STATE-0F-THE-ART MINE SUBSIDENCE EVALUATIONS

STATE-0F-THE-ART MINE SUBSIDENCE EVALUATIONS Authored By: Kanaan Hanna, Jim Pfeiffer, Steve Hodges, and Fred Tolen Zapata Incorporated, Blackhawk Division, Golden, CO. Tel: 303-278-8700 khanna@zapeng.com Dave Hallman and Jeffrey Nuttall Tetra Tech MM, Inc., Golden, CO. Tel: 303-217-5700 Dave.Hallman@tetratech.com Bill Locke and Vicky Zimmerman Wyoming DEQ, Abandoned Mine Lands Division, Lander, WY. Tel: 307-332-5085 BLOCKE@wyo.gov, VZIMME@wyo.gov 30 th Annual National Association of Abandoned Mine Land Programs Conference October 26 29, 2008, Durango, Colorado 1

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Reclamation Issues: Subsidence Evaluation and Control Technologies State-of-the-Art Mine Subsidence Evaluations 1 Kanaan Hanna, Jim Pfeiffer, Steve Hodges, and Fred Tolen Zapata Incorporated, Blackhawk Division 301 Commercial Road, Suite B Golden, CO 80401 khanna@zapeng.com, jpfeiffer@zapeng.com, shodges@zapeng.com, ftolen@zapeng.com 303-278-8700 Dave Hallman and Jeffrey Nuttall Tetra Tech MM, Inc. 350 Indiana St, Suite 500 Golden, CO 80401 Dave.Hallman@tetratech.com Jeff.Nuttall@tetratech.com 303-217-5700 Bill Locke and Vicky Zimmerman Wyoming DEQ, Abandoned Mine Lands Division 510 Meadowview Drive Lander, WY 82520 BLOCKE@wyo.gov, VZIMME@wyo.gov 307-332-5085 Abstract Abandoned mines, or voids, pose a serious threat to the public health and the environment, in terms of the potential for failure and collapse resulting in surface subsidence and adverse impacts in terms of cost and safety. Traditionally, mine void detection and related engineering investigations for mine subsidence risk assessment and mitigation have been greatly dependent on systematic drilling, and grouting backfill programs. This approach includes boreholes drilled on a predetermined closely spaced grid pattern, essentially a blind Swiss cheese/shotgun approach. The small snapshot of subsurface conditions provided only at the specific boring locations is inherently unreliable for accurately characterizing the complexities of subsidence risk assessment. Thus, there is a need to apply innovative and cost-effective solutions to advance the ability to assess subsidence hazards and minimize subsidence risk. Such innovation is essential to advancing the state-of-practice in mine subsidence investigations. The use of state-of-the-art geophysical imaging technologies combined with downhole sonar (for water-filled void) or laser (for air-filled void), and video camera imaging tools provides a cost effective, comprehensive, and 1 Presented at the 30 th Annual National Association of Abandoned Mine Land Programs Conference, October 26 29, 2008; Durango, Colorado 3

accurate alternative to systematic drilling by providing two- or three-dimensional representations of subsurface conditions. ZAPATA, Blackhawk Division and Tetra Tech have developed comprehensive mine subsidence investigations using a variety of high-resolution geophysical technologies integrated with other engineering disciplines. This approach allows for more accurate definition and mitigation of subsidence hazards through a proactive rather than reactive process. This program is being carried out in conjunction with the State of Wyoming, Department of Environmental Quality, Abandoned Mine Land Division (DEQ-AML). The engineering geophysical investigations and subsidence evaluations are currently being conducted in both undeveloped and developed residential areas within the City of Rock Springs, WY. The city is largely undermined by historic multiseam, room and pillar coal mining operations at depths ranging from 50 to 400 ft, posing a significant subsidence hazard. This paper describes the unparalleled success achieved by integrated engineering geophysics in subsidence evaluations using the City of Rock Springs as an example. A summary of significant results with emphasis on subsurface characterization, mine workings delineation, and void detection is presented. Mine Subsidence Risk Assessment Abandoned mines can pose a serious threat to the public health and safety and the environment in terms of their potential for collapse and surface settlement or subsidence. With increased growth of our urban areas and associated infrastructure, new development often requires encroachment into areas undermined by historic mine workings. The presence of old mine workings or voids underlying residential and commercial properties, transportation, and infrastructure facilities can cause subsidence ranging from differential settlements to sinkholes, to catastrophic collapse features (Figure 1). Intrinsic to development of cost effective solutions to minimize subsidence risk is an accurate definition of the subsidence hazards that Figure 1 Sinkhole Development Beneath City Street (note: image is not from Rock Springs) 4

