Rock Material. Chapter 3 ROCK MATERIAL HOMOGENEITY AND INHOMOGENEITY CLASSIFICATION OF ROCK MATERIAL

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1 Chapter 3 Rock Material In all things of nature there is something of the marvelous. Aristotle ROCK MATERIAL The term rock material refers to the intact rock within the framework of discontinuities. In other words, this is the smallest element of rock block not cut by any fracture. There are always some micro-fractures in the rock material, but these should not be treated as fractures. Rock material differs from rock mass, which refers to in situ rock together with its discontinuities and weathering profile. Rock material has the characteristics shown in Figure 3.1. HOMOGENEITY AND INHOMOGENEITY Bray (1967) demonstrated that if a rock contains ten or more sets of discontinuities (joints), then its behavior can be approximated to the behavior of a homogeneous and isotropic mass with only 5% error due to assumed homogeneity and isotropic condition. Also, if a rock is massive and contains very little discontinuity, it could ideally behave as a homogeneous medium. Hoek and Brown (1980) showed that homogeneity is a characteristic dependent on the sample size. If the sample size is considerably reduced, the most heterogeneous rock will become a homogeneous rock (Figure 3.2). In the figure s is a constant that depends on rock mass characteristics as discussed in Chapter 26. Deere et al. (1969) suggested that if the ratio between fracture spacing and opening size is equal to or less than 1/100, the rock should be considered discontinuous and beyond this range it should be considered a continuum and possibly anisotropic. An inhomogeneous rock is more predictable than a homogeneous rock because the weakest rock gives distress signals before final collapse of the rock structure. CLASSIFICATION OF ROCK MATERIAL Ancient Shilpshastra in India classified rocks on the basis of color, sound, and heaviness. ISO (2003) proposed classification of rock material based on uniaxial compressive strength (UCS) as shown in Table 3.1. It is evident that rock material may show a large scatter in strength, say of the order of 10 times; hence, the need for a classification system based on strength and not mineral content. # 2011 Elsevier Inc. All rights reserved. 13

2 14 FIGURE 3.1 Material characteristics of rocks. FIGURE 3.2 Rock mass conditions under the Hoek-Brown failure criterion. (From Hoek, 1994)

3 Chapter 3 Rock Material 15 TABLE 3.1 Classification of Rock Material Based on Unconfined Compressive Strength Ranges for common rock materials Term for uniaxial compressive strength Extremely weak* Symbol Strength (MPa) Granite, basalt, gneiss, quartzite, marble Schist, sandstone EW <1 ** ** Limestone, siltstone Slate Concrete Very weak VW 1 5 ** ** ** ** Weak W 5 25 ** ** ** ** Medium strong MS ** ** ** Strong S ** Very strong VS ** Extremely Strong ES >250 ** *Some extremely weak rocks behave as soils and should be described as soils. ** Indicates the range of strength of rock material. Source: ISO , The UCS can be easily predicted from point load strength index tests on rock cores and rock lumps right at the drilling site because ends of rock specimens do not need to be cut and lapped. UCS is also found from Schmidt s rebound hammer (see Chapter 15). Table 8.13 lists typical approximate values of UCS. There are frequent legal disputes on soil-rock classification. The International Standard Organization (ISO) classifies geological material having a UCS less than 1.0 MPa as soil. Deere and Miller (John, 1971) suggested another useful classification system based on the modulus ratio, which is defined as the ratio between elastic modulus and UCS. Physically, a modulus ratio indicates the inverse of the axial strain at failure. Thus, brittle materials have a high modulus ratio and plastic materials exhibit a low modulus ratio. CLASS I AND II BRITTLE ROCKS Rock material has been divided into two classes according to their post-peak stress-strain curve (Wawersik, 1968). Class I: Fracture propagation is stable because each increment of deformation beyond the point of maximum load-carrying capacity requires an increment of work to be done on the rock. Class II: Rocks are unstable or self-sustaining; elastic energy must be extracted from the material to control fracture.

