Mechanical Properties of Rocks,,....

8/11/2019 Mechanical Properties of Rocks,,....

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10-04-2014

Group # 03

Mechanical Properties of Rocks

Group members

Saqib Ali 12 MN 36

Abdullah 12-11 MN 88

Fahad Akash 12 MN 38

Teacher Name

Sir Agha Shafi Jawaid Pathan


3/23

MECHANICAL PROPERTIES OF ROCKS

1. Engineering design properties .

Uni-axial compressive strength ( E and ).

Tensile strength of rocks. Shear strength of rocks.

Tri-axial- Cohesion and internal angle of friction.

2. Index properties.

3. Intrinsic properties.


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Uni-axial compressive strength

A compressive strength is the stress required to rupture aspecimen in compression.

The compressive strength is the capacity of a material orstructure to withstand (resist) the compressive forces.

The rock sample is unconfined at its side while the load isa lied verticall until failure occurs. In this case thecompressive strength is called unconfined compressivestrength (uni-axial compressive strength).

In a uni-axial strain test, a cylindrical specimen is loaded

axially. As the axial load is applied, the material (typically rock or

concrete) tries to expand.


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Uni-axial compressive strength

It is one of the most important mechanical properties of rockmaterial, used in design, analysis and modelling.

In this test, a standard specimen is gripped between the jaws

movable and fixed jaw, and load is applied.

The compression in the specimenis measured by extensometer.


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Standard conditions for Ucs

P = Load at failure

D = core diameter of specimen

c

= P / ( /4 X D

2

)

c = Uni-axial compressive strength

Axial load on cylindrical specimen

Core diameter of 32-75mm

H/d ratio is 2.5 -3.0

Minimum number of tests 10 -20.


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Uni-axial Compressive Strength: ISRM Standard Terminology

Ucs < 5 MPa Very Low

Ucs_ 5 25 MPa Low

Ucs_ 25 50 MPa Moderate

Ucs_ 50 100 MPa Medium

Ucs_100 250 MPa High> cs > a ery g


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Uni-axial compressive test


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Modes of failure of rocks in Ucs

CRUMBLING SHEAR SLABING STRESS PATTERN CONICAL FAILURE


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Factor affecting the ucs of rock

Ucs decreases with increasing H/d ratio of specimen.

Ucs decreases with increasing moisture % of specimen.

Ucs decreases with increasing relative humidity of specimen.

Ucs increases with increasing density of specimen.

Low strain rate shows lower Ucs of the specimen.

Low porosity rock has high strength.

Strength decreases with increases in slenderness.

Strength decreases with increase in size of specimen.


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Types of Curves Stress Stress

Linear (Igneous) Convex (sandstone)

Strain Strain

Stress stress

Concave (Shale , limestone ) S shaped (Coal, Rock salt)

Strain Strain


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Brazilian Test

It is an indirect tensile test, that consists of applying diametricCompressive stress to the sample.

Tensile stress will be induced in the lateral direction or

perpendicular of the applied load.F

P = Load at failure (MN)

D = Diameter of the disc (m)

t = Thickness of the disc (m)

t = Tensile strength of rock

t

= 2P/D t

F


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Brazilian test

Minimum diameter NX core (54 mm) H/d ratio of 0.5:1

The specimen loaded through a

curved jig with spherical seating.

is recommended.


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Shear strength of rock specimen

The stress or load at which a material fails in shear. The internal resistance of a body to shear stress, typically

including a frictional part (called Angle of internal friction )

and the part independent of friction (called cohesion C).

The most important parameter for planning and designing ofunderground and surface mines.

There are two (02) methods of shear strength determination:

Direct Shear testing.

Tri-axial compression test.


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Direct shear test

A specimen is placed in ashear box. A confining stress is applied vertically to the specimen, and the

upper ring is pulled laterally until the sample fails.

After applying the normal force, the shear force is applied

horizontally. After rupture, the shear force can continue to be applied to

obtain the residual strength values.

normal stress

Rock specimen fracture plane

Shear Rrr

stress


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Direct shear test

This force causes a shear failure (fracture) along the juncturebetween the box sections.

Direct Shear Box assembly for 60mm x 60mm x 25mm thick

specimen

Several specimens are tested at various confining stresses todetermine the shear strength parameters, the rock cohesion (c)

and the angle of internal friction, commonly known as friction

angle (). Normal stressRock specimen

fracture plane

Shear

stress


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Direct shear test

Core diameter of 25-50mm.

H/d ratio 2.0-3.0.

Minimum no of tests 5-10.


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Direct shear test

For a single shear test

A = Area of cross section

=Shear force

= Shear stren th

1 = /A

For a double shear test

A = Area of cross section

=Shear force

1 = Shear strength

= /2A


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Punch test

A cylindrical specimen of 150mm diameter and 150mm height. The test specimen is placed in a clamp such that its upper and

lower surfaces are supported.

A punch type shear tool with a 25.4 mm (1 in) diameter is boltedto the specimen and a load is applied to the punch

1 = Shear strength

= Shear force at failure alongthe shear plane

t= Thickness of disc specimen

D= Diameter of the punch

= /D t


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Punch test

For a double punch

A cylindrical rock is placed vertically between the loading

platens of the machine and is compressed by two steel

punches placed parallel to the top and bottom of end surfaces.


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Tri-axial test

A cylindrical rock core is placed in a cell, subjected to all around(confining) pressure by hydraulic oil acting through a thin

impermeable membrane, and loaded axially to failure.

The specimen is enclosed in an impregnable, airtight flexible

membrane is placed between two hardened spherical seatedplatens.

Confining pressure 3

is applied and held constant during the

test by means of a cell hydraulic fluid.

Axial stress is then applied and continuously increased untilfailure occurs.


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Tri-axial test Parameters

Testing machine capacity 150- 250 MN. Hoek and Franklin type tri-axial cell.

The cell pressure range is equal to

1700 - 3800 kPa).

Flexible membrane :

Rubber tubing 1.6 mm thick wall

and 40-60 Duro hardness.

Cohesive strength and internal

Angle of friction ()


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Mechanical Properties of Rocks,,....

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