exist. This requires determining where mine voids exist, the geometry of the voids, their depth, and the nature of the overlying strata. More often than not this information can not be determined with a high degree of reliability from historic mine maps, particularly in areas where pillar robbing occurred during retreat mining and mapping was not possible. Many historic mining operations were conducted with poor underground mapping control or, in some instances, none at all. Often the actual mine development workings do not match the original mine plans available and no asbuilt maps were ever generated. The existence of an underground mine void does not necessarily represent a significant subsidence hazard. Not only is the depth to mine workings and overburden characteristics important, but also the size and extent of the opening(s), spacing and size of any pillars, and the strength of the remaining rock and pillars. At each specific site the unique combination of characteristics contributing to the subsidence risk must be understood and evaluated in order to accurately assess subsidence risk and thereby develop cost effective methods to eliminate or reduce the risk to acceptable levels. To assess the subsidence risk requires knowledge of the following: General location/boundary of mine workings; Accurate location of the voids (rooms-pillars-rooms); Geometry and condition of mine workings (intact, collapsed, water/air/gob-filled, etc.); Extraction ratios; Size and location of entryways; Subsurface conditions; Strata Geologic setting Depth Size, strength, and stability of the remaining coal pillars; Effects of mine fires, dewatering, etc.; Failure mechanisms (sinkholes or troughing); and Location of the workings relative to surface features. Subsidence from room-and-pillar mines is difficult to predict due to many unknown factors, including the following: Unknown overburden characteristics; Unknown condition of the mine workings; Inaccurate historical mine records; Incomplete mine maps; Nonexistent mine maps; Time-dependent integrity of coal and roof structures; Unknown extraction rate; and 5

Irregular extraction areas with a variety of pillar layouts and remnant sizes. Mine Subsidence Mitigation Approaches The traditional approach to mitigating subsidence risk is typically drilling and grouting backfill programs, i.e., drill and fill. In this approach borings are advanced on a pre-determined closely spaced grid pattern throughout the project areas, followed by injecting high slump grout into any mine voids or rubble encountered. This is generally a blind approach with borings conducted on a predetermined grid, and largely unproductive in determining the true conditions of the mine workings and subsidence risk. Generalized rubble bulking models and the depth to the mine workings are used with a simple rule-of-thumb approach to estimate subsidence risk, e.g. older than 100 years or deeper than 100 ft. The basic premise behind this blind shotgun or Swiss cheese approach is that enough borings within the grid will encounter the mine openings at a suitable number of locations to enable sufficient grouting to reduce the subsidence risk to acceptable levels. However, the void widths can not be determined using this approach and the actual subsidence risk is unknown. The limited snapshot of underground conditions afforded by the borings can not be used to define the geometry of the mine workings or extraction ratios, both of which play a large role in determining subsidence risk. Thus, the blind grid approach can not be used to assess the likelihood or risk of roof collapse or pillar failure and the resulting subsidence hazards. Figure 2 illustrates the impact that a simple shift in the drill hole grid relative to the underground workings can have on the perceived extent of the mine workings, degree of coal Figure 2 Typical Rock Springs Room and Pillar Coal Mine 6