4 16 The introduction of partial confinement, as in short samples when end constraint becomes prominent, is likely to have a satisfactory effect. If end restraint becomes severe, it is possible that a Class II rock might behave like a Class I material. Wawersik (1968) conducted experiments on six rock types to demonstrate the features of Class I and II rocks (Figure 3.3). Typical S-shape stress-strain curves may be obtained for rocks with micro-fractures. Further, the post-peak curve for Class II rocks shows reduction of strain after failure. The lateral strain increases rapidly after peak stress in Class II rocks. Brittle rocks, therefore, may be kept in the Class II category. A deep tunnel within dry, massive, hard Class II and laminated rocks may fail because of rock bursts due to uncontrolled fracturing where tangential stress exceeds the strength of the rock material (see Chapter 13). Hence, it is necessary to test rock material in a Servo-controlled closed loop testing machine to get the post-peak curve. UNIAXIAL COMPRESSION Rock failure in uniaxial compression occurs in two modes: (1) local (axial) splitting or cleavage failure parallel to the applied stress, and (2) shear failure. FIGURE 3.3 Stress-strain curves for six representative rocks in uniaxial compression. (From Wawersik, 1968)

5 Chapter 3 Rock Material 17 Local cleavage fracture characterizes fracture initiation at 50 to 95% of the compressive strength and is continuous throughout the entire loading history. Axial cleavage fracture is a local stress-relieving phenomenon that depends on the strength anisotropy and brittleness of the crystalline aggregates as well as on the grain size of the rock. Local axial splitting is virtually absent in fine-grained materials at stress levels below their compressive strength. Shear failure manifests in the development of boundary faults (followed by interior fractures), which are oriented at approximately 30 degrees to the sample axis. In finegrained materials where the inhomogeneity of the stress distribution depends only on the initial matching of the material properties at the loading platen interfaces, boundary and interior faults are likely to develop simultaneously and appear to have the same orientation for any rock type within the accuracy of the measurements on the remnant pieces of collapsed specimens (basalts, etc.). Local axial fracturing governs the maximum load-carrying ability of coarse-grained, locally inhomogeneous Class I and II rock types. Thus, in coarse-grained rocks the ultimate macroscopic failure mode of fully collapsed samples in uniform uniaxial compression cannot be related to peak stress. In fine-grained, locally homogeneous rock types, which most likely are Class II, the peak stress is probably characterized by the development of shear fractures seen in continuous failure planes. In controlled fracture experiments on very fine-grained rocks, the final appearance of a collapsed rock specimen probably correlates with its compressive strength. However, if rock fracture is uncontrolled, then the effects of stress waves produced by the dynamic release of energy may override the quasi-elastic failure phenomenon to such an extent that the latter may no longer be recognizable. The extent of the development of the two basic failure modes, local axial splitting and slip or shear failure, determines the shape of the stress-strain curve for all rocks subjected to unidirectional or triaxial loading. Partially failed rocks still exhibit elastic properties. However, the sample stiffness decreases steadily with increasing deformation and loss of strength. Macroscopic cleavage failure (e.g., laboratory samples splitting axially into two or more segments) was never observed in the experiments on Class I and II rocks. An approximate theoretical analysis of the sliding surface model, which was proposed by Fairhurst and Cook (1966), revealed qualitatively that unstable axial cleavage fracture is an unlikely failure mode of rocks in uniaxial compression. The dynamic tensile strength of rocks (granite, diorite, limestone, and grigen) is found to be about four to five times the static tensile strength (Mohanty, 2009). Brazilian tensile strength of laminated rocks and other argillaceous weak rocks like marl do not appear to be related to the UCS of rock material (Constantin, personal communication). STABILITY IN WATER In hydroelectric projects, rocks are charged with water. The potential for disintegration of rock material in water can be determined by immersing rock pieces in water for up to one week. Their stability can be described using the terms listed in Table 3.2 (ISO , 2003). Ultrasonic pulse velocity in a saturated rock is higher than in a dry rock because it is easier for pulse to travel through water than in air voids. However, the UCS and modulus of elasticity are reduced significantly after saturation, particularly in rocks with water sensitive minerals. On the other hand, the post-peak stress-strain curve becomes flatter in the case of undrained UCS tests on saturated samples because increasing fracture porosity after failure creates negative pore water pressure.

