IMSRPX 622.209773 IL6msr no. 10 UiP^- Suju»*-V Final Report of Subsidence Investigations at the Rend Lake Site, Jefferson County, Illinois Brenda B. Mehnert Danny J. Van Roosendaal Robert A. Bauer Philip J. DeMaris Nelson Kawamura linois Mine Subsidence Research Program IMSRPX 1997 Cooperating agencies ILLINOIS STATE GEOLOGICAL SURVEY Department of Natural Resources BUREAU OF MINES United States Department of the Interior LIBRARY. Final Report of Subsidence Investigations at the Rend Lake Site Jefferson County, Illinois Brenda B. Mehnert Danny J. Van Roosendaal Robert A. Bauer Philip J. DeMaris Nelson Kawamura linois Mine Subsidence Research Program IMSRPX 1997 ILLINOIS STATE GEOLOGICAL SURVEY William Shilts, Chief Natural Resources Building 615 East Peabody Drive Champaign, IL 61820-6964 The Illinois Mine Subsidence Research Program (IMSRP) was established in 1985 to investi- gate methods and develop guidelines for underground mining operations that aim to maximize coal extraction yet preserve the productivity of prime farmland. The research program was initi- ated by the Illinois Coal Association and the Illinois Farm Bureau. The Illinois State Geological Survey, a division of the Illinois Department of Natural Resources, directed the IMSRP. Participating research institutions included Southern Illinois University at Carbondale, the University of Illinois at Urbana-Champaign, Northern Illinois University, and the Illinois State Geological Survey. A five-year Memorandum of Agreement, signed by the State of Illinois and the Bureau of Mines, U.S. Department of the Interior, ensured collaboration, coopera- tion, and financial support through 1991 . Major funding was also provided by the Illinois Coal Development Board. This publication is one in a series printed and distributed by the Illinois State Geological Survey as a service to the IMSRP. Appendixes to this volume are available upon request as Open File Series 1997-7 ILLINOIS DEPARTMENT OF NATURAL RESOURCES © Printed with soybean ink on recycled paper Printed by authority of the State of Illinois/1997/500 CONTENTS ABSTRACT 1 INTRODUCTION 1 Scope and Purpose of Work 1 Background and Previous Studies 1 Natural resources affected by mine subsidence, method of mining, overburden fracturing, hydrogeologic effects, surface subsidence characteristics Physical Setting 4 Site selection, physiography, surficial geology, bedrock stratigraphy and structure, hydrogeology Mine Characteristics 8 GEOTECHNICAL MONITORING PROGRAM 8 Introduction 8 Instrument layout, mine operations Surface Subsidence and Deformation Monitoring 10 Surveying, horizontal displacements, tiltplates Overburden Characterization 1 1 Exploratory drilling, laboratory testing for intact rock properties Overburden Deformation Monitoring 15 Time-domain reflectometry, inclinometer/sondex Hydrogeologic Investigations 17 Drift piezometers, bedrock piezometers, aquifer characterization RESULTS 20 Surface Subsidence Characteristics 20 Subsidence profiles, strain profile, tiltplates Overburden Characterization 24 Geotechnical core log, geophysical logs, intact rock properties Overburden Deformation Monitoring 27 Time-domain reflectometry, inclinometer/sondex Hydrogeologic Response to Subsidence 29 Drift water level response, bedrock water level response, aquifer characteristics INTEGRATION OF OBSERVATIONS 32 SUMMARY AND CONCLUSIONS 32 Surface Subsidence Characteristics 32 Overburden Characterization 34 Overburden Deformation 34 Hydrogeologic Response 34 RECOMMENDATIONS 34 ACKNOWLEDGMENTS 35 APPENDIXES (available upon request as Open File Series 1997-7) A Total Station Data B Longitudinal Surveys and Subsidence Calculations C Transverse Surveys and Subsidence Calculations D Closures on Controls E Horizontal Strain Calculations F Presubsidence Geotechnical Core Log G Presubsidence Geophysical Logs H Postsubsidence Geophysical Logs I Rock Mechanics Laboratory Data J Split-Spoon Sample Descriptions and Soil Lab Test K Data and Hydrographs of Drift and Bedrock Wells L Sondex Data REFERENCES 35 FIGURES 1 Diagram of the longwall mining technique 2 2 Strata deformation associated with longwall mining 3 3 Site location in Jefferson County, Illinois 4 4 Stratigraphic column of bedrock above the Herrin Coal at the study site 6 5 North-south geologic cross section through the study area 7 6 The study panels and adjacent portions of the Rend Lake Fault System 8 7 Instrument plan over panels 3 and 4 9 8 Frost-free monument design 1 1 9 General tiltplate installation 12 10 Schematic of the TDR installation 16 1 1 Schematic of the inclinometer/sondex installation 18 12 Schematic of the piezometer and pump well installation 19 13 Transverse subsidence profile development 20 14 Longitudinal subsidence profile development with mine face location 21 15 Panel 4 transverse strain profile for coal company's monument line 22 16 Subsidence, tilt, and face advance resulting from the dynamic subsidence wave 23 17 Subsidence, tilt, and curvature results using two monitoring techniques 24 18 Comparison of pre- and postsubsidence core logs 25 19 Presubsidence intact rock properties 26 20 Postsubsidence intact rock properties 26 21 Progression of TDR cable deformation at the centerline of panel 4 27 22 Comparisons between TDR events and sondex changes 28 23 Inclinometer data showing the location of the shear planes 29 24 Piezometric response of the drift piezometers due to precipitation and not mining 30 25 Piezometric response of the bedrock piezometers over panel 3 31 26 Water level in the well in the center of panel 3 31 TABLE 1 Sequence of subsurface events 33 ABSTRACT The purpose of this investigation was to study the amount, extent, and location of overburden frac- turing leading to the surface expression of coal-mine subsidence and to examine the effects of bedrock deformation on the local hydrogeology. Two longwall panels in Jefferson County, Illinois, were characterized before, during, and after subsidence by using core drilling, geotechnical instru- mentation, and in situ testing. Geotechnical instruments, including surface monuments, piezome- ters, a pump well, an inclinometer/extensometer (sondex), and two time-domain reflectometry cables, were used to monitor overburden and ground-surface response over a 5-year period. Ninety percent of the subsidence occurred within the first 3 months of undermining. Over this period, the ratio of subsidence to mined-out height was 63%. The long-term subsidence meas- ured over panel 3 increased the ratio of subsidence to mined-out height to 70%. The residual subsidence is probably induced by the closure of fractures generated during the collapse of over- burden. Static subsidence characteristics were within the range of values previously reported for the Illinois Basin. Drift (shallow) water levels showed no appreciable change during mining. Bed- rock water levels declined as the mine face approached, reached maximum lows when tensile strains (as measured on the surface) passed, showed a temporary recovery spike as the maxi- mum compressive strain passed, and then steadily recovered over a period of 3 months until pas- sage of the mine face of the adjacent panel. Above the panels, aquifer characteristics, such as hydraulic conductivity and storativity, were improved as a result of mining-induced fracturing. INTRODUCTION Scope and Purpose of Work High-extraction mining techniques are being used more frequently in Illinois to