ENGINEERING GEOLOGY/Rock Properties and Their Assessment 571 Figure Representative stress strain curve for rock in uncon fined compression, showing hysteresis Figure Theoretical strain curve at constant stress (creep curve) Table Classification of deformability which takes place when a load is applied, is represented by OA There follows a period of primary or transient creep (AB) in which the rate of deformation decreases with time If the stress is removed, the specimen recovers At first this is instantaneous (BC), but this is followed by a time elastic recovery, illustrated by curve CD On the other hand, if the loading continues, the specimen begins to exhibit secondary or pseudo-viscous creep This type of creep represents a phase of deformation in which the rate of strain is constant The deformation is permanent and is proportional to the length of time over which the stress is applied If the loading is continued further, then the specimen suffers tertiary creep during which the strain rate accelerates with time and ultimately leads to failure Elastic Properties Young’s modulus, E, is the most important of the elastic constants and can be derived from the slope of the strain–stress curve obtained when a rock specimen is subjected to unconfined compression (i.e., static loading), it being the ratio of stress to strain The strains are measured by attaching strain gauges to the test specimens, or by displacement transducers, and recording their outputs Strain measurements on specimens less than 50 mm in diameter, however, are high and not representative of the material behaviour Most crystalline rocks have S-shaped stress–strain curves (Figure 7) At low stresses the curve is nonlinear and concave upwards, that is, Young’s modulus increases as the stress increases The initial tangent modulus is given by the slope of the stress-strain curve at the origin Gradually a level of stress is reached where the slope of the curve becomes approximately linear In this region Young’s modulus is defined as Class Deformability (MPa  10 3) Description Less than 5 15 15 30 30 60 Over 60 Very high High Moderate Low Very low Reproduced from Bulletin International Association Engineering Geology, No 19, 364 371, 1979 the tangent modulus or secant modulus At this stress level the secant modulus has a lower value than the tangent modulus because it includes the initial ‘plastic’ history of the curve A classification of deformability has been suggested by the IAEG and is given in Table In addition to their non-elastic behaviour, most rocks exhibit hysteresis Under uniaxial stress the slope of the stress–strain curve during unloading initially is greater than during loading for all stress values (Figure 7) As stress is decreased to zero a residual strain, OR, is often exhibited On reloading the curve RS is produced that, in turn, is somewhat steeper than OP Further cycles of unloading and reloading to the same maximum stress give rise to hysteresis loops, which are shifted slightly to the right The non-linear elastic behaviour and elastic hysteresis of brittle rocks under uniaxial compression is due to the presence of flaws or minute cracks in the rock At low stresses these cracks are open but they close as the stress is increased and the rock becomes elastically stiffer, that is, E increases with stress Once the cracks are closed the stress–strain curve becomes linear When a specimen undergoes compression it is shortened and this generally is accompanied by an increase in its cross-sectional area The ratio of lateral