extraction and level of subsidence risk. Such a shift could occur because of an offset in the original mine grid system relative to current geospatial controls and high precision survey equipment, resulting in significantly different success rate for borings encountering voids. This, in turn, affects the interpretation of extraction ratios, subsidence hazards and proposed locations for grout placement, amount of grout to be placed, and level of mitigation provided. Nonetheless, in both examples, the initial subsidence risk was identical. In summary, with the previous blind grid-drilling approach, the existence and geometry of the voids remains uncertain, and the subsidence risk is therefore poorly understood. As a result grout placement is uncontrolled and not easily verified, and ultimately the level of mitigation afforded is questionable. The unknown location and condition of abandoned underground mines represents a significant challenge to geologists and engineers in accurately evaluating subsidence hazards and developing appropriate mitigation measures. The problem must first be accurately defined before an appropriate solution can be developed. State-of-the-art geophysical imaging techniques integrated with other engineering disciplines can be confidently and practically applied to characterize overburden conditions, delineate mine workings and detect voids, thereby allowing subsidence hazards to be more accurately defined. This in turn enables targeted cost-effective mitigation efforts to be employed in a proactive rather than reactive manner. Such innovation is essential to advancing the state-of-practice in mine subsidence investigations. Recent successful applications of this engineering geophysical approach at several abandoned mine sites proved effective in providing subsurface information with greater detail and accuracy than traditional grid drilling methods. Engineering Geophysical Investigation Methodology A comprehensive engineering-geophysical program was developed by ZAPATA, Blackhawk Division and Tetra Tech in conjunction with the Wyoming DEQ-AML to investigate mine subsidence problems in the City of Rock Springs. The investigations are currently being conducted in both undeveloped and developed residential areas within the city. The objectives of the initial portions of the multi-phase investigations are to: Characterize subsurface conditions, geologic characteristics and strength of the subsurface materials; Identify subsurface anomalies, weak zones, and filled or collapsed (rubble) zones related to mine workings, Delineate the location and extent of mine voids; Determine subsidence mechanisms; Determine the appropriate locations of borings aimed at focusing the subsequent exploratory drilling program and the highresolution geophysical surveys; 7

Evaluate subsidence risks within the investigated areas; and Optimize any required mine subsidence mitigation efforts. To achieve these objectives, a variety of high-resolution state-of-the-art geophysical technologies and logging are applied. These techniques are the complemented by the use of conventional drilling and sampling and various tethered robot downhole tools. The basic principles of these methods and techniques are described by Hanna and Pfeiffer (2007); and include: Multi-channel Analysis of Surface Waves (MASW) Method and Common Offset (CO) Sections The MASW techniques provide a complementary look at the shallow subsurface characteristics (up to 80 ft deep), bulk estimate of the shear wave velocities with depth, weak or disrupted zone anomalies related to possible mine workings and identify areas for subsequent exploratory drilling and detailed geophysical surveys. The MASW produces a 2-D shear wave velocity profile under each survey line. The CO sections are basically a single-trace profile of the subsurface, showing the variations in surface waves and body waves along each line, yielding additional subsurface information. These techniques are not designed to produce high-resolution images of the mine voids. Therefore, subsurface mine workings can be expected, theoretically, to manifest themselves as low velocity zones or velocity inversions (high over low velocity). The MASW method employs a surface energy source (such as accelerated weight drop, sledgehammer, or low impact vibrator) as a seismic signal source, and a surface receiver array (geophones). Reverse Vertical Seismic Profiling (RVSP) Method The RVSP method provides high-resolution 2-D profiles of the subsurface characteristics, including images of the localized old mine works and voids (rooms-pillars-rooms). This information is can then used to determine the offset and rotation of the existing mine works from the geo-referenced historical mine map. This method requires at least one borehole. A seismic source (such as an airgun) is lowered into a borehole and data are recorded from a geophone array deployed at the surface. The borehole is cased using 4-in ID, schedule 40, solid PVC, capped in the bottom and filled with water. The annulus between the borehole wall and casing is grouted. A linear array (line) is surveyed at specified boreholes to provide a profile or panel. Multiple panels can be obtained from a single borehole depending on the level of detail required. The depth of investigation may vary from approximately 70 ft to several hundred ft. Guided Waves Method The guided waves method provides information between two boreholes to determine if discontinuities (such as voids, faults, washouts, etc.) exist within the 8

