large diameter
underground high-pressure natural gas pipeline was installed
utilizing horizontal directional drilling (HDD) methods.
Objectives of the investigation were to evaluate the impacts
of: 1) pipeline installation on existing ground-collapse
features, 2) potential induced ground subsidence and 3)
possible effects on water bodies and building structures. The
site was investigated utilizing geophysical testing (electrical
resistivity), standard penetration test (SPT) borings, and
ground vibration monitoring during pipeline construction. In
the investigation, subsurface conditions indicative of possible
preexisting weakened soil and rock materials associated with
incipient raveled zones in overburden soils and soil-filled
conduits in limestone bedrock were found in proximity to the
pipeline corridor. During the HDD boring and pipeline
installation, noticeable ground vibrations occurred, along with
formation of several ground settlement/collapse features. The
data suggest two mechanisms of induced sinkhole formation:
erosion of weak zones in overburden soils by the high pressure
drilling mud and/or erosion of weak, soil-filled conduits in
limestone bedrock. In addition to current settlement impacts to
the property, the investigation found a potential for future
ground subsidence associated with undetected eroded and raveled
zones that may in the future propagate to the land
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 79 those triggered by water level declines, typically from ground water withdrawals (pumping), or those related to Florida, sinkhole formation and ground subsidence accompanying heavy ground water pumping are common occurrences and are typically associated with rapid declines in the potentiometric surface of the Floridan aquifer. This causes an increase in effective stress over formed by dissolution. Increased pumping can alter the downward raveling of overburden soils. Pumping can incipient raveled soil zones in the subsurface. Sinkhole formation can also be triggered by construction activities such as water impoundment in reservoirs and retention basins, ground loading, ground vibrations from heavy equipment, changes to natural drainage patterns by diversion of stormwater, and drilling of borings and water wells. The mechanisms activating sinkhole formation would include increased ground rocks associated with incipient sinkhole conditions. This paper presents a case history of a geophysical and geotechnical investigation conducted at a site in northern investigation was to evaluate impacts from installation karst subsidence feature(s), potential for induced ground subsidence, and impacts on building structures Abstract Induced sinkholes are a known geologic hazard and may be associated with construction activities that soils and rocks. This study describes the geophysical and geotechnical investigation of a site in northern installed utilizing horizontal directional drilling to evaluate the impacts of: 1) pipeline installation on bodies and building structures. The site was investigated utilizing geophysical testing (electrical resistivity), standard penetration test (SPT) borings, and ground vibration monitoring during pipeline construction. In the investigation, subsurface conditions indicative of possible preexisting weakened soil and rock materials associated with incipient raveled zones in overburden found in proximity to the pipeline corridor. During the vibrations occurred, along with formation of several two mechanisms of induced sinkhole formation: erosion of weak zones in overburden soils by the high pressure in limestone bedrock. In addition to current settlement impacts to the property, the investigation found a potential for future ground subsidence associated with undetected eroded and raveled zones that may in the future propagate to the land surface. Introduction Induced sinkholes are caused or accelerated by human activities and are associated with two broad conditions: INDUCED SINKHOLE FORMATION ASSOCIATED WITH INSTALLATION OF A HIGH-PRESSURE NATURAL GAS Ted J. Smith, P.G., C.P.G. Earth & Water Resources, LLC, 2000 S. Florida Ave., Lakeland, Florida, 33803 USA, email@example.com George C. Sinn, Jr., P.E. Central Florida Testing Laboratories, Inc., 12625 40 th St., Clearwater, Florida, 33762 USA, firstname.lastname@example.org
NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 80 clayey sediments and carbonate rocks of the Miocene landforms including sinkhole lakes, cypress domes, and broad wetland basins. Numerous lakes and swamps are were created by karst processes resulting in broad and water bodies near the property. The study methods included an electrical resistivity survey of the pipeline corridor, subsurface testing by standard penetration test (SPT) borings, and ground vibration monitoring during pipeline installation. Location and Geologic Setting approximately 20 acres in size and is bordered by and vegetated wetland areas on the northern side. The with ground elevations ranging from approximately 22 Lake to the south and east, and the fringing wetland areas on the north side. Two large residential structures and associated outbuildings are currently located on the property (Figure 2). Geology Karst Plain (Scott, 2005), which encompasses much of The geomorphic province is formed by a series of Pleistocene age marine terraces developed on sandy and Figure 1. Location Map, Hillsborough County, Florida. Figure 2. Geophysical Testing and Boring Location Plan showing pipeline easement, building structures, and testing locations.
