Interstate Technical Group on Abandoned Underground Mines
Fourth Biennial Abandoned Underground Mine Workshop
3-D Seismic Tomographic Imaging for Abandoned Mine Subsidence Characterization : Case Histories
Khamis Haramy, Geotechnical Engineer
Department of Transportation
Federal Highway Administration
Central Federal Lands Highway Division
Denver Federal Center, Bldg. 52,
P.O. Box 25246, HTS-16.6
Denver, Colorado 80225
Phone: (303) 716-2192
Geotechnical engineers have struggled for years with the difficult task of accurately locating abandoned underground opening that affect the design and stability of various transportation infrastructures. Commonly, designs are based on data from borings that may or may not encounter underground opening.
Subsidence above mine openings is generally divided into two classifications: trough subsidence and chimney or sinkhole subsidence. Trough subsidence results from a widespread collapse of mine workings, and can be caused by a combination of roof caving, pillar failure, and punching of pillars into soft mine floor. Trough subsidence is typically associated with full extraction mining methods such as longwalling or full pillar retreat. Sinkhole subsidence is associated with the failure of isolated areas or individual openings and is caused by roof failure migrating upward through weak overburden. This migration stops when the bulked volume of caved material is sufficient to fill the original mine void. If the workings are shallow enough, subsidence at the surface occurs as circular or elliptical depressions. Therefore it is essential to map the location, size and number of openings in the subsurface prior to predicting its effect on surface subsidence.
Developing the means to cost-effectively, yet comprehensively characterize site conditions during the geotechnical evaluation or construction of a project is critical to minimize project risk and ultimately, project costs. This paper describes a 3-D tomographic imaging method that has been successfully used on many projects to map abandoned mines in the subsurface. Case studies will be provided of how both risks and costs were reduced through the application of this technology for major transportation projects.
Case Study 1: Location of a collapsed tunnel beneath the foundation of an overpass over I-25 in Colorado:
During the planning and construction of a new highway overpass on Interstate 25, the Colorado Department of Transportation (CDOT) became concerned that the foundation stability of the bridge could be compromised by the existence of a rock tunnel connected to an abandoned coal mine beneath the bridge caissons. Information from Colorado Geological Survey (CGS) indicated the possible existence of a rock tunnel and crossing the interchange below Pier 3, which is under construction. The exact tunnel alignment and conditions with respect to the location of the caissons were not clear. If the tunnel has collaped, itscollapsedne may extend to the bearing horizon, reducing the bearing capacity of the foundation and the skin friction along the caissons significantly. In an attempt to better define the location and alignment of the tunnel and to assess its condition or state of collapse, a 3-dimensional crosshole tomographic survey was performed. The purpose of the survey was twofold:
- To image the subsurface geologic structure including strata and cavity or rubble resulting from past mining operations.
- To evaluate the extent of the subsidence zone that may occur due to mine entry/tunnel collapse.
A total of six vertical boreholes, defining a zone/volume approximately 110 ft wide by 275 ft long by 200 ft deep, were used to conduct a cross-borehole seismic survey. All boreholes were drilled and cased with perforated PVC pipe. The crosshole seismic survey was conducted using similar procedure as discussed in the previous section. Color-coded 2-D and 3-D tomographic images were constructed, including horizontal and vertical cross-sections, at 5-ft increments to delineate the location of the tunnel.
Tomographic imaging results are depicted in several cross-sections, as shown in Figure 2. The location of the suspected tunnel was established at a depth between 90 and 95 ft. The results from the three-dimensional images indicate that the tunnel is not intact, and that a caved zone extends from the tunnel horizon to near the ground surface. Figure 1 shows three distinct zones: (1) an unconsolidated, low-velocity zone (< 20 blows); (2) an intermediate consolidated zone (>30 blows); and (3) a consolidated, high-velocity zone (> 50 blows).
Based on field studies of sinkhole subsidence previously performed in Boulder County (Matheson, 1987), the expected extent of the rubble zone and angle of draw were superimposed on the tomography results and related to the planned location and bearing horizon of the bridge caissons.
Figure 3 shows that if further sinkhole-type subsidence is to occur, the bearing capacity of Column 1 will be affected. Based on this information, CDOT is currently exploring options for the geotechnical design phase, including relocation of the caissons, changes to caisson design and depth, and/or rehabilitation of the rubble zone near the bearing horizon.
