Engineering Properties of Rocks

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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

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

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Engineering Properties of Rocks

• During Engineering planning, design and
  construction of works, there are many rock
  mechanics issues such as
• Evaluation of geological hazards
• Selection and preparation of rock materials
• Evaluation of cuttability and drillability of
  rock
• Analysis of rock deformations
• Analysis of rock stability
• Control of blasting procedures
• Design of support systems
• Hydraulic fracturing, and
• Selection of types of structures.
• For this lecture we will confine our study to
  thefactors that influence the deformation and
  failureof rocks.

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Engineering Properties of Rocks

• Such factors include
• Mineralogical composition and texture
• Planes of weakness
• Degree of mineral alteration
• Temperature and Pressure conditions of rock
  formation
• Pore water content, and
• Length of time and rate of changing stress that a
  rock experiences.
• Mineralogical Composition and Texture.
• Very few rocks are homogeneous, continuous,
  isotropic (non directional) and elastic.
• Generally, the smaller the grain size, the
  stronger the rock.

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Engineering Properties of Rocks

• Texture influences the rock strength directly
  through the degree of interlocking of the
  component grains.
• Rock 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.
• When cleavage has high or low angles with the
  principal stress direction, the mode of failure
  is mainly influenced by the cleavage.
• Anisotropy is common because of preferred
  orientations of minerals and directional stress
  history.
• Rocks are seldom continuous owing to pores and
  fissures (i.e. Sedimentary rocks).
• Despite this it is possible to support
  engineering decisions with meaningful tests,
  calculations, and observations.

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Engineering Properties of Rocks

• Temperature and Pressure
• All rock types undergo a decrease in strength
  with increasing temperature, and an increase in
  strength with increasing confining pressure.
• At high confining pressures, rocks are more
  difficult to fracture as incipient fractures are
  closed.
• Pore Solutions
• The presence of moisture in rocks adversely
  affects their engineering strength.
• Reduction in strength with increasing H2O content
  is dueto lowering of the tensile strength, which
  is a functionof the molecular cohesive strength
  of the material.
• Time-dependent Behavior
• Most strong rocks , like granite show little
  time-dependent strain or creep.

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Engineering Properties of Rocks

• Since 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.
• Index Properties of Rocks
• Porosity- Identifies the relative proportions of
  solids voids
• Density- a mineralogical constituents parameter
• Sonic Velocity- evaluates the degree of
  fissuring
• Permeability- the relative interconnection of
  pores
• Durability- tendency for eventual breakdown of
  components or structures with degradation of
  rockquality, and
• Strength- existing competency of the rock fabric
  binding components.

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Engineering Properties of Rocks

• Porosity 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 lt 1-2) occurs as planar fissures.With
  weathering this increases to gt 20. Porosity is
  therefore an accurate index of rock quality.
• Density 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.
• Sonic Velocity Use longitudinal velocity Vl
  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

• Permeability 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.
• Durability Exfoliation, hydration, slaking,
  solution, oxidation abrasion all lower rock
  quality. Measured by Franklin and Chandras
  (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 waterbath for 10
  minutes. The drum is internally divided by
  asieve mesh (2mm openings) and after the 10
  minutesrotation, the percentage of rock (dry
  weight basis) retainedin the drum yields the
  slake durability index (Id). A sixstep ranking
  of the index is applied (very high-very low).

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Engineering Properties of Rocks

• Strength- 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.
• IS P/D2 , where P load at rupture D distance
  between the point loads and I s is the point load
  strength.
• The test is standardised on rock cores of 50mm
  due to the strength/size effect
• Relationship 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.
• All 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

• Engineering Classification Systems for Rock
• Use of classification systems for rock remains
  controversial.
• Bieniawskis 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.
• While point load test values give rock strength,
  drill corequality is rated according to rock
  quality designation(RQD) introduced by Deere
  (1963). The RQD of a rockis calculated by
  determining the percentage of core in lengths
  greater than twice its diameter.
• Spacing of Joints is determined from available
  drill core.