coal seam. This technique is a relatively cost-effective means to identify whether or not mine workings are present. With this method, a seismic source (usually an airgun) is placed in one drill hole at the level of the coal seam. A geophone string (usually a hydrophone) is placed within and around the level of the coal in a second borehole located a distance of up to 700 ft away. If the coal seam is continuous between the holes, the seismic signal generated by the airgun is relatively strong and readily detected in the second hole. If a void exists in the coal between the two holes the seismic signal has difficulty propagating through the seam to the distant borehole. However, this method does not provide highresolution in that the position of the void between the two holes cannot be determined, only the fact that a void or discontinuity exists somewhere in the seam between the two boreholes. Geophysical Logging and Drill Logs Borehole logging surveys provide detailed in-situ information on physical properties immediately around the borehole walls. The results obtained from the MASW profiles and CO profiles (sections) are used as reconnaissance tools for focusing the drilling program into areas most likely to contain old mine working and un-mined coal areas. Based on the interpretation of MASW and CO profiles, borehole locations are selected. The geophysical logging suite included natural gamma (count per second), gamma-gamma density (lb/cu ft), electrical log (Ohm-M) sonic (ft/sec), and borehole deviation surveys. The drill logs provide a record of the physical lithological information from each borehole, including formation depths, formation thickness, lithology and stratigraphy, geomechanical properties, hydrological data, and general drill log data including geographical location. The information obtained from logging is used to improve the understanding of the subsurface strata, delineate geology with depth in the borehole; and augment the geophysical data processing, analysis, and interpretation. Tethered Robot Downhole Imaging Tools Downhole imaging tools included sonar, laser, and video camera. They provide information related to mine void conditions, including the presence of the confirmed voids, content of the voids, geometry of the voids, alignment of mine entries with respect to the historical mine map; and focusing the subsequent drilling operation. The sonar tool is designed to image water-filled voids, while the laser tool is designed to image air-filled voids and cavities. The sonar tool can take range measurements up to 300 ft in 360-degree horizontal planes referred to as scans. Scans are referenced using the depth from ground surface, borehole surface coordinates, and the view of the magnetic compass. By taking enough scans at different elevations, a 360-degree 3-D model of the void can be produced and can be geo-referenced. 9

The laser tool can take range measurements up to 500 ft in distance, in 360-degee horizontal as well as vertical planes. These scans are then used to create a 3-D volume of the void. The borehole camera provides video records of the void conditions (such as the size of the collapsed/fill materials, nature and characteristics of the pillar/roof/floor, etc.) at a distance range of approximately 15 ft. Video from the camera is displayed in real time and can be recorded. Areas of Investigation Site Description The present geophysical investigations of mine subsidence in Rock Springs, Wyoming have been divided into three areas. The first locale is a residential area known as the Tree Streets neighborhood, located just south of downtown Rock Springs. The Tree Streets neighborhood is bounded on the west and north by Blair Avenue, on the east by A Street, and by Willow Street to the south. In addition, two undeveloped areas, known as Tracts B and H, are included in the study. Tracts B and H encompass approximately 23.5 and 37.6 acres to be reclaimed, respectively. Tract H adjoins the Tree Streets neighborhood, lying directly to the southwest. It is bounded on the east by Walnut Street and to the north by Willow Street. Tract B is located approximately 2,000 feet east of the Tree Streets neighborhood. Figure 3 depicts the relative locations and configurations of the sites and illustrates the respective geophysical layouts within each site. Geology Structural Setting. The minable coal seams in the vicinity are contained within the Rock Springs Formation, on the western edge of the Rock Springs Uplift (Schultz, 1910; and Keith, 1965). The strike of the coal-bearing Rock Springs Formation around the city is north-northeast, and the dip is typically 8 to 12 degrees to the west-northwest. Although three coal seams were extensively mined beneath the city, the structural orientation of the seams relative to one another precluded the extraction of multiple seams beneath the same location, except in one section of Rock Springs (Colaizzi, et. al., 1981), as shown in Figure 4. The Tree Streets area and Tract H are both underlain by mine workings on both the No. 1 and No. 7 coal seams. Faulting is common in the area, and in the Tree Streets neighborhood, the trace of the Alder Fault trends approximately N70E along Alder street and separates two historic mine workings into two distinct mine blocks.. Similar faults also cross the Tract H area and offset the mines and mined horizons on either side of the fault. 10

