Strain monitoring of borehole diameter changes in heterogeneous jointed wall rock with chamber excavation; estimation of stress redistribution

Strain changes of vertical diameter in horizontal boreholes drilled in the wall rock at two different sized openings were monitored under conditions of progressive excavation in a Mesozoic porphyrite rock mass. For a small chamber measuring 8 m wide, 7 m high and 32 m long, a diagram of the strain changes measured with 12 remodeled borehole-type strain gauges showed two peaks at distances of 1.91 m and 5.63 m from the wall. For a large cavern measuring 24.0 m wide, 46.6 m high and 134.5 m long, a diagram of these changes by 11 gauges showed two peaks at distances of 5.5 m and 8.5 m from the wall. These two peaks were made stepwise by progressive excavations, indicating that stress increases due to excavations were supported in the two portions by the interlocking of jointed blocks. However, a conventional elastic theory predicts that the maximum compression occurs just behind a stress relaxed region, and gradually decreases as a function of the distance from the chamber wall; thus, this theory cannot be applied to the stress redistribution of a heterogeneous jointed rock mass.