6 18 TABLE 3.2 Rock Material Stability in Water Term Description (after 24 h in water) Grade Stable No changes 1 Fairly stable A few fissures are formed or specimen surface crumbles slightly 2 Unstable Many fissures are formed and broken into small lumps or specimen surface crumbles Specimen disintegrates or nearly the whole specimen surface crumbles 3 4 Source: ISO , The whole specimen becomes muddy or disintegrates into sand 5 CLASSIFICATION ON THE BASIS OF SLAKE DURABILITY INDEX Based upon his tests on representative shales and clay stones for two 10-minute cycles after drying, Gamble (1971) found the slake durability index varied from 0 to 100%. There are no visible connections between durability and geological age, but durability increased linearly with density and inversely with natural water content. Based on his results, Gamble proposed a classification of slake durability as seen in Table 3.3. The slake durability classification is useful when selecting rock aggregates for road, rail line, concrete, and shotcrete. Rock in field is generally jointed. It was classified by core recovery in the past and later in the 1960s by modified core recovery (RQD), which will be discussed in Chapter 4. TABLE 3.3 Slake Durability Classification Group name % retained after one 10-minute cycle (dry weight basis) Very high durability >99 >98 High durability Medium high durability Medium durability Low durability Very low durability <60 <30 Source: Gamble, 1971, % retained after two 10-minute cycles (dry weight basis)

7 Chapter 3 Rock Material 19 REFERENCES Bray, J. W. (1967). A study of jointed and fractured rock. Part I. Rock Mechanics and Engineering Geology, 5 6(2 3), Deere, D. U., Peck, R. B., Monsees, J. E., & Schmidt, B. (1969). Design of tunnel liners and support system (Final Report, University of Illinois, Urbana, for Office of High Speed Transportation, Contract No , p. 404). Washington, D.C.: U.S. Department of Transportation. Fairhurst, C., & Cook, N. G. W. (1966). The phenomenon of rock splitting parallel to the direction of maximum compression in the neighborhood of a surface. In: Proceedings 1st Congress, International Society of Rock Mechanics, Lisbon, pp Gamble, J. C. (1971). Durability Plasticity classification of shales and other argillaceous rocks (p. 159). Ph.D. Thesis. University of Illinois. Hoek, E. (1994). Strength of rock and rock masses. ISRM News Journal, 2(2), Hoek,E.,&Brown,E.T.(1980).Underground excavations in rocks. Institution of Mining and Metallurgy (p. 527). London: Maney Publishing. ISO (2003). (E). Geotechnical investigation and testing Identification and classification of rock Part 1: Identification and description (pp. 1 16). Geneva: International Organization for Standardization. Mohanty, B. (2009). Measurement of dynamic tensile strength in rock by means of explosive-driven Hopkinson bar method. In Workshop on Rock Dynamics, ISRM Commission on Rock Dynamics. Lausanne, Switzerland: EPFL, June. Wawersik, W. R. (1968). Detailed analysis of rock failure in laboratory compression tests (p. 165). Ph.D. Thesis. University of Minnesota.

Module 5: Failure Criteria of Rock and Rock masses. Contents Hydrostatic compression Deviatoric compression

Module 5: Failure Criteria of Rock and Rock masses. Contents Hydrostatic compression Deviatoric compression FAILURE CRITERIA OF ROCK AND ROCK MASSES Contents 5.1 Failure in rocks 5.1.1 Hydrostatic compression 5.1.2 Deviatoric compression 5.1.3 Effect of confining pressure 5.2 Failure modes in rocks 5.3 Complete