Case Study 2: Detection of abandoned coal mine entries beneath a retaining wall foundation along US Rout 24 near Peoria, Illinois:
During the installation of soldier piles for a retaining wall in Peoria County, IL, the Illinois Department of Transportation (IDOT) encountered underground cavities filled with water. It was determined that the cavities were related to the existence of an abandoned underground coal mine under the retaining wall foundation and US Route 24. Cross-hole seismic velocity tomography was performed to image subsurface conditions beneath the retaining wall. The objective was to identify and map the extent of the underground mine workings and any other weak, permeable, or highly fractured zones that may have been present.
The survey was accomplished by drilling 15 vertical boreholes and pairing an airgun source within a borehole with a string of hydrophones in an adjacent borehole. A set of surface-mounted geophones was also used to provide additional information. Air gun and hydrophone positions were varied to create adequate ray path coverage between the boreholes, allowing a three-dimensional tomogram to be produced for a volume approximately 26 ft wide by 720 ft long by 60 ft deep.
A summary of the seismic tomography results is shown in Figures 4 and 5. Figure 4 is a 3-D image of the relatively low velocity zones, and figure 2 is a 2-D crosscut through the middle of the surveyed area. The results in Figure 4 are presented as solid, three-dimensional velocity contours of the lower velocity surface that delineates less competent material or surface soils layers. Figure 5 shows both the velocity tomography results and the interpretation in terms of coal mine entry and collapsed zone locations. The figures reveal the presence of three collapsed mine entries, with their associated caved zones. Based on those results and on-site observations, it is concluded that the entries are about 20 ft wide by 8 ft high. Each entry and the triangular area overlaying the entry (figure 2) are interpreted as filled with fractured material, indicating a caved zone extending to the surface.
Once the entry locations were identified, IDOT developed a rehabilitation strategy wherein approximately 3-ft-diameter pipes were placed vertically through the identified mining horizon and fracture zone, with soldier piles placed and grouted within the pipes. The tomography survey allowed the rehabilitation effort to proceed without the need for expensive and time-consuming probe drilling. The locations of the three coal mine entries correspond with locations where IDOT drilling penetrated water-filled voids. Thus, the results of the tomography study were assigned a high degree of confidence, and remediation efforts proceeded with no delays based on survey results.
Case Study 3: Evaluation of the Effectiveness of Jet Grouting to Improve the Quality of a Bridge Pier Foundation:
The Bill Emerson Memorial Bridge is being constructed over the Mississippi River between Missouri and Illinois. The river bottom consists of karstic limestone; voids and structural defects within the limestone necessitated reinforcement of the foundation by high-pressure jet grouting. To evaluate the effectiveness of the jet-grouting program, tomographic surveys prior to and subsequent to jet grouting were performed.
From a construction platform in the river, six boreholes were drilled in close proximity to one another to a depth of about 70 ft below the river bottom. Pairs of adjacent boreholes were surveyed using an air gun source and hydrophone receivers in a manner that allowed a three-dimensional tomogram to be produced. The volume surveyed was about 11 ft long by 10 ft wide by 70 ft deep.
Figure 6 shows sections of the resulting tomogram before and after jet grouting operations. The "Pre-Grouting" and "Post-Grouting" images are rotations of the vertical prism that made up the surveyed volume. The image labeled "Well-Log Images" in Figure 3 illustrates how well log data may be presented in a three-dimensional image that is compared to tomography results. In this example, data from a well-site geologist was used to construct a three-dimensional image of the borehole logs. Material property values were assigned to each lithologic unit identified and the volumetric composite log constructed. These data may be compared to Tomo2 of the pre-grout seismic velocity survey. Acceptable agreement was obtained. After grouting in this volume was completed, the survey was repeated, using the same boreholes and data collection procedure. Results of this survey are shown in the right-hand portion of Figure 6. The gross effect of grouting was to increase the overall seismic
velocity of the volume. This may be interpreted as an improvement in the overall rock quality and strength. These results allowed Missouri Department of Transportation (MoDOT) to confirm that the jet grouting effort have improved the overall rock quality but also showed few zones that were not adequately grouted for the pier foundation as designed.