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Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock
Properties
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SUGGESTED METHODS FOR DETERMINING SOUND VELOCITY
Scope
This test is intended as a method to determine the velocity of
propagation of
elastic waves in laboratory rock testing. Three different
variations of the method are given. These are:
1. The high frequency ultrasonic pulse technique. 2. The low
frequency ultrasonic pulse technique. 3. The resonant method.
Apparatus
The same rock or even the same sample can be used for all three
method. The electronic components should be impedance matched and
have shielded leads to ensure efficient energy transfer. To prevent
damage to the system allowable voltage inputs should not be
exceeded.
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Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock
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PROCEDURE
1. Care should be exercised in core drilling, handling, sawing,
grinding and lapping the test specimen to minimize mechanical
damage. The surface area under each transducer shall be
sufficiently plane to provide good coupling.
2. Drying of specimens may be carried out by using a desiccator.
Saturated specimens shall remain submerged in water up to the time
of testing.
This method is for the determination of velocities of
compressional (dilatational, longitudional, P-) and shear
(rotational, transversal, S-) waves in rock specimens of
effectively infinite extent compared to the wave length of the
pulse used. The condition of infinet extent is satisfied if the
average grain size< wave length of the pulse< minimum
specimen dimension.
The taransmitter is pressed to the center of a plane normal to
the direction of
wave propagation by a stress of about 10 N/cm. Position the
receivers Increase the voltage output of the pulse generator, the
gain of the amplifier
and the sensitivity of the oscilloscope and counter the optimum
level, given a steeper pulse front the permit more accurate time
measurements.
The oscilloscope is used with the time-delay circuit to display
both the direct pulse and the first arrival of the transmitted
pulse, and to measure the travel time.
The counter is triggered to start by the direct pulse applied to
the transmitter and is triggered to stop by the first arrival of
the pulse reaching the receiver.
Determine the zero time of the circuit including both
transducers and the travel time measuring device and applied the
correction to the measured travel times.
Since the first transmitted arrival is that of the compression
wave, its detection is relatively easy.
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Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock
Properties
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The amplitude of the shear wave relative to the compression wave
may be increased and its arrivial time determined more accurately
by means of thickness shear-transcuder elements.
Calculation First and second Methods Velocities are calculated
from travel times measured and the distance, d, between transmitter
and receiver by using the equations: Vp=d.tp Vs=d.ts Vp= velocity
of the longitudinal wave Vs= = velocity of the shear wave tp , ts =
times which the P-and S- wave, took the travel the distance d.
Use of pulse velocity
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Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock
Properties
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This method is intended to measure uniaxial tensile strength of
rock specimens indirectly by the Brazil test.
APPARATUS
PROCEDURE
Test specimen should be prepared with clean water. Surface of
the specimen should be free from any irregularities Water content
should be controlled. Specimen diameter shall not be less than 54mm
(NX) Thickness should be approx. Equal to radius. Specimen shell be
wrapped around its periphery with one layer of the
masking tape. Loading rate 200 N/s recommended Number of
specimens per sample tested should be determined from practical
considerations, normally 10 are recommended.
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Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock
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Calculation
The tensile strength of the specimen t shall be calculated by
the following formula: t =0.636*P/(D*t) MPa Where;
P= load at failure (N)
D = diameter of the test specimen (mm)
t = the thickness of test specimen measured at the center
(mm)
Reporting results
The following information shall be reported:
(a) Lithologic description of the rock. Source of sample,
including: geographic location, depth and orientations.
(b) Type of specimen (core, blasted or broken sample, in situ).
Size and shape of core or block specimen.
(c) Date of sampling, date of testing and condition of storage
(i.e. exposure to temperature extremes, air drying, moisture,
etc.).
(d) Orientation of the hammer axis in the test. (e) Method of
clamping sample (V-block or clamps). (f) The Schmidt Hardness value
obtained as in the Calculations section above.
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Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock
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This test is intended to measure strength of cylidrical rock
specimens subject to triaxial compression. This provides the values
necessary to determine the strenghth envelope and from sthis the
value of
the internal friction angle () and the apperant cohesion (c) may
be calculated.
APPARATUS
A triaxial cell A Loading device for applying axial load
Equipment for generating and measuring the confinin pressure
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Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock
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Preparation of sample
a)Test specimens shall be right circular cylinders having a
height to diameter ratio of 2.5-3.0 and a diameter preferably of
not less than core size approximately 54 mm. b)The ends of specimen
shall be flat to 0.02mm and shall not depart from perpendicularity
to the axis of specimen by more than 0.001 radian or 0.05mm in
50mm.
c)The sides of specimen shall be smooth and free of abrupt
irregularities and straight to within 0.3mm over the full length of
specimen. d)The use of mapping materials or end surface treatments
other than machining is not permitted. e)The average diameter shall
be used for calculating the cross-sectional area.The height of the
specimen shall be determined to nearest 1.0mm. f)Samples shall be
stored for no longer than 30 days in such a way as to preserve the
natural water content,as far as possible,and tested in that
condition. h)The maximum load on the specimen shall be recorded in
Newtons. )The number of specimen tested sould be determined from
practical considerations but at least five are preffered.
Procedure
1. Axial load & the confining pressure must be incresed
simultaneously & in such a way that axial stress &
confining pressure be apprximately equal, until the predetrmined
test level for the confining pressure is reaced.
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Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock
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2. Load on the specimen shall be applied continuously at a
constant stress rate such that failure will occur with 5-10 min of
loading. Loading rate of 0.5-1.0 MPa/s.
3. The maximum axial load and the corresponding confining
pressure on the spsciment shall be recorded.
Calculation
The compressive strength of the specimen shall be calculated by
dividing the maximum load carried by the specimen during the test
by the original cross-sectional area.
The confining pressure and the corresponding strength values for
the different specimens are poloted with the confining
pressure.
11-marcsin m
2cossin-1c b
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Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock
Properties
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This tests measures peak and residual direct shear strength as a
function of stress normal to the sheared plane.
The results of it useful for example: limiting equilibrium
analyses of slope stability problems stability analyses of dam
foundations. A shear strength determination should preferably
comprise at least five tests
on the same test horizon with each specimen tested at a
different but constant normal stress.
Arrangement of direct shear test in laboratory (shear box)
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Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock
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DIRECT SHEAR STRENGTH TEST
Graphs of peak and residual shear strength normal stress are
plotted from the combined results for all test specimens.
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