maximize coal min- ing productivity and to decrease the cost of the delivered product. Underground coal extraction by these techniques causes rapid collapse of the overburden and subsidence of the ground surface. Farmland and water resources may be affected by this surface subsidence. The Illinois Mine Sub- sidence Research Program (IMSRP) was created to address these concerns. This study is one of several projects performed under the IMSRP with funding from the U.S. Bureau of Mines (USBM), Illinois Department of Energy and Natural Resources Coal Development Board, and the Office of Surface Mining Reclamation and Enforcement. The purpose of this investigation was to study the amount, extent, and location of overburden frac- turing that leads to the surface expression of coal-mine subsidence and to examine the effects of the bedrock deformation on the local hydrogeology. Two longwall panels in Jefferson County, Illinois, were characterized before and after subsidence by using core drilling, geotechnical instru- mentation, and in situ testing. Geotechnical instruments, including surface monuments, piezome- ters, a pump well, an inclinometer/extensometer (sondex), and two time-domain reflectometry (TDR) cables, were used to monitor overburden and ground-surface response. The Illinois State Geological Survey (ISGS) monitored the instruments before, during, and after subsidence. North- ern Illinois University (NIU) assisted the ISGS in characterizing the hydrologic changes in the overburden. This report summarizes the geotechnical monitoring program and the results from monitoring throughout a 5-year period. Background and Previous Studies Natural resources affected by mine subsidence Coal and farmland are both important to the state's economy. Illinois is the second largest producer of agricultural commodities and the fifth largest producer of coal in the United States. Problems sometimes exist in ensuring that both farmland and coal resources are used to their maximum potential. Depending on the original topography of the land, subsidence-induced ground movements can modify surface drainage, possibly affecting crop yields of gently rolling farmland. Mine operators need to understand the impact of underground mining on near-surface hydrology and surface-drainage patterns. Method of mining The modern longwall mining system has been used in Illinois since 1976 (Janes 1983) and has been used in the study mine since 1987. Longwall mining produces imme- diate planned subsidence over each longwall panel. The longwall panels are laid out using tradi- tional room-and-pillar methods to form entryways. When panel layout is complete, specialized longwall equipment is moved in, set up, and mining of the defined block of coal begins (fig. 1). Gradually advancing under movable hydraulic shields, the longwall shearer removes all of the coal across a wide working face. The room-and-pillar entryways, which are used for ventilation 1 side view shield movement roof "face" of panel _ moveable shearer with ' floor rotating cutter drum conveyor Figure 1 Diagram of the longwall mining technique. and transportation, are left in place between the panels. As the mine face advances, the overbur- den behind the shields is left unsupported and collapses into the void. Vertical and horizontal movements are propagated by gravity up through the bedrock and surficial materials; thus, sur- face subsidence quickly follows this collapse. A general model for this process is shown in figure 2 (Peng and Chiang 1984, New South Wales Coal Association 1989). Overburden fracturing Coe and Stowe (1984), Ming-Gao (1982), and Whitworth (1982) investi- gated the location and amount of fracturing in subsided bedrock over longwall operations in Ohio (U.S.A.), Jiangsu Province (China), and South Staffordshire Coalfield (United Kingdom), respec- tively. Before the IMSRP began, Conroy (1980) conducted the only study concerning fracturing above a high-extraction mining operation in Illinois. He grouted two time-domain reflectometry (TDR) cables into boreholes that extended from the ground surface into a 625-foot-deep longwall mine and found that the bedrock movements severed one of the cables to within 100 to 150 feet of the surface. More recently, Bauer et al. (1991) investigated fracturing using a TDR cable and an MPBX over a high-extraction mine in Williamson County. They found that differential movements were associ- ated with interfaces between materials of contrasting strength, such as bedrock and drift, or near the boundaries of a fragipan within the soil where the TDR cable was sheared. Hydrogeologic effects Fracturing of the overburden may affect water-bearing formations by creating voids and increasing secondary permeability, as found in the Appalachian Plateau of Pennsylvania by Booth (1986). In a study in England, Garritty (1982) suggested that fracturing of the bedrock up to the surface may hydrologically connect an aquifer or surface water body with the mine. In Illinois, Cartwright and Hunt (1978) observed localized, open, vertical joints due to faulting in a mine roof; they speculated that these joints could provide a direct passage for water from higher strata in the immediate roof. These fractures were discontinuous, however, and did not provide any hydrologic connection to the surface. In another study in Illinois, Nieto (1979) found no leakage into mines with faults located 600 feet under the Rend Lake reservoir. Sub- sequent studies in Illinois, including this one, also have shown that discontinuous, localized frac- turing caused by strains in the bedrock occurs without any hydrologic connections to the surface. Before this study was initiated, the hydrogeological effects of subsidence on aquifers had been investigated by several researchers, including Coe and Stowe (1984), Duigon and Smigaj (1985), maximum compression maximum tension maximum subsidence original ground surface angle of draw coal not to scale h = mined out height Zone I = caved zone thickness = 2 to 8 times h Zone II = fractured zone thickness = 28 to 42 times h Zone III = continuous deformation zone Zone IV = near surface Figure 2 Strata deformation associated with longwall mining (modified from Peng and Chiang 1984 and New South Wales Coal Association 1989). Garritty (1982), Owili-Eger (1983), Pennington et al. (1984), and Sloan and Warner (1984). In studies of the Appalachian Plateau, Coe and Stowe (1984), Booth (1986), and Pennington et al. (1984) observed water level drops in wells that were undermined by the longwall mining method. Water levels partially or completely recovered, however, several months after being undermined by the longwall method in the Appalachian Plateau (Owili-Eger 1983). Subsequent studies in Illi- nois by Pauvlik and Esling (1987) have also observed a similar response of wells to mining. Surface subsidence characteristics The longwall method of mining produces planned subsi- dence. As the longwall face advances, a subsidence trough develops at the surface; it is both longer and wider than the area of total coal extraction at the mine level. The face advance at mine level produces a dynamic traveling wave at the surface directly above and slightly behind the face position. The subsidence trough is typically characterized by two types of profiles: static and dynamic. Static profiles are generally measured transverse to the panel to show the final shape of the subsi- dence trough. In