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Discussion of Z.T. Bieniawski's paper The effect of specimen size on compressive strength of coal
lnt J. Rock Mech. Mm Sex Vol.7, p. 231. Pergamon Press 1970.Printedm GreatBritain
DISCUSSION Discussion ofZ. T. BIENIAWSKI'SPaper* The Effect o f Specimen Size on Compressive Strength o f Coal by P. R. Sm~OREYt I READ with great interest the above paper by Blemawski. I am the leader of the research project on strength and behaviour of rocks at the CentralMining Research Station, Dhanbad. We are also considering the determination of the in situ strength of coal. While comparing Protodyakonov's formula for size effect [equation (1), p. 329 of the paper] with his experimental data, Bleniawskz has calculated the Protodyakonov constants, cry, m and b, by solving three simultaneous equations after putting three values from data into equation (1). The values obtained by hzm are: ~u=80psi,
m=
13.5, b = 6 0 i n .
After calculations by the same method I have, however, obtained the following values after choosing three sizes from Bieniawski's data: cru = 4 3 3 p s i , m =
18, b =
1.4in.
These constants subsequently give the value of cro as 7794 psi. For sizes of I ft and higher, Protodyakonov's formula appears to fit reasonably well, if these constants are used. Protodyakonov's formula itself, while taking into account the distance between discontinuities, does not, however, indicate that the strength should remain constant for sizes smaller than the constant b. The experiments on coal by EVANS and POMrROY [1] also indicate a marked strength variation for such small size ranges as ~ in. to 2 in. The third point I wish to mention is that equation (3) suggested for specimens smaller than 5 ft (p. 333) gives values of strength much lower than those obtained experimentally (Table 1) when it is applied to cubical specimens smaller than 1 ft. Has equation (3) been derived only from the data for specimen sizes greater than 1 ft? I would very much appreciate comments on the above points. REFERENCE 1. EvANsI. and POMEROYC. D. The Strength of Cubes of Coal m Uniaxial Compression, The Mechanical Properties of Non-Metal Brittle Materials, pp. 5-25, Butterworths, London (1958).
* Int. J. Rock Mech. Min. Sci. 5, 325-335 (1968).
t Central Mining Research Station, Dhanbad, Bihar, India. 231
DISCUSSION Discussion ofZ. T. BIENIAWSKI'SPaper* The Effect o f Specimen Size on Compressive Strength o f Coal by P. R. Sm~OREYt I READ with great interest the above paper by Blemawski. I am the leader of the research project on strength and behaviour of rocks at the CentralMining Research Station, Dhanbad. We are also considering the determination of the in situ strength of coal. While comparing Protodyakonov's formula for size effect [equation (1), p. 329 of the paper] with his experimental data, Bleniawskz has calculated the Protodyakonov constants, cry, m and b, by solving three simultaneous equations after putting three values from data into equation (1). The values obtained by hzm are: ~u=80psi,
m=
13.5, b = 6 0 i n .
After calculations by the same method I have, however, obtained the following values after choosing three sizes from Bieniawski's data: cru = 4 3 3 p s i , m =
18, b =
1.4in.
These constants subsequently give the value of cro as 7794 psi. For sizes of I ft and higher, Protodyakonov's formula appears to fit reasonably well, if these constants are used. Protodyakonov's formula itself, while taking into account the distance between discontinuities, does not, however, indicate that the strength should remain constant for sizes smaller than the constant b. The experiments on coal by EVANS and POMrROY [1] also indicate a marked strength variation for such small size ranges as ~ in. to 2 in. The third point I wish to mention is that equation (3) suggested for specimens smaller than 5 ft (p. 333) gives values of strength much lower than those obtained experimentally (Table 1) when it is applied to cubical specimens smaller than 1 ft. Has equation (3) been derived only from the data for specimen sizes greater than 1 ft? I would very much appreciate comments on the above points. REFERENCE 1. EvANsI. and POMEROYC. D. The Strength of Cubes of Coal m Uniaxial Compression, The Mechanical Properties of Non-Metal Brittle Materials, pp. 5-25, Butterworths, London (1958).
* Int. J. Rock Mech. Min. Sci. 5, 325-335 (1968).
t Central Mining Research Station, Dhanbad, Bihar, India. 231