ABSTRACT

Sound design of structures founded in or upon rock requires that the deformability of each rock member involved be characterized. This can be done by means of loading tests (plate bearing, fiat jacks, borehole jacks, pressure chambers, etc.) or through convergence measurements (tunnel convergencer,o of sag, etc.) in exploratory works. Analysis of data will be qualitative (degree of nonlinearity, creep behavior, scattering of results over a number of sites, etc.) and quantitative (modulus of elasticity or deformation). Some of these tests involve a very bulky apparatus and are expensive to perform. On the other hand, for the purpose of mapping and describing the various rock members which occur within a given site, many feet of diamond drill holes are frequently completed. Even with the optimum equipment and expert drillers, however, quantitative characterization of the rock units on the basis of tests on returned samples is apt to be misleading; the softer and weaker components of the rock tend to be lost and the fabric of the rock blockfracture system in situ is not sampled. Laboratory moduli of elasticity have proved to be consistently higher than in-situ values selected for engineering purposes.

This led to the design of instruments which would operate in boreholes, combining the advantages of a reduced size and a deeper investigation, from the surface to several tens of feet.

BOREHOLE DEFORMABILITY MEASURING INSTRUMENTS

In the last 10 to 15 years, a number of testing devices have been constructed which can be inserted into a borehole to apply a load and mea- sure the direct response of the walls of the borehole. As summarized in Table 1, these devices are of three types: (1) instruments that supply a uniform internal pressure in the borehole by pressuring a fluid in an expandablej acket (boreholed ilatometers), (2) devicest hat supply a unidirectional pressure to a portion of the circumference of the borehole by forcing apart circular plates (borehole jacks), and (3) devices that force a small indenting pin into the wall rock (borehole penetrometers).

Borehole Dilatometers

Two types of full circle radial expansion devices have been developed. In the Menard pressuremeter,1, 2 a central measuring cell filled with water is inflated between upper and lower guard cells introduced to minimize end effects. The inflation pressure is obtained from a gas pressure bottle; the borehole displacement is calculated from the changing diameter during pressuring. The Geoprobe instrument 3 is a somewhat similar device. In the second type of dilatometer, displacements are measured by differential transformers placed across one or more diameters. Instruments of this type known to the authors are the LNEC (Laboratorio Naeionale de Engenharia Civil) device4, the Janod-Mermin device4, Come's cell, 6 the tube deformeter, 7 and the sounding dilatometer. 8, 9 The most elaborate of these is the tube deformeter which has three groups of 8 LVDT's giving the diametral deformation every 60 ° around the circumference in the center and at end of the loaded area.

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