general, a "maximum possible subsidence" cannot exceed the thickness of the mined-out height. For geological conditions prevailing in Illinois, the maximum subsidence is approximately 60% to 70% of the mined-out height. Three conditions of the subsidence trough's final shape were defined by a width-to-depth ratio (W/D) (Whittaker and Reddish 1989) for the United Kingdom coal field. Subcritical extraction would correspond to a W/D <1.4; critical extraction would correspond to a W/D = 1 .4; and super-critical extraction would correspond to a subsidence W/D >1 .4. These conditions are dependent on the overburden's strength characteristics and its bridging ability; therefore, these ratios may not hold true for the Illinois Basin or the Appalachian area. The vertical movements are accompanied by horizontal displacements that act toward the area of maximum subsidence. The magnitude of the horizontal displacements is a function of the gradient of the subsidence profile (Whittaker and Reddish 1989, Tandanand and Triplett 1987). Dynamic profiles are documented as surface subsidence occurs. A longitudinal survey line over the length of the panel is used to document the dynamic traveling subsidence wave that develops on the ground surface behind the advancing face. At the sides of the trough, the progression from dynamic to static subsidence occurs rapidly and produces a complex pattern of surface cracks (Van Roosendaal et al. 1990, 1991). Both types of profiles are presented in this report. Physical Setting Site selection Several factors were considered during the site selection process. First, a long- wall mine had to be available for study within a reasonable time frame. It was essential that instru- ments be installed well before the site was undermined so that site characterization and baseline data collection could take place. Next, the full cooperation of the mine operators and surface own- ers was required. Formal agreements with these parties were negotiated prior to initiation of work. Finally, the site had to be accessible and well suited for both instrument installation and long-term monitoring. The above criteria were used to select two longwall panels in southwestern Jefferson County, Illinois (fig. 3). The panels are located about 12 miles southwest of Mt. Vernon in Sections 19 and 20, T4S, R2E. The site lies between Illinois State Route 148 and the west side of Rend Lake, 4 miles south of Waltonville. Physiography The study site is located in the Mount Vernon Hill Country physiographic division of Illinois. The geomorphology of the area is characteristic of a maturely dissected, sandstone- shale plain of low relief under a thin mantle of lllinoian drift. Restricted uplands and broad alluvi- ated valleys occur along the larger streams (Leighton et al. 1948). Alexander Pulaski [•] site location Figure 3 Site location in Jefferson County, Illinois Surface topography above the panels is gently rolling, with elevations between 420 to 460 feet above mean sea level. The topography in the area is primarily bedrock-controlled (Horberg 1950). Bedrock features are modified, however, by glacial action and somewhat subdued by a thin mantle of deeply eroded drift that covers the region (Leighton et al. 1948). Dendritic drainage is predominantly bedrock-controlled in the vicinity of the panel site (MacClin- tock 1929). The panels are located in the drainage basin of the Big Muddy River, which drains into the man-made Rend Lake Reservoir. Any surface water over the panels generally drains east into the west arm of the reservoir. Surf icial geology The Bluford silt loam is the predominant modern soil over the study area (Dar- mody 1990), which is a somewhat poorly drained upland sloping gently to the northeast. Slopes range from 1% to 2% over most of the area and reach 4% in the shallow drainageway on the north side. The low slopes here have minimized erosion of the original 2 to 4 feet of Peoria loess that the modern soil developed on. The Peoria loess in this area overlies the Sangamon paleosol, which developed atop the lllinoian glacial drift deposit (L. Follmer, personal communication, 1993), here only 4 to 8 feet thick. The drift is quite rich in clay and locally contains rounded pebbles and organic debris. Below the drift is deeply weathered bedrock; weathered remnants of bedrock, such as siltstone lenses, are found as much as 4 feet above coherent bedrock. The Bluford soils (subtypes change as slopes vary from 0° to 6°) were originally forested and are generally well suited to growing corn, soybeans, small grains, hay, and pasture (Soil Conserva- tion Service 1988). A weakly expressed fragipan in the B horizon inhibits root penetration and becomes brittle in the dry season. Bedrock stratigraphy and structure The coal mine operates in the Herrin Coal seam at a depth of about 700 feet. Only the Herrin and Springfield Coals are considered suitable for under- ground mining in this township; the Herrin Coal is the uppermost of the two coal seams. The Her- rin Coal, which ranges from 7 to 1 1 feet thick in the mine, is typically 8 to 9 feet thick in the study area. The bedrock is of Pennsylvanian age and composed of rock units of the Carbondale, Shel- burn, Patoka, and Bond Formations (fig. 4; Greb et al. 1992). A detailed geologic log of this col- umn is given in appendix F. The immediate roof of the Herrin Coal is the Energy Shale, which is more than 100 feet thick. The Energy Shale is predominantly silty shale and thinly bedded siltstone; it has some thin zones of sandstone. Near its base, the shale is less silty and locally finely carbonaceous; it generally con- tains small plant compressions and, in some areas, contains both fallen and upright tree trunks. The study area is 5 to 6 miles east of the Walshville paleochannel, which was active contempora- neously with Herrin peat deposition. The Walshville paleochannel was the source of the thick clastic deposits that became the Energy Shale. The Herrin Coal near the paleochannel is split by shale interpreted as flood deposits (DiMichele and DeMaris 1987), and the thick deposits of En- ergy Shale are interpreted as overbank flood deposits (Bauer and DeMaris 1982). Energy Shale deposits more than 100 feet thick, such as those found here, probably formed from channel avul- sion in response to gradual marine transgression (DiMichele and DeMaris 1987). The typical Her- rin Coal roof units (Anna Shale and Brereton Limestone) found elsewhere in the Illinois Basin are missing in this study area. Their absence is apparently due to nondeposition. The bedrock interval from the Bankston Fork Limestone to the Carthage Limestone contains few minable coal seams and is less studied than the Carbondale Formation. The interval contains no thick sandstone units or other likely aquifers; two thin coals of uncertain stratigraphy are found near the top of the interval. The Bond Formation (all bedrock here above the Carthage Limestone) is of interest because of groundwater supplies. Thin sandstone intervals are present, but the Mt. Carmel Sandstone chan- nels provide the best local water sources. The study site is located over the northwest flank of one of these channels (fig. 5), and several piezometers were placed within the channel interval. The Mt. Carmel Sandstone is a sandstone/siltstone unit that is widespread in southeastern Illinois. As the cross section indicates, the Mt. Carmel Sandstone shows an abrupt change (over a dis- tance of less than 1 mile) from a thin (less than 30 feet thick) sheet facies to the thick sandstone- dominated channel facies seen in drill hole T401. Drill holes within the study area show the Mt. Carmel Sandstone is as much as 80 feet thick. The bottom 50 feet consists predominantly of sand- stone interbedded with some shale and siltstone; the upper 30 feet contains some sandstone and System Formation Q z O CO z < 2 z g § I _J > CO z z LLI Q_ ■ ? z DC D m _i LU X CO LU _l < Q Z o CD DC < O Named Members Mt. Carmel ? Sandstone (channel fades) Carthage Limestone (Shoal Creek Limestone) Chapel Coal Piasa Limestone Danville Coal Bankston Fork Limestone Energy Shale Herrin Coal _ _ - _ 100- 3ririTZ 200- - - - - _ _ _ _ _- - r . - . 