INTRODUCTION

This paper deals with a solution to the problem of measuring changes in the strength properties of weak rocks, associated with changes in moisture content and or the passage of time.

One method of quantitatively assessing changes of strength involves having a large number of rock samples, with essentially identical strengths and initial moisture contents. By destructively testing one specimen at the end of each increment of time or incremental change in moisture content, a graph of strength versus time or moisture content can be plotted.

This approach is not valid where the number of available specimens is small, or where the natural variability of the test population is large.

In the latter case the variations in strength with time or moisture content may be less than the variations with specimen number or position: causal relationships would be difficult to discern.

TESTING OPTIONS

The progressive reduction of strength in a single specimen can be assessed by:

  • measuring specimen degradation or slaking, as manifested by loss of mass during the course of the test program;

  • measuring loss of strength, by repeated non-destructive testing during the course of the test program.

The latter is more difficult in the Melbourne mudstones. Although the measurement of ultrasonic pulse velocities is quite feasible for less weathered specimens, even at the highest magnifications on the available cathode-ray-oscilloscopes, no reliably discernible pulse arrivals can be detected after travel through the softer, more intensely weathered materials. Also the pressure necessary to be applied between the transducer heads and the rock, to ensure pulse transmission, is sufficient to cause compressive failure of the weaker specimens. Rebound hardness can also be measured. A small hard-tipped hammer is dropped through a constant distance, and the height of rebound of the hammer is used as a measure of the rock's hardness. This technique can cause minor crushing, over an area of about a millimetre diameter on the surface of a weak rock. Sequential tests can avoid the damaged spots from previous tests, while still conducting many tests within a few millimetres of each other.

The rebound hardness values measured upon the weak weathered Melbourne mudstones are so low (Shore hardnesses less than 10) as to be considered unreliable, for detecting any small variations.

The principle of hardness testing was, however retained in the present investigation. This was to approximate a non-destructive test with a test which damaged an insignificantly small volume of the test specimen, so that replicate tests could be conducted upon an effectively undamaged specimen. Following a review of alternative hardness testing methods it was concluded that the N.C.B. Cone Indenter appeared promising.

THE N.C.B.CONE INDENTER

This is a small portable rig developed at the Mining Research and Development Establishment of the British National Coal Board(Szlavin, 1974).

It measures the indentation hardness of rock specimens in a similar fashion to metallurgical hardness testers.

(Figure in full paper)

The specimens which are about 25 mm diameter and 6 mm thick, are placed between a standard hardened steel conical point and a flat spring-steel beam.

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