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Engineering Properties of Rocks
Associate Professor John Worden
DEC
University of Southern Qld
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 all
geological materials, including soils.
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 the
factors that influence the deformation and failure
of rocks.
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.
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.
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 due
to lowering of the tensile strength, which is a function
of the molecular cohesive strength of the material.
 Time-dependent Behavior
 Most strong rocks , like granite show little
time-dependent strain or creep.
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 rock
quality, and
 Strength- existing competency of the rock fabric
binding components.
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 <
1-2%) occurs as planar “fissures”.With weathering this increases to > 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 (%).
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 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 (Id)”. A six
step ranking of the index is applied (very high-very low).
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.
Engineering Properties of Rocks
 Engineering Classification Systems for Rock:
 Use of classification systems for rock remains controversial.
 Bieniawski’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.
 While 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.
 Spacing of Joints is determined from available drill core.
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 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.
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 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).
 Rocks are non ideal solids and exhibit hysteresis during unloading.
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 of“creep”deformation.
Solids are termed “brittle”or “ductile”depending 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.
 Young’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.
 Deere and Miller (1966) identified six stress-strain types.
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, 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.
 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)
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 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.
 For 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?
 Theoretical direction of shear failure is not always in
agreement with experimental observations, nor does it occur at peak strength.
Engineering Properties of Rocks
 Mohr (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.
 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 them
to enlarge and presumably develop into fractures.
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 Griffith’s
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 type
dependent 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.
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 Young’s 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 ratio;
high – medium – low.
 Rocks are therefore classed as BH (high strength- high
ratio); CM (medium strength – medium ratio), etc.
 This data should be included with lithology descriptions and RQD values.