Mca admission in india

MCA Admission in India

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Engineering Properties of Rocks

Associate Professor John Worden

DEC

University of Southern Qld




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.admission.edhole.com



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.




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.admission.edhole.com



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.admission.edhole.com



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.admission.edhole.com



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.admission.edhole.com



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 (%).admission.edhole.com



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).admission.edhole.com



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 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.admission.edhole.com



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.admission.edhole.com



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.admission.edhole.com



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.admission.edhole.com



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)admission.edhole.com



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.admission.edhole.com



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 themto enlarge and presumably develop into fractures.




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.admission.edhole.com



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. admission.edhole.com


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