Stress and slabbing in massive rock

Abstract

Introduction The measurement of in situ stresses is now much easier and more reliable than seven or eight years ago. This is primarily due to improvements, both in the solid state electronics of read out devices and in the epoxy glues used to attach strain gauges to rock. In addition the micro computer now facilitates immediate evaluation of field test data. This paper discusses the results of a field investigation into the measurement of locked-in stress in a massive sandstone. In addition, the geological factors affecting the behaviour of the sandstone, including changes of loading and geometry due to a road cutting excavation, are examined. Finally, a remedial treatment is proposed to overcome problems which developed after construction and as a consequence of the locked-in stress. A major road cutting constructed by the Department of Main Roads, New South Wales is located south of Wyong within the Sydney Basin of eastern Australia (Fig. 1). The general profile of the road cutting incorporating the site stratigraphy is shown in Fig. 2.> Thirty-five metres of shale overlie a massive sandstone exposed for 25 m of a known 30-m thickness. Underlying the sandstone is additional shale. Excavation of the cutting was achieved by ripping in the shale layers and by blasting in the massive Wyong sandstone. The sandstone was presplit prior to blasting. Two years after excavation, and before the road has been opened to public traffic, continual spailing of the 1/4:1 sandstone batter has occurred and become steadily worse. Over a three