11 Figure 3 Gephysical Investigation Sites

Figure 4 Typical Section of the Coal Seams under the City Stratigraphy The near-surface stratigraphic sequence in the area consists of a mantle of unconsolidated alluvium overlying bedrock. The alluvium mainly consists of very fine-grained sand, some silt, clay and gravel. While this unconsolidated material is up to 40 feet in thickness locally, it ranges from 20 to 30 feet in the vicinity of Tract B, and ten feet or less in the vicinity of the Tree Streets neighborhood. The alluvium is generally non-cohesive, and is subject to flow and collapse if not contained (JFC, 2000). Underlying the alluvial cover is the Upper Cretaceous Rock Springs Formation, comprised locally of a series of coal seams, carbonaceous shale, siltstone, claystone, and sandstone. Three mineable coal seams were exploited from beneath the city, both of which are high-quality, low sulfur bituminous coal. The uppermost mineable coal in the Tree Streets neighborhood is the No. 1 seam, which is present at depths ranging from 30 to 150 feet below ground surface (bgs), and at thicknesses ranging from three to ten feet. The No. 1 seam is also present beneath Tract H but is absent in the vicinity of Tract B. 12

In the Tree Streets neighborhood and Tract H mining also occurred on a lower coal seam, the No. 7 seam. In these areas the No. 7 seam ranges in thickness from three to ten feet, and is separated from the overlying No. 1 seam by approximately 250 feet of interburden consisting principally of sandstone, siltstone and shale. In the area of Tract B, the No. 7 seam is present in thicknesses ranging from three to ten feet, at depths ranging from 60 to 100 feet bgs. Mine Subsidence Problems Shallow No. 1 seam workings are present in the vicinity of the Tree Streets neighborhood within the John Park, Excelsior, and Union Pacific No. 2 mines. Subsidence features associated with these abandoned coal mine workings have been documented in the neighborhood since 1950 with multiple episodes occurring since that time. In response to this problem, a series of efforts aimed at mitigating the effects of mine subsidence in the area have been undertaken since 1976. The mitigation efforts involved various traditional drilling and slurry or grout injection approaches. However, ongoing ground movements are occurring in the area which necessitated additional investigation to determine the cause. Extensive deeper mine workings are also present beneath the Tree Streets neighborhood on the lower No. 7 coal seam in the form of the Central Coal & Coke No. 2 mine. The extent to which these deeper workings have contributed to subsidence problems in the neighborhood is also under investigation. The Tract B and Tract H study areas are undeveloped land parcels which are undermined. These parcels have been proposed for development to help address housing shortages in Rock Springs but have been identified as having high subsidence risk that restricts land use. A subsidence investigation and mitigation design program was authorized at the request of the Governor s office to address the high subsidence risk. Tract B is underlain by workings in the Central Coal & Coke No. 2 mine in the No. 7 coal seam. Tract H is underlain by workings of the John Park and Excelsior mines in the upper No. 1 coal seam and Central Coal & Coke No. 2 mine in the deeper No. 7 coal seam. Historic subsidence events have been documented in Tract H. Data Acquisition Data acquisition and related field activities at the investigational sites have been conducted beginning August 2007 to the present. Figure 5 presents photographs of the field activities as typical examples of the work conducted. 13