300- -J-Z-Z-Z- £ 400- Q. -~'-~-~'-~ .. 7*7 —| fc= | — 600- I I I I l l l 700- _-_ -_--•- Predominant Lithology top of bedrock shale and siltstone, weathered at top shale, black, fissile shale, silty, fining upward sandstone, massive at top, silty toward bottom shale, silty, interlaminated with siltstone; rare sandstone interbeds sandstone, massive, rare shale interbeds, scattered fossil foliage in bottom third; pebble lag deposit at base; unconformably overlying: siltstone, fining upward to nonsilty shale limestone, fossiliferous, over black shale shale, silty coal, 0.44 ft thick siltstone. fining upward into shale coal, 1.20 ft thick shale, carbonaceous, with calcareous nodules; some slickensides near base siltstone, sandy at top, with clay interlaminations at bottom shale, black, marine fossils coal, 0.68 ft thick claystone, weak, with shale nodules, fining upward siltstone, fining downward shale, gray, silty at top, carbonaceous around 540 ft siltstone - shale, black, marine fossils sandstone, unconformably overlying: claystone, slickensided limestone over black shale coal, 3.0 ft thick claystone, slickensided limestone siltstone, with some sandstone zones at top, laminated siltstone in lower half, fining downward section is estimated below TD of 699 ft siltstone, fining downward to shale with plant debris at base coal, bright-banded, typically 8 to 9 ft thick Figure 4 Stratigraphic column of bedrock above the Herrin Coal at the study site. Formation boundaries follow Grebetal. (1992). thousand ft Figure 5 North-south geologic cross section through the study area, modified from Pattee (1994). silty shale, although siltstone predominates. The lower 50 feet is largely poorly cemented sand- stone, but it contains some short intervals of calcite cemented sandstone. At least 60 to 65 feet of sediment was eroded by the channel at the study site (fig. 5), and the erosion cut through rela- tively low porosity shale and impure siltstone beds. The bedrock below the study is relatively undeformed, producing relatively flat-lying strata. The regional dip of the strata is approximately 1 .9 to 2.8 m/km (10 to 15 ft/mile) to the east-north- east. The Rend Lake Fault Zone passes through this and neighboring mines (Keys and Nelson 1980). The fault zone consists of north-striking, parallel, normal faults with moderate to low dis- placements. At the mine level, the faults are often found in en echelon patterns, making it difficult to lay out mine panels to avoid them. The Rend Lake Fault Zone begins to trend slightly to the north-northwest beginning several miles south of the mine, and it strikes roughly N20°W adjacent to the study area. The longwall study panels, aligned east-west, were laid out to avoid these faults (fig. 6). The position of the fault just to the east of the study panels determined the eastern extent of both panels 3 and 4. Because this fault dips westward, no fault effects were expected in the subsided overburden. Hydrogeology Groundwater resources are limited and are primarily used for domestic and household needs. Surface water reservoirs, such as Rend Lake Reservoir and smaller artificial impoundments, supply water to the larger communities. Water for livestock, minor irrigation, and other farming purposes is usually obtained from small ponds found in the study area. Groundwater is pumped from large diameter (typically 2-12 ft) dug wells that provide water for domestic purposes. These wells typically end near the top of bedrock, thus accessing sand and gravel lenses in the glacial drift overlying the bedrock surface. A survey of farmstead wells (Illinois State Water Survey 1934) showed that 4- to 5-foot-diameter wells dug from 10 to 23 feet deep were sufficient for domestic uses. Smaller diameter modern wells have also obtained ground- water from the glacial drift. Yields, however, were generally much smaller than those obtained from the dug wells. Figure 6 The study panels and adjacent portions of the Rend Lake Fault System. Bedrock aquifers of the Pennsylvanian Series in the study area are limited primarily to sandstone units; some fractured limestone aquifers were also reported (Pryor 1956). Static water levels in these wells ranged from about 10 to as much as 80 feet below the ground surface. The Mt. Carmel Sandstone aquifer, about 60 to 150 feet below ground surface, is most often tapped. Mine Characteristics The mine operates in the Herrin Coal at a depth of about 720 feet. The Herrin Coal ranges from 7 to 1 1 feet thick in the mine and is typically 8 to 9 feet thick in the study area. The study area consisted of longwall panels 3 and 4, their adjacent entries, and the ground surface above them. Panels 3 and 4 (fig. 6) are 600 feet wide, run east-west, and approach 1 mile long. The total unsupported panel width is 617 feet, giving a panel width versus mine depth (W/D) ratio of 0.86. The panels were mined from east to west. The carbonaceous base of the roof shale is often mined with the coal, producing actual mining heights of 9 to slightly more than 10 feet in the study panels. Longwall mining in panel 3 began on June 15, 1988. The transverse monument line and piezome- ters were undermined in late August, and mining was completed on December 15, 1988. Longwall mining began in panel 4 on November 23, 1988. The transverse monument line was undermined in early January 1989. The piezometers and a nearby instrument cluster were undermined in early February, and the mining was completed on April 25, 1989. GEOTECHNICAL MONITORING PROGRAM Introduction The instrumentation for this study was selected to measure the geotechnical and hydrological effects of subsidence on the overburden. The ISGS proposed an instrumentation plan that was reviewed and accepted by the USBM Twin Cities Research Center, and the mine company. The ISGS was responsible for subcontracting the drilling and installation of instruments. The drilling contract was awarded to Raimonde Drilling, Inc. (RDI) of Chicago, Illinois. RDI was responsible for installation of seven bedrock piezometers, one pump well, two time-domain reflectometry (TDR) 8 control instruments - ^ P200 .P306 Tf™„„o« ^+_ c o (/> 0.0000 -0.0002 - -0.0004 TV - ! 