Size and shape effects in point load tests of irregular rock fragments |
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| Authors: | L A Panek T A Fannon |
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| Institution: | (1) Mining Engineering Michigan Technological University, Houghton, Michigan, U S A;(2) Present address: 8 Hillside Drive, 80215 Denver, CO, U S A |
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| Abstract: | Summary Point-load tests were performed on three hard rocks of the Lake Superior district, ironformation, metadiabase, and ophitic basalt. More than 500 irregular, mine-run fragments ranging in diameter up to about 250 mm were tested in the field, using a specially designed, semi-portable test rig. Results were analyzed by multiple regression techniques, seeking a  best expression for the point-load strength in terms of a size effect and shape effects. Standard unconfined compression tests and Brazilian tests were also performed on the metadiabase and the basalt, three core sizes of each, in order to determine their respective size effects. The size-effect exponents for compression were found to be a variable characteristic of rock type, as previously reported for other rocks by the senior author, whereas the size-effect exponent in the point-load test was constant over all three rocks.Notation
A
Load-bearing area of prismatic compression specimen, mm2
- 
Probability level of a statistical significance test
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b, w
Long and short prism dimensions, mm, in plane perpendicular to load
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c
i
,c
L
Exponent in equation for strength (coefficient, in the log-linear form)
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d
Diameter, mm
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D, H
Initial and final (at rupture) distance between the load points, mm
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E
Young's modulus, MPa
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F
Axis of the fracture surface normal toD, mm, point-load test
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G, J
Semiminor, semimajor, specimen axis, mm, perpendicular to direction of point load
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h
Height of prismatic compression specimen, mm
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K
Constant in linear regression model, basic rock strength parameter
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L, M, S
Long, intermediate, and short axes, of point-load-test specimen
- log
Natural (Napierian) logarithm
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n
Number of tests
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v
Poisson's ratio
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P
Resistance of test specimen at rupture, Newtons
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Q
Compressive strength, MPa, determined from unconfined compression test
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R
Multiple correlation ratio for a multilinear regression analysis
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SE
Standard error of estimate for a multilinear regression analysis
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T
Tensile strength, MPa, determined from diametral compressive test of core
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t
Thickness, mm
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U
Geometric mean diameter, mm, of the minimum cross section through the load points=(DG)1/2
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V
Geometric mean diameter, mm, of point-load specimen=(LMS)1/3
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W
Geometric mean diameter, mm, of the specimen midsection perpendicular to the point-load direction=(GJ)1/2
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X, Y
Geometric mean diameter, mm, of the fracture surface, estimated by (DF)1/2, by (HF)1/2
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Z
LM
,Z
RA
, ...
Differences between values of logK, used to express category effects, those attributed to specimen orientation and/or rock type |
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| Keywords: | |
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