rock joints-direct shear strength-laboratory method of determination
1989 Edition

IS 12608 pertains to the procedure for measuring rock hardness, recommending the use of small, individual rock pieces that can be firmly fixed to a stable base to prevent any movement during testing. The orientation of the test surface relative to geological features like bedding or foliation should be documented when relevant. Suitable specimens should be representative of the rock mass in terms of lithology, mineralogy, and grain size, adequately sized for secure clamping, and prepared with a flat and smooth surface. This method is applicable to a broad range of rock types encountered in geotechnical and engineering geology investigations.

Prior to testing, the Schmidt hammer must be calibrated using a designated calibration anvil. This involves taking ten rebound measurements on the anvil and calculating their average. If this average deviates beyond acceptable limits from the standard value of the anvil, adjustments or maintenance of the hammer are necessary. The hammer used should be Type L with an impact energy of 0.74 Nm. This calibration ensures the precision and repeatability of hardness measurements during the actual testing process.

The standard mandates conducting no fewer than 25 individual hardness tests on a single rock specimen. These test points must be spaced at least twice the diameter of the hammer’s plunger apart to avoid localized effects. The final hardness value is determined by averaging these measurements. If any test causes visible damage such as cracking, both the test and specimen are invalidated. Specimens must be securely held in place during testing to ensure accuracy and repeatability.

Hardness readings are influenced by the orientation of the hammer relative to the specimen—whether vertical (up or down) or horizontal within ±5°. When testing cannot be performed at these standard orientations, such as in tunnel settings, measurements taken at other angles should be corrected using factors from IS 11315 (Part 5):1987. These correction factors adjust for the effects of gravity and rebound variations due to orientation differences. The orientation and any applied corrections must be documented in the test report to ensure consistent and comparable results.

The Schmidt hammer method uses a spring-driven steel piston impacting a metal anvil on the rock surface, with the rebound height serving as an empirical hardness measure suitable for both field and laboratory use. In contrast, the Shore scleroscope employs a diamond-tipped indenter dropped onto the specimen, primarily for laboratory measurement of mineral hardness. The Schmidt hammer contacts a larger surface area and provides a single hardness value per test, making it suitable for quick field assessment. The Shore scleroscope requires multiple tests to account for mineral heterogeneity and provides an averaged hardness value. Thus, the Schmidt hammer offers a rapid empirical test, while the Shore scleroscope delivers more precise laboratory-based hardness evaluation.