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Engineering Properties of Rocks Associate Professor John Worden DEC University of Southern Qld

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Engineering Properties of Rocks lAt this point in your course, you should appreciate that rock properties tend to vary widely, often over short distances. lA corollary of this is that during Engineering practice, the penalties for geologic mistakes can be severe. lWe will therefore briefly review factors that “quantise” rocks. lThe study of the Engineering Properties of Rocks is termed Rock Mechanics, which is defined as follows: v“ The theoretical and applied science of the mechanical behaviour of rock and rock masses in response to force fields of their physical environment.” vIt is really a subdivision of “Geomechanics” which is concerned with the mechanical responses of all geological materials, including soils.

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Engineering Properties of Rocks lDuring Engineering planning, design and construction of works, there are many rock mechanics issues such as: vEvaluation of geological hazards; vSelection and preparation of rock materials; vEvaluation of cuttability and drillability of rock; vAnalysis of rock deformations; vAnalysis of rock stability; vControl of blasting procedures; vDesign of support systems; vHydraulic fracturing, and vSelection of types of structures. lFor this lecture we will confine our study to the factors that influence the deformation and failure of rocks.

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Engineering Properties of Rocks lSuch factors include: vMineralogical composition and texture; vPlanes of weakness; vDegree of mineral alteration; vTemperature and Pressure conditions of rock formation; vPore water content, and vLength of time and rate of changing stress that a rock experiences. lMineralogical Composition and Texture. vVery few rocks are homogeneous, continuous, isotropic (non directional) and elastic. vGenerally, the smaller the grain size, the stronger the rock.

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Engineering Properties of Rocks vTexture influences the rock strength directly through the degree of interlocking of the component grains. vRock defects such as microfractures, grain boundaries, mineral cleavages, twinning planes and planar discontinuities influence the ultimate rock strength and may act as “surfaces of weakness” where failure occurs. vWhen cleavage has high or low angles with the principal stress direction, the mode of failure is mainly influenced by the cleavage. vAnisotropy is common because of preferred orientations of minerals and directional stress history. vRocks are seldom continuous owing to pores and fissures (i.e. Sedimentary rocks). vDespite this it is possible to support engineering decisions with meaningful tests, calculations, and observations.

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Engineering Properties of Rocks lTemperature and Pressure vAll rock types undergo a decrease in strength with increasing temperature, and an increase in strength with increasing confining pressure. vAt high confining pressures, rocks are more difficult to fracture as incipient fractures are closed. lPore Solutions vThe presence of moisture in rocks adversely affects their engineering strength. vReduction in strength with increasing H 2 O content is due to lowering of the tensile strength, which is a function of the molecular cohesive strength of the material. lTime-dependent Behavior vMost strong rocks, like granite show little time-dependent strain or creep.

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Engineering Properties of Rocks lSince there are vast ranges in the properties of rocks, Engineers rely on a number of basic measurements to describe rocks quantitatively. These are known as Index Properties. lIndex Properties of Rocks: vPorosity- Identifies the relative proportions of solids & voids; vDensity- a mineralogical constituents parameter; vSonic Velocity- evaluates the degree of fissuring; vPermeability- the relative interconnection of pores; vDurability- tendency for eventual breakdown of components or structures with degradation of rock quality, and vStrength- existing competency of the rock fabric binding components.

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Engineering Properties of Rocks lPorosity: Proportion of void space given by- n = p / t, where p is the pore volume and t is the total volume. Typical values for sandstones are around 15%. In Igneous and Metamorphic rocks, a large proportion of the pore space (usually 20%. Porosity is therefore an accurate index of rock quality. lDensity: Rocks exhibit a greater range in density than soils. Knowledge of the rock density is important to engineering practice. A concrete aggregate with higher than average density can mean a smaller volume of concrete required for a gravity retaining wall or dam. Expressed as weight per unit volume. lSonic Velocity: Use longitudinal velocity V l measured on rock core. Velocity depends on elastic properties and density, but in practice a network of fissures has an overriding effect. Can be used to estimate the degree of fissuring of a rock specimen by plotting against porosity (%).

