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Ultrasonic Pulse Velocity
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Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock Properties
1
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.
Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock Properties
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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.
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
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.
Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock Properties
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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.
Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock Properties
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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
Seven Lecture: 14 / 04 / 2015 Course of: Measurement of Rock Properties
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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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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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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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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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