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 81 are common sites of paleo or relic sinkhole activity and and Floridan aquifers. Karst and Sinkhole Development sediments. They also constitute a geologic hazard since greatly depending on the degree of consolidation of and conduits in the limestone formation, and the hydraulic connection with the deeper aquifers. Lakes and wetland and may serve as recharge areas to the Floridan Aquifer. Pipeline Construction horizontal directional borehole located within an property. The easement is approximately 15 meters wide to east alignment extending from the northwest corner Island Lake. The easement passes in close proximity to the main residential building and outbuildings (Figure 2). The borehole for the pipeline was completed using wetland basins and lakes formed by multiple coalescing sinkhole and subsidence features. Peninsular Florida is underlain by a thick sequence of Paleogene carbonate rocks that form the Florida platform, which is capped by a thin series of Miocene sediments. Important geologic and hydrogeologic units Table 1. Descriptions of each unit are taken from Arthur et al. (2008), Campbell (1984), and Scott, (1988). Hydrogeology Two principal hydrogeologic units are present in the is hosted primarily by permeable sandy soils within Upper Floridan aquifer, which is hosted primarily in the Suwannee Limestone and deeper limestone formations (Ocala Limestone and Avon Park Formation). potentiometric surface corresponding with the water table, which occurs at depths of less than 1.5 meters (5 the Upper Floridan Aquifer system in the area occurs at permeable sandy sediments. Lakes and wetland basins System Series Lithostratigraphic Unit Lithology Hydrostratigraphic Unit Quaternary Undifferentiated Sands and Clays minor silty, clayey and organic soils, local deeper clayey soils and shell beds Pleistocene Tertiary Miocene Tampa Member: Arcadia Formation White to tan, quartz sandy, locally clayey, fossiliferous limestone and dolostone, per portion Upper Floridan Aquifer Oligocene Suwannee Limestone grainstone Table 1. Regional Geologic and Hydrogeologic Units, Northern Hillsborough County.
NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 82 The modeled data were then analyzed with respect to identifying anomalous electrical resistivity signatures associated with potential subsurface karst features based on the following criteria: Columnar patterns of higher resistivity materials extending to the depth of apparent limestone bedrock. zones of soil raveling. Patterns indicative of localized increased depth of features. soils overlying moderate to low resistivity materials correlated with limestone occurring at approximate portion of the survey area located north and northwest of the main residential building. The anomalies were characterized by areas of higher resistivity materials at depth representing possible zones of downward raveling sands extending to limestone bedrock. The anomalies were subsequently tested by SPT borings Geological Conditions Standard penetration test (SPT) borings were used to investigate subsurface conditions and geophysical anomalies within the pipeline corridor. The borings were conducted using conventional mud rotary drilling methods, in general accordance with ASTM performed along the alignment of the proposed gas pipeline and advanced to depths ranging from ground surface. The boring locations are shown on Figure 2. The subsurface geological and geotechnical data were used to construct a geologic profile of the pipeline corridor, depicted in Figure 4. Three generalized subsurface units or soil strata were encountered in the borings: the easement for the new pipeline. Although the construction details and depth of the older pipeline are not known, it was presumed that it was installed using concern at the site is an existing ground collapse feature located near the existing smaller gas pipeline (Figure 2). The collapse feature is oval in shape with dimensions of opinion that it is an induced sinkhole collapse associated with the installation of the existing gas pipeline. Minimal geotechnical data were collected by the pipeline contractor, consisting of two SPT borings drilled at each end of the pipeline alignment and advanced to depths of Electrical Resistivity Survey An electrical resistivity imaging (ERI) survey was conducted over accessible portions of the pipeline corridor and surrounding area to assist in characterizing the general subsurface geology and identify potential anomalous subsurface geological features for further investigation. The ERI survey consisted of four transect lines using up transects oriented approximately west to east were located within the pipeline corridor, with lengths of approximately 270 and 250 meters (885 ft. and 820 ft.). Additionally, two shorter transects, approximately 128 and 41 meters the pipeline corridor, were completed in the eastern and western portions of the property. The western transect was located near the western side of the masonry outbuilding and the existing ground collapse feature. The locations of the ERI transects are shown on Figure 2. The resistivity data were collected using a combined
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 83 was encountered at variable depths, ranging from 10 to the site. The limestone bedrock deepens to the west to depths in excess of 24 meters (80 ft.). The subsurface soil and rock materials exhibited a variable density and consistency over the depth of the SPT borings. Isolated zones of very loose and very soft soil and rock materials over intervals of 0.5 to 2 meters (1.5 to 6.5 ft.) were encountered in several borings in the western and eastern portions of the site. The weak zones within the limestone bedrock. circulation losses typically occurred in association with very loose zones within the clayey soil unit (Stratum 2) within the limestone bedrock. Pleistocene deposits. The limestone bedrock correlates Stratum 1 Sand, Sand with Clay, Peat Stratum 2 Sand with Clay, Clayey Sand, Sandy Clay sand, with localized near surface deposits of peat (1 to sand (Stratum 2). The soil unit has a variable thickness, extending to depths ranging from approximately 7 to 14 meters (22 to 47 ft.) in eastern and central portions the entry point of the gas pipeline at the northwestern corner of the property. Sandy soils similar in appearance portion the property. Stratum 2 consists of variably clayey sand and minor sandy clay and was encountered as local shallow lenses zones within the limestone. Figure 3. Electrical Resistivity Profiles.
NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 84 Figure 4. Geologic section of pipeline alignment.
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 85 employed to maintain neutral buoyancy in the pipeline. was apparently utilized to facilitate the installation of the pipeline through the borehole, resulting in noticeable ground vibrations in the area. Monitoring of site conditions were conducted over the The monitoring consisted of a review of available daily as well as periodic inspections of the property. Several boring and installation of the gas pipeline. These included features and associated discharges of drilling mud near the pipeline easement in the northwestern portion of the site (Figure 5). Noticeable ground vibrations also occurred largely during installation of the pipeline within the borehole. Ground Vibration Monitoring concerns regarding potential damage to building structures. Excessive ground vibrations and ground The ground vibration monitoring utilized a remote seismograph system, which measures peak particle velocity, frequency, and air overpressures produced by vibration sources. The system records on a continuous basis in a histogram recording at a rate of 1000 samples per second with a maximum peak particle velocity with Suwannee Limestone (Oligocene). A thin section of the Tampa Member of the Arcadia Formation (Miocene) also appears to occur in the upper portion of the limestone Suwannee Limestone (Arthur et al., 2008). Monitoring of Pipeline Construction As described, the gas pipeline was installed through a process involving the following phases: drilled. The borehole orientation is controlled by varying the angle of the drill bit. During drilling, the location of the borehole is surveyed using electromagnetic methods. Reaming Process: Upon completion of the pilot hole, the borehole is enlarged. Reaming tools with increasing diameters are alternatively pushed and pulled in multiple diameter. a mud pass or packer reamer is passed through the directional borehole to clean and remove soil and rock back of the steel pipeline. through the reamed borehole. A weld cap and swivel are welded to the end of the pipeline, which is then attached to the drill string to prevent rotation of the pipeline as it is pulled through the borehole. Depending on the Figure 5. Photographs of blowout and ground collapse features.
NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 86 protection of strawberry crops. These sinkholes caused substantial damages to private and public buildings and infrastructure. Sinkhole formation in karst areas can also be triggered by construction activities such as water impoundment in reservoirs and retention basins, loading and ground vibrations from heavy equipment, changes to natural drainage patterns by diversion of stormwater, and drilling of borings and water wells. Examples of sinkholes have frequently formed in stormwater retention basins, typically following heavy rain known to occur during drilling of water wells and even geotechnical borings. In addition, the authors are familiar with an occurrence of induced sinkhole activity in 2007 during construction of the Land O' Lakes water reuse reservoir. At the site, numerous bentonite seepage cutoff wall within a dike associated with the reservoir. The sinkholes likely developed in response to the increased hydraulic loadings imposed voids near the dissolutioned limestone bedrock surface. Induced Sinkhole Formation collapse features or induced sinkholes formed in to greater than 10 ft. in diameter and were located within or near the pipeline easement in the western portion of the site (Figures 2 and 5). Drilling mud was discharged indicating a hydraulic connection with the underlying These induced sinkholes are theorized to have been triggered by increased stresses on preexisting weakened soil and rock materials associated with incipient sinkhole conditions in the subsurface. Based on the SPT borings, such conditions appear to exist as weakened or partially raveled zones within the overburden soils above the dissolution conduits within the limestone bedrock. These induced sinkholes which formed in association with the previous and current pipeline construction, and (PPV) recorded every minute. Data are transmitted via a cellular telemetry system. The geophone records vibrations on the longitudinal, transverse, and vertical pipeline easement near one of the masonry outbuildings (Figure 2). to evaluate ground vibrations, vibration data are often compared to U.S. Bureau of Mines (USBM) criteria for evaluating damage to building structures from blasting in mines and quarries (Siskind et al., 1980). While ground vibrations can be perceived by humans at very low levels cause damages to building structures are much higher, vibration levels were measured during the monitoring period. Maximum