1 1 \s>^ "i 1 r >~^ V values from subsidence (vertical displacement) measurements values from tiltplate measurements Figure 17 Subsidence, tilt, and curvature results using two monitoring techniques. Figure 17 shows a series of plots showing the development of the dynamic parameters through time. The tilt and curvature for both the tiltplates ("points") and survey monuments are equal. The slight differences are probably due to the fact that calculations of tilt and curvature via the survey monuments used the average distance between monuments. Overburden Characterization Geotechnical core log Figure 18 shows a comparison of pre-and postsubsidence core logs. Although drilling through the postsubsidence overburden was more difficult because of the loss of drilling fluid in some highly fractured zones, core recovery was excellent before and after subsi- dence. Changes in the RQD of the respective cores were not unique to any particular lithology but more a function of the position of mining-induced fracture zones within the overburden. Under- mining caused the fracture frequency in the lower bedrock to increase dramatically, whereas only smaller increases were noted toward the surface. The larger increases occurred in the stronger materials, which deform more like brittle material (Bauer 1984). Geophysical logs A plot of subsidence-induced changes in shear-wave velocity (fig. 18) shows four spikes that represent velocity decreases of 12% to 18%. These spikes directly correlate with coals and thin calcareous zones where more fracturing and bed separations occurred within the overburden. The general decrease of 1% to 10% in the shear-wave velocity throughout the rest of the overburden is the result of wave attenuation through a fractured medium filled with fluid. Comparison of this plot with the postsubsidence fracture frequency and hydraulic conductivity logs supports this conclusion. The other geophysical logs (caliper, density, and gamma ray log) did not show any appreciable changes as a result of subsidence. Intact rock properties Intact rock core specimens were tested in the laboratory and classified following the methods of Deere and Miller (1966). Figures 19 and 20 show plots of laboratory- 24 • 10^ to £ 3 > CO c ft «; 10' 6 C OJ o E o 10 -10 i-tXlO^ £ 10' 6 Q. 10 10 03 CD £- -1- o o c gj Co- rn 3? c CO C 1 o 20- c co 3 .o >»>- n tu O u5 3 O n 3"±; .fcO ^^ >- t/> 5 ® CO y 3 -U to ~ 3 *^ to CO 01 ^C- Q. >ioH 8 6 4 2-1 >10 8 6 4 2 Q — #" to .a 3 0) to o Q. 100 80 50 n 3 to CD 100 80 50 100 to 80 3 ? IS 40 o CL CD 100 o CO 80 O 3 $ 40 V II — T~T — I I I i — i — i — r -HTihr bn "i — i — i — r tbfl \m nn^n -T^n- K^\- i i I i I i i I i i I i r i i I i i r — i — i — r o 3 o> en CO o 0) XI TJ "5 Q. ^_ Q) C >» O) O O CO o o o o c CD ■D 'to X) 3 OT to O Q. ■D C CD "o w >> O o r: CD 13 £ ■° a ° I •!! • £25co2 c, o,o> (1) o CO c TS C a> a> 2 CO (/> E co to j2« ra 5S ;= .C Q =0 to o wo c in _CD o) _co co 19 S* to x: o™ .c 0)OO CO r? CD CO CD CD C -k o o o o O (i\ u> n IS -!i TS : 2 o o o o 2co-£o>o) wto u) - ^ ^ ^CD - ^ ^*CD CD C ° o) I- 15 2 -£lo &•£ o m Si CO™ E O™ E To \/ / (W) Mldap 18 \\v^^ 0) o C O to 'C co Cl E o O w 3 Li. 25 *£> E D c B A very low low medium high very high strenqth strength strength strength strenqth 20.00 ■ shale / s* 10.00- + sandstone ** yS _ ° limestone ** s* ^ - o siltstone ^ ^ S* x claystone / " C o shale/limestone «•"' X" S" s" s* / o s* ^ ** m 1.00- /■ -> x ■ m s~ - s* ■ ■ i •^ ^ ■ ■ m s" /■ ^ s* 0.25 i i 1 8 16 32 uniaxial compressive strength (psi x 10 3 ) Figure 19 Presubsidence intact rock properties. 20.00 10.00 E very low strength D low strength C medium strength B high strength A very high strength ■ shale + sandstone o limestone 9^ 3 -o o E CO 'at c c CD at 1.00 0.25 4 8 16 uniaxial compressive strength (psi x 10 3 ) 32 Figure 20 Postsubsidence intact rock properties. 26 time domain reflectometry signals 100 200- 300 £ 400 500 600 700 ^t^t^> progression of shear deformation of cable 190 80 — i r -10 -80 face position (ft) -120 -185 -250 Figure 21 Progression of TDR cable deformation at the centerline of panel 4. tested specimens from pre- and postsubsidence drill cores. The uniaxial strength of a particular specimen is plotted against its tangent modulus taken at one-half the ultimate strength. Presubsi- dence plots show typical ranges of values for the given rock types. Data from the postsubsidence drill cores are limited, but they suggest that the rock strength decreased somewhat. Any effects to the modulus are indistinct. Subsidence may increase microfracturing or disrupt bedding planes or laminae, causing a decrease in strength. Strength information is best evaluated using the informa- tion shown in figures 19 and 20. Complete rock strength test data may be found in appendix I. Overburden Deformation Monitoring Time-domain reflectometry TDR cable 99 feet inside panel edge As mining progressed, this cable failed in shear at a depth of 262 feet, in tension at 138 feet, in shear at 1 1 1 feet, and finally failed at a depth of 3 feet. Figure 21 shows the signal amplitude changes and the digitized signal outputs for the cable readings. The failure at a depth of 1 1 1 feet occurred in the upper part of the Mt. Carmel Sandstone, and the last reflection amplitude was -58 mp. The failure at 138 feet is close to the base of the Mt. Carmel Sandstone, which is in contact with a shale unit. The failure at a depth of 262 feet occurred near a 0.3-foot-thick coal seam and had a maximum signal ampli- tude of -52 mp. Failures in this cable occurred near contacts between strong and weak lithologic units, and near weak units, such as thin coal. The 52 and 58 mp amplitude changes represent about 0.12 inch of displacement between the inner and outer conductors of the cable before fail- ure, as determined by the relationship between shear deformation and reflection coefficient for a 0.5-inch-diameter cable (fig. 8 in Dowding et al. 1989). TDR cable at centerline of panel The cable in the center of the panel failed in tension at a depth of 1 17 feet on or just before February 9, 1989. No distortions were detected before this break. The failure at 1 17 feet corresponds to the top of the Mt. Carmel Sandstone in contact with a shale unit. A tensile strain of about 1% was calculated in the depth interval from 107.3 to 126.3 feet 27 using the sondex. After the failure at 1 17 feet, the cable deformed at depths of 51 and 113 feet, with reflection (signal) amplitudes of as much as -20 and -33 mp, respectively. Inclinometer/sondex Sondex As the longwall face approached the inclinometer/sondex hole, the upper 50 feet of overburden yielded 0.2 foot of vertical extensional movement. The lower 500 feet of overburden experienced very small changes. When the mine face advanced to within 50 feet of undermining the sondex, the upper bedrock continued to yield, while the lower portion of the bedrock underwent small (0.01 foot) compressional displacements. By February 9, 1989, when the longwall face was 38.5 feet past the borehole, both the sondex and inclinometer sondes could not be lowered below 423 feet, which is the depth of a 0.8-foot-thick coal seam over a claystone unit. The next day the sondex could not be lowered below 245 feet. On February 14, the instruments could not be lowered below 145 feet; 1 week later, the sondex passed this point only to be stopped at the 245-foot point again. The amount of strain between each sondex ring ranged from +2.7% (extension) to -1 .4% (com- pression). The changes in vertical strain were mostly related to lithologic changes. The largest vertical strains developed in the upper part of the bedrock, very near the ground surface. The smaller strains occurred in the Mt. Carmel Sandstone. A plot of vertical changes for each ring (without taking into account subsidence) helps to identify zones of bedrock separations (fig. 22). As the longwall face approached, vertical extensional separations occurred near the top 50 feet of overburden. After the longwall face had advanced 100 feet, "beams" approximately 100 feet thick began to separate. This response can be interpreted from the "S" shaped graph of changes ver- sus depth. Appendix L contains the sondex data. TDR events in T400 (tensile zone) T400 geologic column Sondex changes (ft) heave i settlement TDR events in T401 (centerline) -0.2 o.o 100 200- 300 a. ID 400 500 600- 700 shear in base of silt- stone, 106 ft -v shear and break in sand- — • stone,111ft if shear in sandstone, 123 n/f. break in sandstone, 1 38 ft shear in interiaminated\ siltstone, 205 ft extension at base of limestone, 235 ft '"' shear and break just "^ below thin coal, 262 ft extension in siltstone,- 345 ft E3 coal drift Y^A limestone I I sandstone C---3 shale E3 siltstone top of bedrock at 25 ft ^-- shear at base of black fissile shale, 50 ft , shear in Mt. Carmel Sandstone, 111 ft PL break in Mt. Carmel Sandstone, 115 ft 100 200 300 a. •o 400 500 600 700 Figure 22 Comparisons between the TDR events and sondex changes occurring near the upper part of the bedrock. 