Low Pressure Abrasive Waterjet Use for Precision Drilling and Cutting of Rock

Construction of a second theater underground at the Gateway Arch in St. Louis required a quiet, non-intrusive means of rock removal, capable of cutting the vertical walls of the theater down to a depth of 4.5 meters, within a vertical accuracy of 2.5 cm. A low pressure (35 MPa) abrasive laden waterjet was developed for this; and, also modified as an abrasive waterjet drill to emplace 25 mm diameter resin-bonded rock bolts in the rock walls. The main volume of the rock within the room was concurrently excavated using a method known as CUSP, where the rock is drilled to a depth of 70 cm, split using a DARDA rock splitter, and the resulting crack grown out to delineate 400 kg blocks using a 7.5 kw impact breaker attached to 743 series Bobcat. This system proved effective in breaking out blocks in rock ranging from plain dolomite to layers which included beds of chert several inches thick. Production rates within the room were constrained by the limited space available, but exceeded 5 cubic meters/hour. On the basis of this experience, the paper describes the practical advantages and problems of using this new method of construction, stress the advantages of the relatively small, lightweight, maneuverable tools that were developed; discussed the way in which these tools can best be used in mining and tunnelling operations, and provides an indication of the costs which might be anticipated for such use.