daily peak particle velocities (PPVs) correlate, in part, with installation of the gas pipeline use of the hammering device and perceived ground ground vibrations were well below USBM criteria for building damage, ranging from 1% to 2% of the limits not met, neighboring homes felt the vibrations associated with advancement of the pipe and complained of the noise created by the hammering efforts associated with installation of the pipe through the borehole. Geologic Hazards Analysis Geologic Hazards Based on local geologic conditions at the site, induced numerous examples of induced sinkholes related to ground water pumping in the region, including the vicinity of the Section 21 and South Pasco well recently, large numbers of sinkholes developed in 1998 development of a large capacity water production well. In 2009 and 2010, multiple sinkholes formed in association with heavy irrigation pumping for freeze
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 87 any overburden soils below the boring and into the limestone bedrock. These weakened soil zones would remain in the subsurface and may trigger future ground settlement as the drill mud dissipates with time. There is also a probability that additional zones of eroded and raveled soil zones are present in the subsurface that have not yet propagated to the land surface. The subsurface data from the geotechnical investigation are favorable for incipient sinkhole conditions over much of the pipeline corridor. An abrupt increase in depth to limestone bedrock along with zones of weak soils was found in SPT borings located in the northwestern portion of the site. The large wetland basin in the area appears to correspond with the underlying basin feature in the limestone, suggesting the presence of a large strength overburden soils and very loose and soft soil borings. Collectively, these data suggest a probability limestone. Depending on the degree of erosion, these weakened zones may have developed to the point to serve as potential locations of future ground settlement. would encompass the pipeline easement and potentially affecting building structures on the site. Acknowledgements We gratefully acknowledge support of the property owner who funded the investigation. The geophysical survey and ground vibration monitoring were performed & Scientists, Inc., who employed the senior author during References Arthur JD, Fischler C, Kromhout C, Clayton JM, Kelley the Southwest Florida Water Management District. Florida Department of Natural Resources Bureau indicate that induced sinkholes should be considered a and pipeline construction in the region. Two mechanisms of induced sinkhole formation appear to be occurring at the site, depending on whether the limestone. In the western portion of the site, where the soils, the large diameter borehole entailed the removal lead to further weakening of any incipient raveled zones volumes of drilling mud utilized in the drilling would the weak soil zones, and triggering further raveling. The larger of these weak zones eventually propagated upward to the land surface resulting in ground settlement and collapse. boring reached its target depth and was advanced through features within the Suwannee Limestone, occurring as that often connect with deeper cavern systems (Yon the abundance of these features in the region and their presence in the SPT borings at the site, it is probable these structures were encountered during advancement of the the drilling mud would cause erosion of weaker zones in near the bedrock surface and triggering raveling of the overburden soils. These conditions could also result in ground settlement or collapse if the raveling propagates close to the land surface. events at the site, there is an elevated risk of further and pipeline installation. It is important to note that the induced sinkhole features that occurred in association connection between the limestone, the pipeline bore, and the land surface. These connections allowed the drilling mud from the borehole to discharge to the surface during drilling operations, but may have also extended through
NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 88 Publication No. 122. ASCE, Reston, VA. Survey Bulletin No. 59. Scott TM. 2005. Revisions to the geomorphology of Florida focusing on the Eastern Panhandle and RL. 1985. Types, features, and occurrence of sinkholes Siskind DE, Stagg MS, Kopp JW, and Dowding produced by ground vibration from surface mine blasting. US Bureau of Mines Report of In Land subsidence in the United States. US Wilson WL, Shock EJ. 1996. A description of quantitative methods for modeling new sinkhole frequency, size distribution, probability, and risk, based on actuarial statistical analysis of the new sinkhole data spreadsheet. Unpublished report by Subsurface Evaluations, Inc., Winter Park, Florida.