As part of an investigation of ground movements around a zone of caving ground, measurements were made of the deformation of a 90- meter length of mine tunnel. The tunnel lay astride the path of an approaching caved zone 64 meters wide, which eventually undermined the tunnel. The approach of the caving zone caused the rock mass around the tunnel to expand laterally and downward toward the cave, resulting in a gradual sinking of a portion of the tunnel and distortion of the tunnel section, which led to the fracturing of the concrete lining. Longitudinal extension and bending of the axis of the tunnel were measured, as well as the distortion of the tunnel cross section, using extensometers and tiltmeters of simple design. The approach was to monitor basic deformation components that could be conveniently measured by mine personnel, making maximum use of off-the-shelf hardware, to show that definitive data can be generated thereby. Especially notable was the demonstrated capability to determine from a triangular configuration of cross tunnel extensometer measurements the principal directions of deformation within the surrounding rock mass. By increasing the number of gage points from 2 to 3, three times as much information is obtianed as by measuring roof-to-floor convergence.
Proc. 22D U.S. Symp. on Rock Mechanics, Cambridge, Massachusetts, June 29- July 2, 1981, PP. 328-334