28 Inclinometer Six sets of inclinometer readings were obtained between January 24, 1989, when the panel face was located 485 feet away from the inclinometer, and February 23, 1989, when the face was 637 feet past it. As the face approached the inclinometer, no significant horizontal displacements were detected until the face was between 125 and 60 feet away; at this time, noticeable horizontal displacements were measured. Major differential displacements of about 1 inch each were measured at four different depths when the face was 60 feet away. The relative displacement at a depth of 175 feet was located within an interlaminated siltstone. The next dis- placement occurred at 260 feet, just below a thin coal. The other two displacements took place at deeper locations: at 530 feet within a shale layer and at 590 feet, probably through a slickensided claystone. The magnitude of displacements corresponded to the northwest-southeast direction, which is about 45° relative to the mining direction (fig. 23). When the face was 218 feet past the inclinometer, two substantial relative displacements were measured. The shallower displacement occurred at a depth of 50 feet, through the base of a black, fissile shale; this displacement had a magnitude of 7 inches. This shear plane was also detected by the centerline TDR cable. At a depth of 150 feet, close to the contact between the Mt. Carmel Sandstone and the interlaminated siltstone, the inclinometer casing had a displacement large enough to block the downward passage of the probe. Therefore, the magnitude of the cumu- lative displacement measured after this event is unknown because the fixed reference at the bot- tom of the casing was lost. Unfortunately, total station surveys were not performed each time the inclinometer was read, so a fixed reference at the top of the inclinometer was not available. The last two readings (fig. 23) cannot be compared with each other, nor can they be related to previous readings because the reference was no longer fixed. They can be analyzed individually, however, and do provide relative displacements with depth, a relationship that is indispensable in identifying sliding surfaces. Hydrogeologic Response to Subsidence Drift water level response Drift water levels and local water-supply wells showed no apprecia- ble change during mining. Water level response was more a function of seasonal effects (Freeze and Cherry 1979) (fig. 24). Water levels in the drift piezometers were low during periods of high evapotranspiration (early spring to early fall), and water levels were high during low evapotranspi- ration (fall and winter). Hydrographs for all drift piezometers and daily precipitation are presented in appendix K. 3 a. o _<» — 1/24/89 1/31/89 O 2/7/89 A 2/14/89 x 2/23/89 I I I 1 1 - 100 150 200 250 300 depth (ft) Figure 23 Inclinometer data showing the location of the shear planes found by the TDR cable and the dynamic movement of the bedrock with the advancing longwall face. 29 (/> C o 0) Q) o c (0 Q. ■D ■2- -6- "WU -8- 0) -10- -12 UuJ North Panel 4 Panel 3 • Panel 2 Piezometer P202 (continuous record) August 1988 November February 1989 May August Figure 24 Piezometric response of the drift piezometers due to precipitation and not mining. Bedrock water level response Hydrographs from Mt. Carmel Sandstone piezometers located on the centerline and chain pillars of panel 3 are shown in figure 25. Water levels declined as the mine face approached the instruments, reached maximum lows when tensile strains (as meas- ured on the surface) passed, showed a temporary recovery spike as the maximum compressive strain passed, and then steadily recovered for 3 months until passage of the mine face of the adja- cent panel. In each piezometer, maximum tensile strains were measured when the mine face was 50 to 100 feet past the instruments; this correlates with the maximum surface aperture measured when the face was approximately 70 feet past the surface cracks. Water levels in piezometers on the adjacent panel began to decrease when the approaching mine face was about 1 ,500 feet away. Piezometric drops are caused by increased secondary porosity resulting from developing and opening fractures in the tensile part of the wave. Recovery occurs when the compressive part of the subsidence wave passes the piezometer, the fractures partially close, and the cone of depres- sion associated with the advancing longwall face (tensile event) moves away. The association between water level fluctuations and dynamic-subsidence strains has also been documented by Walker (1988). Bedrock water levels recovered within 2 to 3 months of undermining. 30 -60 -70 -80 I -90 ■o c 3 2 5 ]> -100 I a) g -no 5 -120- -130 220 1st panel mining 2nd panel mining maximum tensile events 260 1 r 300 Julian days Figure 25 Piezometric response of the bedrock piezometers over panel 3. 340 380 300 N Julian date 1 988 380 Figure 26 Water level in the well in the center of panel 3. Arrows point to days when thera was no mining After day 340, note the drop in water level caused by the mining of panel 4. # 31 As the mine face approached each well, water levels showed steps or plateaus related to tempo- rary mine work stoppages (i.e., on Sundays; indicated by arrows on fig. 26). The effect of no mining is shown by a step or plateau after the day off. The declining curve shows partial recovery during work stoppage, and the recovery curve slows down when work starts up again. The gen- eral rate of decline as the face advanced was approximately 1 .5 feet/day, whereas the recovery rate was 0.7 foot/day. The deep shale piezometer P305 over panel 4 responded to undermining quite differently from the sandstone piezometers (appendix K). The water level declined from 70 feet below the ground level (BGL) as a result of the passage of panel 3; it then slowly recovered to about 60 feet BGL by January 1989. The P305 water level did not decline as panel 4 approached, but rather it rose rapidly about 25 feet 2 weeks before subsidence. This may be due to the "Noordbergum Effect" (Verruijt 1969, Booth 1992), which is a short-term dynamic effect caused by ambient stress from lowered water pressure in units below the piezometer. Aquifer characteristics Presubsidence hydraulic conductivities of 10" 7 to 10" 6 cm/s (shales) and 10" 6 to 10" 4 cm/s (sandstone) were measured by pump, slug, and