Borehole extensometers

Radial displacements were measured by means of borehole extensometers installed in borings SIl and SI2. They were located close to the position of the stress cells. Each extensometer consists of graphite rods with independent anchoring points located at a depth of 1.0, 3.0 and 7.0 m into the borehole. Measured and computed evolutions of temperature in Boring SF 21, at a radial distance of 1.20m are shown in Figure 23. Most predictions are quite accurate. Two of them, however, depart in a significant way. Prediction of temperature depends... 

In Figure 5 and 6, calculated displacements for the two nonlinear models are compared to the measured ones for three different multiple-point borehole extensometers. One general trend concerning the calculated results is an increasing relative displacement, from Anchor 1 to Anchor 4, with a delay in the development of the latter caused by the delay in the propagation of the thermal front. Non-linearities are more pronounced when using the ubiquitous joint model, whereas the brittle model leads to weakly nonlinear behaviour, close to the pure elastic one. 

In order to understand the abutment pressure distribution and its effects on the failure of extremely thick coal seam, underground measurements using pressure sensors and multi-position borehole extensometers were performed. The results are shown in Figures 2 and 3. 

Holes were drilled across the joint planes and foliations of the rock mass in the tunnel wall to install borehole extensometers. Conventional... 

Figure 5. Schematic of installation locations of geophones, borehole extensometers and borehole camera inspection holes...

ISRM. (1992). Commission on Testing methods. Suggested methods for installation of borehole extensometer. International Society for Rock Mechanics. 

Measurements are taken on the three principal joint directions and in 6-cm diameter boreholes. Each measurement point corresponds to two boreholes, both perpendicular to the joints and 20 cm apart (see Fig. 3). A pressure gauge for measuring hydrostatic pressure is placed in the first borehole and a vibrating wire extensometer is installed in the second in order to monitor deformation normal to the joint. This extensometer is positioned within a concrete cylinder. The deformation measurement is thus that of the cylinder with a 6-cm diameter and 50-cm length. The recorded deformations are then transformed into normal displacements by referring to the basic measurement length, i.e. the 15-cm extensometer length. 

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