In the present study, the work of Gstalder and Raynal on the correlation of physical properties of rocks with drilling performance properties of rocks with drilling performance has been extended and some new methods of measuring rock drillability have been proposed. Their conclusions that sonic velocity is a good indicator of rock drillability have been substantiated, provided a mineralogical factor is taken into account.

Rock drillability [ ], defined in the present work as the volume of rock drilled per present work as the volume of rock drilled per unit of energy input, has the same dimensions as Gstalder and Raynal's "Effritement Specifique" as obtained from a Schreiner hardness test. A good correlation is obtained between these two quantities but it is apparent that 6 to 34 times as much rock breakage occurs by penetration tests as by air drilling tests for the same expenditure of energy. The value of drillability [ ] varies with bit type and may be a good indicator of the most efficient bit type for a given rock.

Drilling strength., as defined earlier by Somerton, and drillability [ ] both correlate well with sonic velocity but both require a mineralogical factor. However, drilling strength shows a linear relation to triaxial strength, which is only slightly dependent on mineralogical character.

Triaxial elastic moduli values correlate well with rock drilling parameters. This is to be expected since the relations between elastic moduli and sonic velocity are well known. In fact the evidence all seems to indicate that a function of sonic velocity could be introduced into rock drilling equations to account for the rock strength term.

Drilling Strength

In drilling performance equations one quantity that has been found difficult to evaluate is the resistance of the rock to drilling. Whether it be "drilling strength" as defined by Somerton, a variable strength exponent as used by Galle and Woods, or an intercept value on a drilling rate-bit weight plot as proposed by Bingham, this quantity plot as proposed by Bingham, this quantity cannot be evaluated by a single simple test.

Simon has proposed that drilling strength be defined by an impact loading test as the ratio of energy input to volume of rock broken.

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