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Engineering Properties of Rocks lPermeability: As well as the degree of interconnection between pores / fissures, its variation with change in normal stress assesses the degree of fissuring of a rock. Dense rocks like granite, basalt, schist and crystalline limestone possess very low permeabilities as lab specimens, but field tests can show significant permeability due to open joints and fractures. lDurability: Exfoliation, hydration, slaking, solution, oxidation & abrasion all lower rock quality. Measured by Franklin and Chandra’s (1972) “slake durability test”. Approximately 500 g of broken rock lumps (~ 50 g each) are placed inside a rotating drum which is rotated at 20 revolutions per minute in a water bath for 10 minutes. The drum is internally divided by a sieve mesh (2mm openings) and after the 10 minutes rotation, the percentage of rock (dry weight basis) retained in the drum yields the “slake durability index (I d )”. A six step ranking of the index is applied (very high-very low).

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Engineering Properties of Rocks vStrength- Use Point Load Test of Broch and Franklin (1972). Irregular rock or core samples are placed between hardened steel cones and loaded until failure by development of tensile cracks parallel to the axis of loading. vI S = P/D 2, where P= load at rupture; D= distance between the point loads and I s is the point load strength. vThe test is standardised on rock cores of 50mm due to the strength/size effect vRelationship between point load index (I s ) and unconfined compression strength is given by: q u =24I s (50) where q u is the unconfined compressive strength, and I s (50) is the point load strength for 50 mm core. lAll of the above are measured on Lab specimens, not rock masses/ outcrops, which will differ due to discontinuities at different scales.

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Engineering Properties of Rocks lEngineering Classification Systems for Rock: vUse of classification systems for rock remains controversial. vBieniawski’s Geomechanics system uses a rock mass rating (RMR) which increases with rock quality (from 0-100). It is based on five parameters namely, rock strength; drill core quality; groundwater conditions; joint and fracture spacing, and joint characteristics. Increments from all five are summed to determine RMR. vWhile point load test values give rock strength, drill core quality is rated according to rock quality designation (RQD) introduced by Deere (1963). The RQD of a rock is calculated by determining the percentage of core in lengths greater than twice its diameter. vSpacing of Joints is determined from available drill core.

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Engineering Properties of Rocks vIt is assumed that rock masses contain three sets of joints, but the spacing of the most critical for the application is used. vCondition of joints is treated similarly. Covers the roughness and nature of coating material on joint surfaces, and should be weighted towards the smoothest and weakest joint set. vGround water can exert a significant influence on rock mass behavior. Water inflows or joint water pressures can be used to determine the rating increment as either completely dry; moist; water under moderate pressure, or severe water problems. vBieniawski recommended that the sum of these ratings be adjusted to account for favorable or unfavorable joint orientations. No points are subtracted for very favorable joint orientations, but 12 points for unfavorable joint orientations in tunnels, and 25 points in foundations.

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Engineering Properties of Rocks lDeformation and Failure of Rocks: vFour stages of deformation recognised: Elastic;Elastic; Elastico-viscous;Elastico-viscous; Plastic, andPlastic, and Rupture.Rupture. vAll are dependent on the elasticity, viscosity and rigidity of the rock, as well as temperature, time, pore water, anisotropy and stress history. vElastic deformation disappears when responsible stress ceases. Strain is a linear function of stress thus obeying Hooke’s law, and the constant relationship between them is referred to as Young’s modulus (E). vRocks are non ideal solids and exhibit hysteresis during unloading.

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Engineering Properties of Rocks vThe elastic limit, where elastic deformation changes to plastic deformation is termed the Yield Point. Further stress induces plastic flow and the rock is permanently strained. vThe first part of the plastic flow domain preserves significant elastic stress and is known as the “elastico-viscous” region. This is the field of“creep”deformation. Solids are termed “brittle”or “ductile”depending on the amount of plastic deformation they exhibit. Brittle materials display no plastic deformation. vThe point where the applied stress exceeds the strength of the material is the “ultimate strength” and “rupture” results. vYoung’s modulus “(E)” is the most important elastic constant derived from the slope of the stress-strain curve. Most crystalline rocks have S-shaped stress-strain curves that display “hysteresis” on unloading. E varies with the magnitude of the applied stress and transient creep. vDeere and Miller (1966) identified six stress-strain types.

