It is difficult to make specimens of regular shape of weak rocks as they disintegrate and fail along planes weakness while coring or cutting. Various rock indices based on testing irregular lumps provide a valuable means of predicting the strength. Strength of weak rocks determined by testing specimens of irregular shapes have also been found to be close to the strengths determined by testing specimens of regular shapes. The paper deals with the various techniques of determining rock indices and gives the results obtained in the rock mechanics laboratory of the Banaras Hindu University.
Knowledge of engineering properties of rocks is essential for proper design of rock structures, slopes, dam foundations et". Determination of physico-mechanical properties of rocks requires a large number of shaped, regular specimens. The preparation of such specimens is tedious wad time consuming especially for weak, laminated or bedded rocks. Such rocks usually break when an attempt is made either to drill for cores or cut for the prismatic specimens. Difficulties have also been experienced while preparing regular specimens from halides and such other rock which dissolve in water used for flushing the cuttings.
To overcome the difficulties of preparing the regular shaped specimens suggestions have been made in the rock mechanics literature regarding the development of various rocks indices and irregular specimen testing techniques.
An attempt has been made at Banaras Hindu University to evolve techniques of testing various rock indices e.g. Rock Quality Designation (RQD),Shore Hardness, Protodyakonov Index, Impact Strength Index, Point Load Index etc. and to coorelate them with the usual compressive strengths determined by testing regular specimens. Irregular shaped specimens have also been tested in compression and the results coorelated with usual compression tests.
Rock, particularly the weak rocks, have a marked tendency to creep. Arrangements have been made to study the creep properties of weak rocks in single and double lever type loading frames.
The rocks tested were mostly coal-measure rocks which consisted of coarse grained sandstone, gritty sandstone, medium grained and stone, sandy shale, ferroginous sandstone etc. In most cases, cores were obtained during explorotory drilling and complete records were kept regarding lithology, recovery of cores etc. In some cases. rock blocks were obtained from the field and the cores were obtained in the laboratory. Cubical or prismatic specimens were also prepared in case of those rocks which were too fragile to core. Irregular, egg shaped specimens were prepared according to the specifications laid down by the International Bureau of Rock Mechanics (1961. In certain tests, in order to get wide range of results, some stronger rocks were also used.
The percentage core recovery of a rock section of particular litho-logy was obtained by taking into account the distance drilled and the length of the core recovered.
Fig. 1 dhows the plot between compressive strength and percentage core recovery of different types of rocks tested. Percentage core recovery has varied from 15% to 100% and compressive strength varied from 9 kg/sq.cm to 232 kg/sq.cm.
