Index properties of rocks

INTRODUCTION
Engineers or geologists determine index
properties:
 At the field site.
 By hand held or portable equipment.
 Index properties correlated to strength and
deformation properties for design

Reasons for Developing Index
Tests
 1 Laboratory testing elaborate & time
consuming
 2 Delay in assessment
 3 Field tests immediate results
requiring
 Not much specimen preparation
 Use of portable equipment for testing
 Correlated to strength & design properties
 Representing in situ properties
 Borehole logging can be related to index
tests

Brazilian Test
σt = 2P / πDt
 σt= Uniaxial tensile strength
 P = Load at failure on a
portable machine MPa
 D = diameter of the core (m)
 t = Thickness of core (m)

Idealised Condition for Brazilian Test
 Minimum core diameter 54 mm
 Diameter to height ratio1 to 2
 Minimum number tests –5
 Loading rate- 6.5 mm/minute
 Loading through curve jig
 Failure of specimen across loading platen

Point load Index Test
 The point load test was developed by Broch and
Franklin in 1972.
 It is a small hand-portable test apparatus used to
provide an index for the strength classification of
hard rocks in the field; it allows a quick and non-
expensive on-site evaluation of strength of hard
rocks.
 The test consists of squeezing pieces of
rock diametrically between two hardened steel
cones

Point load Index Test:
 Basically, the test method relies on the
principle of inducing tensile stress into the rock
by the application of a compressive force.
 Rock specimens break since tensile cracks
develop parallel to the loading direction.
 The cylindrical rock samples can be loaded
diametrically or axially whereas irregular rock
pieces can also be loaded

Idealised Condition for Point Load Index
Test
 Portable loading machine
 Calibration chart for size correction
 Minimum core diameter 50mm
 L/d ratio = 1.5:1
 Number samples-10-15
 No standard rate of loading
 Platen 60 degree cone with 5mm curvature

Point load Strength Test
 The maximum tensile stress at the centre
of the specimen may be related to the
applied load and to the distance between
the point loads according to the equation:
Is= P/D2
Where,
Where P is the load (MN) at rupture and
D is the core diameter (meters).

Point load Strength Test:
 For irregular rock pieces, an equivalent
diameter should be recorded.
 In general, it has been found that the
value of the
load P at failure depends largely on the core
diameter.
 Hence, the results of point load tests are
usually presented in terms of a reference
diameter equal to 50 mm.
 The unconfined compressive strength σc
is related to the point load
index with 50 mm cores Is50 as follows:

Point load Strength Test:
σc =24Is

MRDE Impact Test
 Test to estimate degradability of coal.
 Face conveyor, transfer points, storage bin,
screening and washing.
 1.8kg standard plunger (42mm diameter) is
dropped.
 on coal by standard distance.
 20 blows on 100 gm of coal.
 One impact every 2 seconds.
 Weight of coal remaining on 3mm sieve is
impact index.

Standard test conditions:
 Specimen diameter > 32 mm
 Number of sample 5 or 6
 Impact rate not faster than 1 to 2 per
second

Schmidt Hammer Rebound Test
 Portable inexpensive
 devise.
 Rock joints or rock
 surface or lab specimen.
 Amount of rebound of
 hammer on the prepared
 surface.
 Rebound number on the
scale can be correlated to UCS.

Schmidt Hammer Rebound Test:
 ISH= 0.5 σc
 ISH= Schmidt rebound number
 σc = UCS MPa
 Coefficient of Correlation = 0.86

Schmidt Hammer Test
 A Schmidt hammer, also known as a Swiss
hammer or a rebound hammer, is a device to
measure the elastic properties or strength of
concrete or rock, mainly surface hardness and
penetration resistance.
 It was invented by Ernst Schmidt, a Swiss
engineer

Index properties of rocks

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Index properties of rocks