Figure 5 Data Acquisition Field Set-up 14

Discussion of Results As previously described, the investigational sites involved multi-seam coal mining at depth ranging from 30 ft to 400 ft bgs. The mine workings at shallow depth are air-filled, and flooded at greater depth. The following summarizes some of the significant results obtained from the geophysical investigations. MASW, RVSP, and GW Interpretations Figure 6 depicts the project location map, the MASW, and the RVSP interpretation for Tract B with the following results: MASW and RVSP survey lines layout, and data interpretations have been superimposed on a digitized historical mine map of No. 7 coal seam. The areas highlighted in light blue are the MASW and Common Offset anomalies interpreted as the lateral extent of the mine workings areas. The areas highlighted in dark blue and yellow are the RVSP anomalies showing the presence and lateral extent of mine openings (pillars-voidspillars). Exploratory Boring Locations. There were 21 borings advanced in the 23 acres of Tract B. These borings were used for subsurface geologic characterization, borehole geophysical logging (see Figure 5), RVSP imaging, confirmation of the geophysical investigation ( ground truthing ), and borehole camera and laser imaging of the detected mine voids (yellow rectangle areas). In addition, the map shows over 150 boring locations (red circle color and text labels) in the residential area north of Tract B that were drilled as part of a previous investigation and grouting program (1997). Historical Mine Map. The map was digitized using several different versions of available old maps. The map layout and position have not been finalized. The MASW interpreted data were used initially to position the exploratory borings for confirmation of the interpretations and to locate the borings for the subsequent RVSP survey. In conducting these borings there was a 100 percent success rate in encountering either void or pillar targets based on the MASW and CO profiles, thus confirming the accuracy and usefulness of these techniques. See Figure 6 for the MASW survey lines and borehole locations. Figure 7 shows the MASW profile line 2 data interpretation for Tract B with the following results: The interpreted mine workings anomalies show a lateral extent of mine workings up to 100 ft. The two borehole drill logs show rubble zones at depth of approximately 50 to 60 ft. 15

16 Figure 6 Tract B MASW and RVSP Interpretation Map

Figure 7 Tract B MASW Profile Line 2 Mine Workings Data Interpretation Borehole B-EX4 shows that the mine floor, immediate roof, and the overburden are stable while borehole B-RV4EX shows that the rubble zone has extended above the immediate roof. The immediate roof had collapsed creating a 2 ft void. These results are consistent with the high and low velocity zones depicted on the MASW profile at these locations, respectively. Figure 8 shows boreholes cross section data interpretation along the MASW line 6 for Tract B with the following results: Borehole lithology data and interpretation of the lateral extent and height of the rubble zone within the mining horizon from the MASW data were consistent with the subsequent borehole data. Borehole B-RV8EX (total depth [TD] 75 ft) shows a rubble zone height of approximately 28 ft above the mine floor. 17

Borehole B-RV6EX (TD 75 ft) shows a 4 ft void. The laser and borehole camera tools were used to image the void conditions, and are discussed separately. Figure 9 shows a borehole cross section data and GW interpretation from the Tree Streets area with the following results: Stratigraphy data from boreholes W-12RV (TD 340 ft) and P-17 RV (TD 100 ft), and the No 1 and No. 7 coal seams. Rubble zones were not encountered in either boring. The boreholes were spaced approximately 275 ft apart and used to obtain RVSP and GW data. Due to drilling difficulties, borehole P-17 RV was not advanced to the deeper No. 7 seam. Guided waves interpretation shows a continuous wave propagating through the coal seam between the two boreholes. This indicates an intact coal seam with no mine workings within the shallow No. 1 coal seam between these two boring locations. Subsidence impacts to overlying residences must be attributed to the deeper No. 7 seam mine workings. Laser, Borehole Camera, and Sonar Interpretations The voids encountered during the drilling program were further investigated to confirm the geometry and condition of the mine workings. Figure 10 shows the borehole camera and laser images from borehole B- RV6EX located in Tract B with the following results: Intact coal rib, indicative of stable coal pillar. Roof fall rocks pilled on mine floor. This rubble is comprised of large slabs of the original 2 to 3 ft thick immediate shale roof and represents a low bulking factor. Void geometry of various sizes up to approximately 45 ft long by 20 ft wide by 4 ft high. The individual scans were used to create a 3-D volumetric image of the mine workings. This type of information will be used to augment the mitigation design program. 18

19 Figure 8 Tract B - Boreholes Cross Section Data Interpretation along MASW Line 6

20 Figure 9 Tree Streets Guided Waves Line 1 Interpretation between Boreholes W12-RV and P17-RV

Figure 10 Laser Scans and Borehole Camera Images in Borehole B- RV6EX in Tract B 21