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Engineering Properties of Rocks lBrittle Failure: vSudden loss of cohesion across a plane that is not preceded by any appreciable permanent deformation. vFor shear failure, Coulomb’s Law applies: = c + n tan , where = the shearing stress; c = the apparent cohesion; n = the normal stress and = the angle of internal friction or shearing resistance. – see diagram. vFor triaxial conditions: = 0.5 ( 1 + 3 ) + 0.5 ( 1 - 3 ) cos 2 and, = 0.5 ( 1 - 3 ) sin 2 , where 1 = stress at failure, & 3 = confining pressure. vSubstitution for n and in the Coulomb equation : 2c + 3 [sin 2 + tan (1- cos 2 )] 1 = --------------------------------------------- sin 2 - tan ( 1 + cos 2 )

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Engineering Properties of Rocks vAs 1 increases, there will be a critical plane on which the available shear strength is first reached. For this critical plane, sin 2 = cos 2 , and cos 2 = sin ; so the above equation reduces to: 2c cos + 3 (1+ sin ) 1 = ---------------------------------- 1- sin v As per Coulomb’s hypothesis, an apparent value of the uniaxial tensile stress, 1 can be obtained from : 1 = 2 cos / 1 + sin , but measured values of tensile strength are generally lower than those predicted by the equation. vFor rocks with linear relationships between principal stresses at rupture, there is agreement, but most rocks are non linear. Perhaps this is due to increasing frictional grain contact as pressure increases? vTheoretical direction of shear failure is not always in agreement with experimental observations, nor does it occur at peak strength.

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Engineering Properties of Rocks lMohr (1882) modified Coulomb’s concept. Mohr’s hypothesis states that when a rock is subjected to compressive stress, shear fracturing occurs parallel to those two equivalent planes for which shearing stress is as large as possible whilst the normal pressure is as small as possible. lGriffith (1920) claimed that minute cracks or flaws, particularly in surface layers reduced the measured tensile strengths of most brittle materials to less than those inferred from the values of their molecular cohesive forces. Although the mean stress throughout the body may be relatively low, local stresses in the vicinity of flaws were assumed to attain values equal to the theoretical strength. vUnder tensile stress, stress magnification around a flaw is concentrated where the radius of curvature is smallest, ie at its end. vConcentration of stress at the ends of flaws causes them to enlarge and presumably develop into fractures.

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Engineering Properties of Rocks vBrace (1964) demonstrated that fracture in hard rocks was usually initiated in grain boundaries, which can be regarded as inherent flaws under Griffith’s theory. vSubsequently Hoek (1968) determined that modified Griffith theories while adequate for prediction of fracture initiation in rocks, could not describe their propagation and subsequent failure of rocks. vHoek and Brown (1980) reviewed published data on the strength of intact rock and developed an empirical equation (subsequently modified in 1997) that allows preliminary design calculations to be made without testing, by using an approximate rock type dependent value (m I ), and determining a value of unconfined compressive strength. vLastly we will briefly examine the Deere and Miller (1966) classification of intact rock.

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Engineering Properties of Rocks lDeere and Miller (1966) Classification of intact rock: vAny useful classification scheme should be relatively simple and based on readily measurable physical properties. vDeere and Miller based their classification on unconfined (uniaxial) compressive strength ( 1 ) and Young’s Modulus (E) or more specifically, the tangent modulus at 50% of the ultimate strength ratioed to the unconfined compressive strength (E/ 1 ). vRocks are subdivided into five strength categories on a geometric progression basis; very high – high – medium –low -very low. vThree ratio intervals are employed for the modulus ratio; high – medium – low. vRocks are therefore classed as BH (high strength- high ratio); CM (medium strength – medium ratio), etc. vThis data should be included with lithology descriptions and RQD values.

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