| Popis: |
This study used several seismic techniques to aid different phases of a subsidence investigation at an abandoned coal mine subsidence site in a community located in the anthracite fields of eastern Pennsylvania. Two surface collapse features, each about 3-5 m (lo-l.5 ft) in diameter, developed on an athletic field underlain by an abandoned room-and-pillar mine 20 m (60 ft) deep. In a multiphase investigation, surface seismic techniques were utilized to characterize mine level and overburden conditions, and to design an effective drill@ pattern for the remediation phase. Subsequently, borehole pressure grouting techniques were used as an abatement procedure to stab&e the ground. During this phase, borehole seismic techniques were used to further characterize subsurface conditions as more than 680 stere (890 cubic yards) of grout was injected. The surface seismic study included walkaway tests followed by a series of common offset lines that traversed the athletic field. A signal enhancement seismograph, lOO-Hz geophones, and a sledge hammer were used for data acquisition. The seismic data combined with information from a mine map and several borehole logs suggested that the athletic field was divided into two zones: the western portion of the field characterized by open mine voids, and the eastern portion of the field characterized by mine voids more likely to be ftied with collapsed rubble. Using these results, a drill& pattern was recommended and more than 20 boreholes were drilled in preparation for the remediation phase. Borehole logs corroborated the predicted trends. All boreholes that would allow placement of grout were selected for the grouting operations and the borehole seismic study was conducted concurrently with grouting. One shot hole and three receiver holes were selected. An air gun served as an energy source and downhole receivers consisted of 4O-Hz geophones arranged in a triaxial configuration. Shot-receiver crosshole traveltimes were acquired for selected depths and these were used to determine crosshole velocity distributions prior to and during injection. Preliminary analyses of the crosshole data indicated a correlation between high velocities and competent mine overburden. In contrast, velocities tended to decrease as the mine level was reached. Both the surface seismic and the crosshole seismic studies provided valuable data and additional subsurface constraints for the characterization and remediation of this abandoned coal mine site. |
