Stabilization of coal mine waste with soil nails

Surface mine operations in the southern Appalachian mountains of the USA have left numerous unstable or marginally stable waste rock slopes. These slopes, which were created by dumping waste material downslope, comprise low-density, random-size material ranging from clay particles to boulders, and often include debris such as trees and stumps. Typical remediation measures include reducing the slope angle or constructing expensive retaining structures. An investigation was conducted to evaluate the effectiveness of soil nailing in these waste rock slopes. A slope that had failed previously was cut back and stabilized with soil nails. The lower portion of the slope was steepened from the existing 1H:1V slope to about 1H:3·7V (75°), thus creating a ‘cut slope’ requiring support to maintain stability. Rather than facing with shotcrete, an erosion control material was placed on the slope, and geogrid facing material was applied over the steeper portions of the slope. The slope was monitored by means of slope inclinometers. Upon completion of the nail installation, a control section of the slope was excavated to the same inclination as the nailed sections, and permitted to fail. Recommendations are provided for stabilizing mine waste slopes with soil nails. Due to the boulders, tree stumps, and debris found in mine waste, nail positions must be changed from the original design. While this flexibility during construction is an advantage of the soilnailing method of stabilization, the as-built nail configuration may bear little resemblance to the design slope. Furthermore, the irregular nail pattern is not amenable to analysis by traditional methods based on constant horizontal and vertical spacing. A method for the determination of an equivalent horizontal and vertical spacing was developed based on the nail density. This method permits the analysis of irregular nailing patterns with traditional twodimensional procedures, and yields the variation in factor of safety across the slope. The method is demonstrated for the project slope.