Page 6 of 8Engineering Materials ScienceMetals LabLEEDS .docx
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1. Lab Report GIS and RS 2012-GE-56
17
LAB: 03
Statement:
To determine the UCS of samples by using point load strength index.
Scope:
This method of test is intended to measure the uniaxial compressive strength of a rock
sample in the form of specimens of regular geometry. The test is mainly intended for
strength classification and characterization of intact rock.
Principle:
The principle of determining the UCS of a given rock sample is based on the fact that the platens provide concentrated compressive load until the rock is broken and its UCS is indicated by a digital meter.
Dimensions:
The core sample whose strength is to be determined should not have the diameter less than 10mm or more than 85 mm. The preferred dimensions are about 50mm.
For axial test the L/d =1.
For diametrical test the L/d = 1.5
For irregular lump test the L/d = 0.5
Apparatus:
A point load tester.
Load measuring System
Vernier calipers
sample
Related Theory
Tensile strength:
Tensile strength is the resistance against an applied load that tends to elongate the material in the axis of the applied load, in other words the resistance against stress caused by pulling the material.
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Compressive strength:
It is the resistance against an applied load that acts to reduce the length of the material in the axis of the applied load, in other words the resistance against stress state caused by squeezing the material.
Rock core sample:
A core sample is a cylindrical section of a naturally occurring substance consistent enough to hold a layered structure.
Stress & strain:
Stress is defined as force per unit area. It has same units as pressure &infact pressure is a special variety of stress. Stress varies both with direction & with surface it acts on.
Ϭ = F/A = N/m2
Strain:
Strain is defined as amount of deformation of an object experiences compared to its original shape & size. It has no unit.
Strain = change in dimension/original dimension
Point load apparatus:
Point load testing is used to determine rock strength indexes in geotechnical practice. It is basically used to determine the strength value of rock sample for site & lab.
Procedure:
Diametral test:
1. The diametric test is conducted on rock core sample. Minimum of 10 test specimens are required to find out the average value of point load strength index.
2. This test can be conducted on the core specimens which are completely dry or after soaking it for 7 days.
3. Measure the total length and diameter (d) of the core specimen. Specimen of l/d=1.5, are considered to be suitable for this test.
4. Place the specimen horizontally between two platens in such a way that the distance between the contact point and the nearest free end (L) is at least 0.75times the diameter of the core (d).
5. Measure the distance between two platen contact points (D) with the help of the scale attached with the loading frame. (Note-In case of diametral test, the diameter of the core (d) and the distance between two platens (D) will be same)
6. Apply load to the core specimen such that failure occur within 10-60 sec. record the failure load P.
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Block or irregular lump test:-
1. This method of test is conducted on rock block or irregular samples. Minimum of 10 test specimens are required to find out the average value of point load strength index.
2. This test can be conducted on the core specimens which are completely dry or after soaking it for 7 days.
3. Place the specimen horizontally between two platens.
4. Measure the distance between two platen contact points (D) with the help of the scale attached with the loading frame. Measure the smallest specimen width (W) perpendicular to the load direction. If the sides are not parallel, then „W‟ is obtained from W1, W2 and W3 as show in the figure and calculated as W= (W1+W2+W3)/3. Measure the distance between platen contact point and nearest free end (i.e. L). The distance L should be at least 0.5D.
5. The distance “De” between the points of contact is read on the scale.
6. 3) The load is increased such that failure occurs within 10 to 60 sec. and the failure load “P” is recorded.
IS = P/D2
F= (D×50)0.45
IS50= F×IS
U.C.S = IS50 × 24
Observations & Calculations:
For axial laoding S.No Load ‘P’ KN Platen diameter ‘m’ Is = P/De2 kN/m2 Correction factor F =(DE/50)0.45 IS(50) = F × IS MPa UCS=Is*24 MPa 1 5.92 0.032 5781.25 0.8180 4.729 113.5
2
4.53
0.032
2033.36
1.72
2.966
71.2 3 24.22 0.032 10871.5 1.47 15.980 384
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For diametral loading
Precautions:
Make sure that needle is at zero before performing experiment.
The ends of the specimen should be flat to 0.02 mm.
The sides of the specimen should be smooth free of abrupt irregularities and straight
To within 0.3 mm over the full length of the specimen.
Load on the specimen shall be applied continuously at a constant Stress rate such that failure occur within 5 -10 min. of loading.
If the sample consists of blocks or irregular lumps, it should be tested as two subsamples, with load first applied perpendicular to, then along the observable planes of weakness.
Applications:
1) The uniaxial compression is used to determine compressive strength of rock specimens, but it is a time-consuming and expensive test that requires specimen preparation. When extensive testing is required for preliminary and reconnaissance information, alternative tests such as the point load test can be used in the field to reduce the time and cost of compressive strength tests.
2) The point load strength test is used as an index test for strength classification of rock materials. The test results should not be used for design or analytical purposes.
3) This test method is performed to determine the point load strength index (Is(50)) of rock specimens, and the point load strength anisotropy index (Ia(50)) that is the ratio of point load strengths on different axes that result in the greatest and least values.
4) Rock specimens in the form of either core cut blocks or irregular lumps are tested by application of concen- trated load through a pair of truncated, conical platens. Little or no specimen preparation is required
S.No Load P KN Platen diameter m Is = P/De2 kN/m2 Correction factor F =(DE/50)0.45 IS(50) = F × IS MPa UCS Is*24 MPa 1 9.35 0.032 5300 0.9245 4.8 115.2
2
5.71
0.032
2670
0.9245
2.4
59.24 3 10.09 0.032 5719 0.9245 5.28 129.9