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Engineering Properties of Rocks

• It is assumed that rock masses contain three sets
  of joints, but the spacing of the most critical
  for the application is used.
• Condition 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.
• Ground 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.
• Bieniawski recommended that the sum of these
  ratingsbe adjusted to account for favorable or
  unfavorable jointorientations. No points are
  subtracted for very favorablejoint orientations,
  but ? 12 points for unfavorable joint
  orientations in tunnels, and ? 25 points in
  foundations.

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Engineering Properties of Rocks

• Deformation and Failure of Rocks
• Four stages of deformation recognised
• Elastic
• Elastico-viscous
• Plastic, and
• Rupture.
• All are dependent on the elasticity, viscosity
  and rigidity of the rock, as well as temperature,
  time, pore water, anisotropy and stress history.
• Elastic deformation disappears when responsible
  stressceases. Strain is a linear function of
  stress thus obeyingHookes law, and the constant
  relationship between themis referred to as
  Youngs modulus (E).
• Rocks are non ideal solids and exhibit hysteresis
  during unloading.

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Engineering Properties of Rocks

• The 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.
• The first part of the plastic flow domain
  preserves significant elastic stress and is known
  as the elastico-viscous region. This is the
  field ofcreepdeformation. Solids are termed
  brittleor ductiledepending on the amount of
  plastic deformation they exhibit. Brittle
  materials display no plastic deformation.
• The point where the applied stress exceeds the
  strength of the material is the ultimate
  strength and rupture results.
• Youngs modulus (E) is the most important
  elasticconstant derived from the slope of the
  stress-strain curve.Most crystalline rocks have
  S-shaped stress-strain curvesthat display
  hysteresis on unloading. E varies with the
  magnitude of the applied stress and transient
  creep.
• Deere and Miller (1966) identified six
  stress-strain types.

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Engineering Properties of Rocks

• Brittle Failure
• Sudden loss of cohesion across a plane that is
  not preceded by any appreciable permanent
  deformation.
• For shear failure, Coulombs 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.
• For 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 .
• Substitution 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

• As ? 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 ?
• As per Coulombs 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.
• For rocks with linear relationships between
  principalstresses at rupture, there is
  agreement, but most rocksare non linear. Perhaps
  this is due to increasing frictionalgrain
  contact as pressure increases?
• Theoretical direction of shear failure is not
  always inagreement with experimental
  observations, nor does it occur at peak strength.

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Engineering Properties of Rocks

• Mohr (1882) modified Coulombs concept. Mohrs
  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.
• Griffith (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.
• Under tensile stress, stress magnification around
  a flaw is concentrated where the radius of
  curvature is smallest, ie at its end.
• Concentration of stress at the ends of flaws
  causes themto enlarge and presumably develop
  into fractures.

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Engineering Properties of Rocks

• Brace (1964) demonstrated that fracture in hard
  rocks was usually initiated in grain boundaries,
  which can be regarded as inherent flaws under
  Griffiths theory.
• Subsequently 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.
• Hoek 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 typedependent value (m I ), and
  determining a value of unconfined compressive
  strength.
• Lastly we will briefly examine the Deere and
  Miller (1966) classification of intact rock.

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Engineering Properties of Rocks

• Deere and Miller (1966) Classification of intact
  rock
• Any useful classification scheme should be
  relatively simple and based on readily
  measurable physical properties.
• Deere and Miller based their classification on
  unconfined (uniaxial) compressive strength (? 1)
  and Youngs Modulus (E) or more specifically, the
  tangent modulus at 50 of the ultimate strength
  ratioed to the unconfined compressive strength
  (E/? 1 ).
• Rocks are subdivided into five strength
  categories on a geometric progression basis very
  high high medium low -very low.
• Three ratio intervals are employed for the
  modulus ratiohigh medium low.
• Rocks are therefore classed as BH (high strength-
  highratio) CM (medium strength medium ratio),
  etc.
• This data should be included with lithology
  descriptions and RQD values.