Rock Displacements Measured During URL Shaft Sinking

Abstract

During sinking of the Canadian Underground Research Laboratory (URL) shaft, borehole extensometers were used to obtain rock displacement measurements and a tape extensometer was used to measure total convergences. The instruments, instrument modifications, and methods used are described. The measurements are summarized and assessed, with particular emphasis on the influence of natural fractures on rock-mass response and the performance of the instrumentation. Displacements varied from 0.09 mm to 1 .75 mm. The frequency of sub-vertical fractures in the rock appeared to be the main factor causing the variation in the measured displacements. Although the displacement instrumentation met certain operational requirements well, lack of precision was a problem. Displacement instrumentation used in future URL experiments should have more measuring points, greater sensitivity, and greater accuracy to better measure small displacements.