Figure 11 shows the laser and sonar images collected from borings H39 and H31 located in Tract H with the following results: Borehole H39 encountered a 9 ft void from 42 to 51 ft bgs. The laser scans revealed that the void extended approximately 85 ft long by 20 ft wide by 9 ft high. This information was used in real time, in the field, to position the drill rig for subsequent borings conducted in a leap frog fashion. The voids encountered in these additional borings were also scanned. These scans provided a three-dimensional image of a portion of the mine workings and allowed us to align the historical mine map. Borehole H31 encountered a void from 393 to 401 ft bgs. The image shows the lateral extent of the void to be approximately 66 ft long by 22 ft wide. The extent of the scans was limited by debris flushed into the void during drilling. In summary, the interpreted data obtained from the historic mine maps, MASW, RVSP, and GW surveys combined with borehole geologic sections, laser/camera images, geophysical logs, and laboratory test data are being used to determine: Geologic setting; Overburden characteristics; Strength and characteristics of subsurface materials Void conditions and geometry; Rubble zone height; Mine workings and layout; Mine workings failure modes (stable or unstable pillar/roof/floor); Subsidence failure mechanism and associated risk; and Shear wave velocities. Evaluation and assessment of the subsurface conditions, utilizing the methods mentioned, are ongoing to determine the subsidence risk and optimum mitigation design program. Final reports of these studies are expected to be available to the public later in 2008. 22

Laser Image Borehole H39b @ ~ 42 ft Depth in Tract H Figure 11 Laser Scans in Borehole H39 and Sonar Scans in Borehole H31 in Tract H 23

Conclusions In contrast to traditional blind techniques typically employed for subsidence mitigation, the Tetra Tech - ZAPATA team concentrates on developing cost effective, focused solutions by: Using a suite of advanced geophysical methods to identify the position, shape and extent of mine voids, including pillar size, spacing, and condition, to accurately define the nature and extent of the potential hazard while minimizing subsurface drilling; Coupling the results of the geophysical surveys, test borings and laboratory data and an understanding of rock mechanics, coal mining operations, and underground coal mine design with empirical and state-of-the-art numerical models to provide detailed evaluation of the subsidence potential, identify specific areas requiring additional ground support or backfilling, and the level of mitigation necessary; and Applying a variety of cost effective grouting approaches suitable for for the type and extent of subsidence risk, depth and geometry of the mine workings, and overburden characteristics. With this approach, ultimately a significantly lower level of risk can be obtained at less overall cost than the traditional approach and will be particularly valuable for areas where the underground workings were poorly documented or mapping inaccurate and the actual extent of potential subsidence hazards are unknown. Acknowledgements The authors would like to express their appreciation to the state of Wyoming DEQ-AML for their undivided attention in guiding and directing this project and for having faith and determination to implement new technologies. Literature Cited Hanna, and Pfeiffer, 2007. Geophysical Technologies to Image Old Mine Works, Paper in 20 th Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP), April 1-5, 2007, Denver, Colorado. JFC Engineers, Architects, and Surveyors, 2000. South Side Belt Route, I-80, 18 and 24 Waterlines, Etc., Final Report for Wyoming Abandoned Mined Land Rock Springs Drill and Grout, Vol. I, unpublished. Mederos, Selena, Tikoff, Basil, and Bankey, Vickey, 2005. Geometry, Timing, and Continuity of the Rock Springs Uplift, Wyoming, and Douglas Creek 24

arch, Colorado; Implications for Uplift Mechanisms in the Rocky Mountain Foreland, U.S.A., Rocky Mountain Geology; December 2005; v. 40; no. 2; p. 167-191. Schultz, A.R., 1910. The Southern Part of the Rock Springs Coalfield, Sweetwater County, WY, Paper in Contributions to Economic Geology, Part II, Mineral Fuels. U.S. Geological Survey Bulletin 381, 1910, pp. 214-281, in Colaizzi, G.J., Whaite, R.H., and Donner, D. L., 1981, Pumped Slurry Backfilling of Abandoned Coal Mine Workings for Subsidence Control at Rock Springs, Wyoming, U.S. Bureau of Mines Information Circular 8846, p. 7. 25