ENGINEERING GEOLOGY/Rock Properties and Their Assessment 569 Figure Correlation chart for Schmidt hardness, relating unit weight, 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) Deformation of Rocks The factors that influence the deformation characteristics and the failure of rock can be divided into two categories: internal and external The internal category includes the inherent properties of the rock itself, whilst the external category is that of its environment at a particular point in time As far as the internal factors are concerned, the mineralogical composition and texture are obviously important, but fractures within a rock and the degree of mineral alteration are frequently more important The temperature-pressure conditions under which a rock exists, significantly affect its mechanical behaviour, as does its pore water content Importantly, the length of time that a rock experiences a change in stress, and the rate at which this is imposed, significantly affects its deformation characteristics The composition and texture of a rock are governed by its origin and geological history Few rocks are composed of only one mineral species and even when they are the properties of that species vary slightly from mineral to mineral Such variations within minerals may be due to cleavage, twinning, inclusions, cracking, and alteration, as well as to slight differences in composition This, in turn, is reflected in the mechanical behaviour of the resultant rocks As a consequence, few rocks can be regarded as homogeneous isotropic substances The size and shape relationships of the component minerals/grains are also significant in this respect, generally the smaller the size, the stronger the rock One of the most important features of texture as far as mechanical behaviour, particularly strength, is concerned, is the degree of