ABSTRACT:

Mining exploration, development, and design require in situ testing instrumentation to measure geotechnical properties on a scale that takes into consideration heterogeneities in the rock mass. An acoustic device that propagates high-frequency (20- kHz) stress waves between boreholes in rock separated by distances from a few to tens of meters was designed, developed, and performance-tested for applications in mining that require assessment of elastic properties and integrity of rock masses. Components of the system include a unique, 5-kv, portable pulser; inflatable, piezoelectric, borehole transmitter and receiver probes; a downhole amplifier; and signal monitoring and measurement components. The probes are designed to operate in either dry or wet, nominally NX-sized boreholes. Results are given for performance tests in a large granite block; in concrete; in deep, vertical boreholes in copper conglomerate at an open pit mime; and in shallow inclined boreholes in a copper porphyry at an underground mine. Several applications of the apparatus in mining research are given.

INTRODUCTION

Safe mine design requires site characterization, with reliable estimates of rock mass behavior. Although laboratory mechanical property tests conducted on small specimens may provide useful information relating to the intact elements of the rock mass, the discontinuous nature of rock masses normally preclude accurate assessment of rock mass behavior by such means (1). The requirement that such tests must be conducted at the field site on a large-scale usually makes them cumbersome, costly, and difficult to interpret. This permits few tests to be conducted, making estimates of reproducibility and reliability difficult. Moreover, information on rock behavior is known only at the specific site of measurement, making it necessary to extrapolate beyond to other regions. Many of these difficulties can be circumvented by using seismic geophysical techniques that provide indirect measures of the mechanical properties of the rock mass.

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