hydraulic injection tests. Postsubsidence values increased approximately two to three orders of magnitude for the shale and one order of magnitude for the Mt. Carmel Sandstone (fig. 18). The increase in hydraulic con- ductivity is attributed to mining-induced fracturing in the overburden (Mehnert et al. 1990, Booth and Spande 1992a, b, Booth 1992). Figure 18 shows the increase in fracture frequency in the postsubsidence hole. INTEGRATION OF OBSERVATIONS The subsurface sequence of events monitored over panel 4 is presented in table 1 . Panel 4 instrumentation first responded to the advancing face in early December 1988, when the face was more than 1 ,500 feet away. The initial response was noted in the piezometers. The water levels in the piezometers gradually declined as the face approached. No other surface or subsurface move- ments were noted at that time. When the face advanced to within 437 feet, separations of 0.2 foot in the upper bedrock were documented by the sondex. The inclinometer and the TDR cables also indicated the advancing face by recording shearing and extension of the bedrock. The inclinome- ter documented some shear zones at 175, 260, 530, and 590 feet. The location of these disrup- tions occurred at interfaces between strong and weak lithologic units. When the mine face was within 50 feet, the centerline TDR cable broke at 1 17 feet, the top of the Mt. Carmel Sandstone unit. The bottom two-thirds of the inclinometer/sondex assembly was lost at this time. The location of the inclinometer/sondex break corresponds to a shale with medium- angled slickensides. When the face was past the instruments, definite shear planes as well as extensional features could be defined. In particular, the TDR cables and the inclinometer defined shear planes at 51 , 113, 150, and 260 feet. These locations were associated with some high-angled joints or litho- logic interfaces. Bedding separations defined by extension in the sondex and TDR cable were at 46, 138, and 184 feet in lithologic interfaces or slickensided joints. The postsubsidence core log- ging complemented the readings of the TDR cables and inclinometer/sondex instruments; water circulation was lost and drill bits were dropped at the defined shear planes and bedding separa- tions. A shear plane at 1 12 feet was defined with the downhole camera. The camera was unable to pass this constricted area. Piezometer obstructions were also prevalent in the shear plane located at 51 feet. SUMMARY AND CONCLUSIONS Surface and subsurface monitoring of two adjacent longwall panels allowed the study of the effects of planned subsidence on the overburden. The following are some of the more important findings from this study. Surface Subsidence Characteristics 1) At this site, 90% of the subsidence occurred within the first 3 months of undermining. During this period, the ratio of subsidence to mined-out height was 63%. Residual subsidence at this longwall site continued for as much as 3 years, probably because of the closure of fractures developed 32 Table 1 Sequence of subsurface events monitored over panel 4. Date Face advance (ft) Instrument Evidence/ depth (ft) Location of movement Complementary instrument 1/25/89 437 sondex tensile/46 2/2/89 173 TDR-edge shear/262 2/7/89 53 inclinometer shear/1 75 shear/260 shear/530 shear/590 2/9/89 38 TDR- centeriine break/117 49 sondex/ inclinometer bottom/423 2/10/89 -106 TDR- centerline shear/51 shear/113 -108 sondex tensile/46 tensile/184 bottom/ 258 inclinometer bottom/258 2/13/89 -140 TDR-edge shear/1 1 1 tensile and break/1 38 2/14/89 -218 inclinometer shear/50 shear/1 50 above calcareous shale between claystone and coal within interlaminated siltstone below a thin coal; shale has highly angled joints siltstone slickensided claystone top of Mt. Carmel Sandstone shale, med. angled slickensides shale, just below calcareous zone sandstone, shale parting just above calcareous shale siltstone interbedded with shale laminae shale, high angled joints shale, high angled joints Mt. Carmel Sandstone Mt. Carmel Sandstone interface where slightly calcareous 7-in. displacement at base of black fissile shale >7-in. displacement in Mt. Carmel Sandstone and interlaminated siltstone see inclinometer on 2/7 and TDR-centerline on 2/10 see inclinometer and sondex on 2/1 and inclinometer on 2/7 see TDR-edge on 2/2 and sondex 2/10 see sondex on 1/25 and inclinometer on 2/14 see TDR-edge on 2/13 see inclinometer 2/14 see TDR-edge on 2/2 see TDR-edge on 2/2 see TDR-centerline on 2/9 see sondex 1/25/89, TDR-centerline on 2/10 during the collapse of overburden. The long-term subsidence measured over panel 3 increased the ratio of subsidence to mined-out height to 70%. 2) The maximum tensile strain calculated was 0.03; this strain occurred 125 feet inside the panel edge (distance/depth ratio of 0.172). The tensile zone on the north side of panel 4 was approxi- mately 1 12.5 feet wide for this "critical" panel. These static subsidence characteristics are within the range of values previously reported for the Illinois Basin (Bauer and Hunt 1982). 3) The tilt parallel to the direction of mining increased with subsidence until the inflection point was reached, and then it decreased as subsidence continued to increase. The north-south pegs did not vary as much, although there was a slight change in the final tilt. The maximum tilt of 33 1 .39° is comparable with that measured at another Illinois site, where tilt was measured directly on a foundation (Triplet! et al. 1992). Tilt and curvature may be calculated using tiltplates or sur- vey monuments; either method gives approximately the same results. Overburden Characterization 1) Increases in hydraulic conductivity were associated with increases in fracture frequency in the core. Postsubsidence hydraulic conductivities increased about two to three orders of magnitude for the shale and one order of magnitude for the Mt. Carmel Sandstone. 2) Fracturing within the overburden resulted in a decrease in shear wave velocity by as much as 18%; more often, it was about 10%. 3) Intact rock properties from pre- and postsubsidence cores showed negligible strength changes. Overburden Deformation 1) The TDR response was correlated with lithology, location in the subsidence profile, and mining advance. TDR deformations in the overburden were commonly documented at the interfaces between strong and weak lithologic layers or within weak, previously fractured layers, as shown in cores retrieved before instrument installation. Tension failures were generally correlated with interfaces between strong and weak lithologic layers, where bedding separations are more likely to occur. 2) TDR failures were observed to progress upward through the overburden as active mine faces advanced past the cable installations. The cable at the centerline of a longwall panel failed mostly in tension, whereas the cable at the panel edge experienced multiple shear failures. 3) The sondex response helped to identify zones of bedrock separations. As the longwall face approached, vertical extensional separations occurred near the top 50 feet of overburden. After the longwall face had advanced 100 feet, "beams" approximately 100 feet thick began to separate. 