568 ENGINEERING GEOLOGY/Rock Properties and Their Assessment Figure Correlation chart for Shore hardness, relating unit weight of rock, unconfined compressive strength and hardness value (After Deere DU and Miller RP (1966) Engineering Classification of Engineering Properties for Intact Rock Technical Report AFWL TR 65 116, Air Force Weapons Laboratory, Kirkland Air Base, New Mexico) The Schmidt hammer is a portable non-destructive device that expends a known amount of stored energy from a spring and indicates the degree of rebound of a hammer mass, following impact, within the instrument (Figure 4) Tests are made by placing the specimen in a rigid cradle and impacting the hammer at a series of points along its upper surface The hammer is held vertically at right angles to the axis of the specimen The specimens should have a flat smooth surface where tested and the rock beneath this area should be free from cracks Test locations should be separated by at least the diameter of the plunger At least 20 readings should be taken from each specimen The lower 50% of the test values should be discarded and the average obtained from the upper 50% This average is multiplied by the correction factor of the Schmidt hammer to obtain the hardness However, the Schmidt hammer test is not a satisfactory method for the determination of the hardness of very soft or very hard rocks Like the Shore scleroscope test, Schmidt hardness Figure The Schmidt hammer Reproduced from Engineering Geology, vol 4, 1979, Elsevier values can be used to derive approximate values of uniaxial compressive strength, as shown in Figure Abrasion tests measure the resistance of rocks to wear The two abrasion tests most frequently used are the Dorry and Los Angeles tests As both these tests are used to assess the resistance to wear of aggregate for road making, they are not dealt with here