ENGINEERING GEOLOGY/Rock Properties and Their Assessment 567 Table Dry density and porosity Class Dry density (Mg m 3) Description Less than 1.8 1.8 2.2 2.2 2.55 2.55 2.75 Over 2.75 Very low Low Moderate High Very high Porosity (%) Over 30 30 15 15 5 Less than Description Very high High Medium Low Very low Reproduced from Bulletin International Association Engineering Geology, No 19, 364 371, 1979 latter generally is determined by using the standard saturation method Alternatively, an air porosimeter can be used In both methods, the pore volume is obtained by saturating with water or with air, the total volume being found by the caliper or bouyancy method The porosity as determined by these two tests does not provide an indication of the way in which the pore space is distributed within a rock, or whether it consists of many fine pores or a smaller number of coarse pores Two tests have been used to investigate the distribution of pore sizes and the microporosity of a rock specimen, namely, the suction plate test and the mercury porosimeter test Microporosity in the suction plate method is defined as the volume of water retained (expressed as a percentage of the total available pore space) when a suction equivalent to 6.4 m of head of water is applied to the specimen In effect, this measures the percentage of pores with an effective diameter of less than mm Such pores are able to retain water against applied suction and influence the amount of damage that can be caused by frost or by the crystallization of soluble salts that a rock used for building stone may undergo Mercury is forced to penetrate the pores of the specimen under an applied pressure in the mercury porosimeter test Obviously, the finer the pores, the higher the pressure that must be used to bring about penetration In this way it is possible to derive the dimensions and pore size distribution from a graph showing the distribution of pores sizes A line is drawn on the curve at the position where 10% of the pore space has been filled with mercury The pores below this size limit can be regarded as the microporosity Hardness Hardness is one of the most investigated properties of materials, yet it is one of the most complex to understand It does not lend itself to exact definition in terms of physical concepts The numerical value of hardness is as much a function of the type of test used as a material property The concept of hardness is usually associated with the surface of a material For Figure The Shore scleroscope instance, the hardness of a rock can be considered as its resistance to a penetrating force, whether static or dynamic, or the resistance to displacement of surface particles by tangential abrasive force As such, hardness is controlled by the efficiency of the bond between minerals or grains, as well as the strength of these two components A number of tests have been used to assess indentation hardness, of which the two most often used in rock mechanics testing are the Shore scleroscope and Schmidt hammer tests The Shore scleroscope is a nondestructive hardness measuring device that indicates the relative values of hardness from the height of rebound of a small diamond-pointed hammer that is dropped vertically onto a securely clamped test surface from a height of 250 mm (Figure 2) Because a rock is not a homogeneous material, several hardness tests must be made over the surface of the specimen and the results averaged Hence, at least 20 hardness determinations should be taken and each point of test should be at least mm from any other The Shore hardness value can be used to derive an approximate value of uniaxial compressive strength from Figure for rocks with strengths in excess of 35 MPa