INDEX PROPERTIES OF ROCKSPrepared By: Ali Wassan

TOPIC:

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

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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