The applicability of Bieniawski and Hoek-Brown empirical strength criteria has been assessed for different groups of coal
and various types of intact rocks by using a vast number of published triaxial test data from various places. Analysis of
individual data sets revealed that the traditional forms of the criteria do not have a perfect agreement with the data. A
strong negative correlation has been observed between B in Bieniawski's criterion and m in Hoek and Brown's criterion
with uniaxial compressive strength of materials. Both criteria have been modified and empirical relationships have been
introduced for coal as follows:
m = 62.903 - 34.213 (log pc) 0.9772
B = 10.152 - 4.709 (log pc )0.8889
Similar relationships have been developed for different rock types as well.
A comparison between the applicability of each of the above approaches with the conventional criteria reveals a very
significant advantage for new approaches and a supremacy for the Bieniawski criterion in all cases particularly in the case
of coal.
The modified Bieniawski criterion fits coal as well as different types of rocks with excellent accuracy.
The modified Hoek-Brown criterion gives a good result for rocks but does not fit coal data quite well. In other words,
Hoek-Brown criterion is not an suitable one for coal.