Uniaxial compressive strength is regarded as one of the most significant factors in designing underground constructions, which can be determined through different factors, one of the simplest and most nondestructive of which is using Schmidt Hammer. Schmidt Hammer comes in different types, the N and L are the most useful ones, though. In this study, the N and L types were used to determine compressive strength of rock mass in Emamzadeh Hashem Tunnel lot II. The uniaxial compressive strength of rock mass was calculated. The standards presented by ISRM and ASTM were used to achieve a logical relationship between the uniaxial compressive strength and the rebound factor. The results showed that Schmidt Hammer type L had better uniaxial compressive strength estimation in Emamzadeh HashemTunnel lot II.
Schmidt hammer was first invented in 1948 to estimate concrete hardness and then it was used to evaluate compressive strength of rock mass. Availability, low cost, quick in situ and in laboratory control and reliability of results are among factors can be named for its increasing use. Schmidt hammer comes in different types, types N and L were used in this research to first understand the compressive strength of rock mass and second choose the best type using statistical results.
Schmidt hammer is used for evaluating compressive strength of rock mass and concrete materials. In order to have a reliable estimate of compressive strength, Schmidt hammer should be first calibrated using a calibration test anvil, the mean of 10 readouts should be calculated on the standard anvil and used to determine the correction factor. In order to do that, plunger is placed on the sample; by pressing it, plunger goes into the hammer. It presses the spring inside the hammer. The spring toggle is released at a specific compression energy level and hits (impacts) the piston placed on the plunger. The rebound height of piston is read on the ruler and used as a criterion for determining hardness. The floating piston affects the surface and the sample returns to the initial state. The rebound distance is dependent on energy absorbed by the affected surface. The rebound value of floating piston is read as the rebound number directly from the numerical criterion available on the device body. Under figure show function Schmidt hammer in rock mass of tunnel.