4) The inclinometer response helped to identify shear zones. Although a few shear zones in the inclinometer hole could also be identified by the TDR cables, the use of such a deep inclinometer was limited in this setting because the bottom reference was lost, and large shearing prevented the sonde from passing below 245 feet. Hydrogeologic Response 1) Drift (shallow) water levels showed no appreciable change during mining. Changes in water level were more a function of seasonal effects. 2) Bedrock water levels declined as the mine face approached, reached maximum lows when tensile strains (as determined on the surface) passed, showed a temporary recovery spike as the maximum compressive strain passed, and then steadily recovered over a period of 3 months until passage of the mine face of the adjacent panel. Bedrock water levels recovered within 2 to 3 months of being undermined. This response may be site-specific because of the lateral extent and thickness of the aquifer monitored (Mt. Carmel Sandstone). 3) Aquifer characteristics such as hydraulic conductivity and storativity were improved; this improvement is attributed to mining-induced fracturing in the overburden. The hydraulic conductiv- ity in the sandstone was increased by an order of magnitude. RECOMMENDATIONS We offer the following recommendations for monitoring and instrumentation. 1) The TDR should be monitored frequently during periods of active subsidence. An intense moni- toring schedule will enable the calculation of the rate of shear and completely document progres- sive failure in the overburden. A minimum frequency of monitoring of three readings per day is suggested. The best method would be to develop an automatic and/or telecommunicated data acquisition system that would permit continuous monitoring. 2) The total station was sensitive to ground movements during surveying; therefore, rapid surveys were required in the influenced area. If the total station instrument was set up in the influenced area and the surveying was delayed, enough subsidence movement could take the total station 34 instrument off level. The elevation of all monuments should be determined with a level rather than with the total station. The level is a more precise instrument in measuring elevation changes. The total station may still be used to determine coordinates with good accuracy. 3) Gathering data from an inclinometer/sondex assembly of such great lengths as was used in this setting can be time consuming. Although an automatic readout unit and a powered reel for lowering the inclinometer were used, it took 8 to 10 hours to read the inclinometer. In addition, as the panel face approached, the inclinometer/sondex assembly became inoperative for depths exceeding 250 feet because large shear and tensile displacements occurred. An inclinometer length of about one-third of the overburden thickness would have sufficed to determine the shear planes developed with undermining. Because the entire inclinometer casing was installed within the overburden where ground movements are induced by mining, a top reference must be selected. By using the top as a fixed reference, cumulative displacements can be calculated down to the bottom of the casing. Coordinates of the top of the casing need to be taken with the total station every time the inclinometer is read. Cumulative displacements at the top of the casing can be com- puted by comparing these coordinates to the initial coordinates determined before mining. Just as important as a fixed reference are (1) the direction of the inclinometer grooves, which should be aligned parallel and perpendicular to the mining direction, and (2) the possibility of the inclinome- ter casing spiraling because of its great length and the temperature/season of its installation. ACKNOWLEDGMENTS This report was prepared by the Engineering Geology Section of the Illinois State Geological Sur- vey under USBM Cooperative Agreement CO267001 . The Cooperative Agreement was initiated under the Illinois Mine Subsidence Research Program. It was administered under the technical direction of the USBM Twin Cities Research Center; Larry Powell acted as Technical Project Offi- cer. Kent Charles and Jose Martinez were the contract administrators for the USBM. This report is a summary of the work completed as part of this contract during the period October 1 , 1986, to September 30, 1993, and submitted to the USBM in January 1994. The authors acknowledge the work of David F. Brutcher, Joseph T. Kelleher, and Christine E. Ovanic for their assistance in the field; Billy A.Trent assisted with manuscript preparation. The authors would also like to thank the landowners (Ernie Martin, Paul Kiselewski, and James Wil- son) and the mining company for their assistance and cooperation. Paul B. DuMontelle, as former Director of the IMSRP, assisted in securing this research site. In addition to funding from the USBM, the IMSRP also received funding from the Illinois Coal Development Board, under the administration of the Illinois Department of Energy and Natural Resources-Office of Coal Development and Marketing. APPENDIXES Available upon request as Open File Series 1997-7 REFERENCES American Society for Testing and Materials, 1988, Annual Book of Standards, Section 4 Construc- tion, Vol. 04.08 Soil and Rock, Building Stones; Geotextiles, 951 p. Bauer, R.A., 1984, Subsidence of Bedrock Above Abandoned Coal Mine in Illinois Produces Few Fractures: Society of Mining Engineers of AIME preprint 84-400, 8 p. Bauer, R.A., and S.R. Hunt, 1982, Profile, strain and time characteristics of subsidence from coal mining in Illinois, in S.S. Peng, editor, Proceedings of the Workshop on Surface Subsidence Due to Underground Mining: Morgantown, WV, p. 207-218. Bauer, R.A., and P.J. DeMaris, 1982, Geologic Investigation of Roof and Floor Strata: Longwall Demonstration, Old Ben Mine #24: Final Technical Report, Part 1: Illinois State Geological Survey, Contract/Grant Report 1982-2, 49 p. Bauer, R.A., C.H. Dowding, D.J. Van Roosendaal, B.B. Mehnert, M.B. Su, and K. O'Connor, 1991, Application of Time Domain Reflectometry to Subsidence Monitoring: Final Report to Office of Surface Mining, 48 p.; NTIS No. PB91 -228411. 35 Bauer, R.A., and D.J. Van Roosendaal, 1992, Monitoring problems— Are we really measuring coal mine subsidence?, in S.S. Peng, editor, Proceedings of the Third Workshop on Surface Subsidence due to Underground Mining: Morgantown, WV, p. 332-338. Booth, C.J., 1986, Strata-movement concepts and the hydrogeological impact of underground coal mining: Ground Water, v. 24, no. 4, p. 507-515. Booth, C.J., 1992, Hydrogeologic impacts of underground (longwall) mining in the Illinois Basin, in S.S. 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McCann, 1979, Use of geophysical logging techniques in the determination of in-situ geotechnical parameters: Transactions of the 6th European Symposium of SPWLA, Paper G, 19 p. Freeze, R.A., and J.A. Cherry, 1979, Groundwater: Prentice-Hall, Englewood Cliffs, NJ, 604 p. Garritty, P., 1982, Water percolation into fully caved longwall faces, in Proceedings of the Sympo- sium on Strata Mechanics, Newcastle-upon-Tyne: Developments in Geotechnical Engineer- ing, v. 32, p. 25-29. GeoTechnical Graphics System, 1991, Software (version 3.1): GeoTechnical Graphics System, Berkeley, CA. Greb, S.F., D.A. Williams, and A.D. Williamson, 1992, Geology and Stratigraphy of the Western Kentucky Coal Field: Kentucky Geological Survey Series 11, Bulletin, 77 p. Horberg, C.L, 1950, Bedrock Topography of Illinois: Illinois State Geological Survey Bulletin 73, 111 p. 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