In situ testing of young shotcrete subjected to vibrations from blasting

A criterion for how close, in time and distance, to young shotcrete blasting can take place will be an important tool in planning for safe and economical tunnelling projects. As a first step, in situ tests with young, plain un-reinforced shotcrete have been conducted in a Swedish mine. Shotcrete panels were projected on tunnel walls and exposed to vibrations from explosive charges detonated inside the rock at shotcrete ages between I and 25 h. The shotcrete was tested to investigate growth of compressive strength and also to determine final compressive and adhesive strengths. The response of the rock was measured with accelerometers, giving signals that were later numerically integrated to particle vibration velocities. An average scaling relation of distance between explosive charges and the point of observation and weight of explosives was also derived. The recorded vibration levels showed that the shotcrete had withstood high particle velocity vibrations without being seriously damaged, even when the rock material was fragmented and ejected. Drumminess of shotcrete appeared, indicating that the major failure mechanism was sudden loss of adhesion at the shotcrete-rock interface. No differences in performance between shotcrete of different ages were observed. It was, thus, confirmed that young shotcrete without reinforcement can also survive high vibration levels without being seriously damaged. (C) 2004 Elsevier Ltd. All rights reserved.