Index properties of rocks
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Transcript of Index properties of rocks
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