INSTRUMENTATION TO MONITOR BEHAVIOR OF SHOTCRETE SUPPORT SYSTEMS

In this discussion which comments on the construction conditions (which are directly related to the instrumentation results), and the general behavior of shotcrete-rock bolt and shotcrete-steel rib support systems, emphasis is placed on the rock instrumentation program which was conducted in the Washington, D.C. subway tunnels. The types of instruments used and their performance is described, as well as the instrumentation program and the preliminary results of the studies. The effects of rock geometry, rock block movements, shotcrete-rock bond, as well as the shrinkage properties of the shotcrete on development of tensile and compressive strains in a rock bolt-shotcrete lining are illustrated. Conclusions regarding design considerations of shotcrete support systems are presented. The instrumentation consisted of the use of extensometers to monitor general rock behavior in the tunnels and stations, and extensometers and strain gages placed in test sections to measure rock movements and strains in the shotcrete lining. The basic approach used was to correlate local geologic conditions and construction procedures with these measurements. The extensometers consisted of mechanical rod-type, double-position and six-position units. The bonded resistance gages used here, were read using a Wheatstone bridge, and the vibrating wire embedment gages were read using a frequency counter. Rock movements in steel supported sections of the tunnel were as much as five times those in the shotcrete-lined reaches. Tension stresses were developed in locations where chloride seams and bedding plane orientation was such that tension would develop. Rock bolt-shotcrete combination support system was used in these areas.