ENGINEERING GEOLOGY/Rock Properties and Their Assessment 575 Figure 11 (A) Graph derived from shear test, showing curves derived for both peak and residual strength (B) Other forms of shear strength tests 54 mm in diameter, with a thickness that is approximately equal to the radius of the specimen The tensile strength of the specimen is obtained as follows: Tb ¼ 0:636P=DH ½9 where H is the thickness of the specimen The Brazilian test is useful for brittle materials but for other materials it may give erroneous results In the point load test the specimen is placed between opposing cone-shaped platens and subjected to compression This generates tensile stresses normal to the axis of loading and the indirect tensile strength (Tp) is then derived from: Tp ẳ P=D2 ẵ10 Loading can take place across the diameter of the specimen, as in Figure 12, or along the axis Rocks that are anisotropic should be tested along and parallel to the lineation Irregular-shaped specimens can also be tested, but at least 20 tests should be made on the same sample material and the results averaged to obtain a value The point load test is limited to rocks with uniaxial compressive strengths exceeding 25 MPa (i.e., point load index above MPa) The effect of the size of specimens is greater in tensile than compression testing because in tension, cracks open and give rise to large strength reductions, whilst in compression the cracks close and so disturbances are appreciably reduced This is especially the case in the axial and irregular lump point load tests Accordingly, a standard distance between the two cones of 50 mm has been recommended, to which other sizes should be corrected by reference to a correction chart Once determined, the point load index can be used to grade the indirect tensile strength of rocks, as shown in Table Finally, in the flexural test a cylindrical specimen of rock is loaded between one lower and two upper supports until the sample fails The flexural strength gives a higher value of tensile strength than that determined in direct tension Durability of Rocks Durability refers to the resistance that a rock offers to the various processes that lead to its breakdown and therefore durability tests can be used to provide a general impression of how a rock will behave in relation to weathering, especially mechanical weathering Durability tests most frequently are used to assess the behaviour of suspect rocks, that is, those that tend to breakdown more readily such as mudrocks, some chalks, and certain basalts and dolerites There are a large number of tests that have been used to assess the durability and many of them are used to determine the durability of rock as a material for building purposes, for aggregate, or for armourstone The latter types of test are not dealt with here Some of the more simple tests include the water absorption test, the wet and dry test, the freeze-thaw test, and soak tests The water absorption test involves oven drying a rock specimen at 105 C until it has attained a constant weight and then saturating it under vacuum The percentage saturation is determined and reflects porosity As rocks break down, their porosity increases and so the water absorption test has been used to indicate the degree of