ABSTRACT

The modulus of rigidity of rock can be determined by a fairly simple procedure in which a cylindrical pressure cell is first calibrated by expanding it inside two metal test cylinders and then is expanded inside a drillhole in rock. The two test cylinders have different expansion characteristics (different material and/or different wall thickness), which make possible the determination of two calibration constants. Change of diameter of the pressure cell is determined from the volume of fluid pumped into it. The essential apparatus consists of a cylindrical pressure cell, a Bourdon pressure gage, a volume metering fluid pump, and two test cylinders. Some advantages of the method are that a rock sample is not required, a diamond drill is not required, and the rock is tested in place.

INTRODUCTION

The cylindrical pressure cell (CPC) is a device for determining the modulus of rigidity G of rock by direct measurement from an in situ test inside a small drillhole. The CPC is essentially a copper tube which is expanded inside the drillhole by pumping fluid into it at pressures up to 10,000 psi. More tests to date have been made in 1.5-inch diameter drillholes using 8-inch-long pressure cells. This report deals only with the 1.5-inch CPC, although the principles are the same for any size cell. Change of volume of the drillhole per increment of applied hydrostatic pressure was measured during the test. The results were interpreted by means of the classical thick-wall cylinder equations for an elastic body. The objective was to develop a convenient apparatus and a technique for making routine field determinations of G in mine and tunnel rocks. The method has the usual advantages of an in situ test, namely that no test specimen need be drilled out, transported to the laboratory, prepared (shaped, gages applied), and tested. Laboratory tests are time consuming and are open to the objections that the rock properties may be affected by exposure to the atmosphere, and the test specimen necessarily does not include major defects. The ratio E/ (1 + ?) appears frequently in equations involving the convergence of an opening in a strained medium, the design of a tunnel lining, and the like (1, 4, 8) 4 . This method yields the value of G = E/2(1 + ?) from a single test in rock, as compared to conventional testing in which the modulus of elasticity E and Poisson's ratio ? must be separately determined. Because E can be calculated from G and ( 1 + ? ), and ( 1 + ? ) typically has a value of about 1.3, a large error in ? has relatively little effect on ( 1 + ? ), which means that E can be calculated with relatively little error by using only a rough estimate of ?. In this sense the method is a practical one for determining E as well as G. Of course if the value of ? is known, the method determines E as accurately as G.

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