Rock extensometer

#This invention relates to devices used to measure small amounts of movement in a rock or similar formation over time.

#Natural rock strata often are exposed to deforming stresses under circumstances in which it is desirable to monitor any resulting deformations. In underground mining removal of material leaves galleries and other void spaces in the strata. The remaining rock is placed under a different set of stresses which can lead to both cracking and crushing of the remaining rock. In an extreme case, excavation failure can occur. Similar problems can arise when excavations are made in close proximity to the foundations of an existing building or other structure.

#It is often of importance to be able to monitor what is happening within a rock stratum, such as in the walls and roof of a mine gallery. The easiest way to monitor changes in a rock stratum is to observe any dimensional changes which may take place within the rock itself: a rock under sufficient tension will fail by cracking, and a rock under sufficient compression will crush. In both cases, deformation of the rock should be detectable by a suitably located linear measurement.

#The extensometer will now be described in one embodiment by way of reference to the attached Figures in which:

#FIG. 1 shows a single radial measuring device in place in a borehole;

#FIG. 2 shows the measuring device of FIG. 1 from the direction A in FIG. 1;

#The embodiment shown in the Figures incorporates a linear motion position transducer. There are several varieties of these available which utilize a variable resistance. Other devices which utilize changes in inductance, in capacitance and which utilize materials whose electrical characteristics change with the applied stress (commonly called strain gauges) are also known. In the extensometer shown in the FIGS., a variable resistance device with a maximum travel of 15 cms (6 inches) is used. Movement of the linking rod results in a change in the value of the variable resistance, which can be measured by a suitable electrical circuit. The transducers used were units made by Waters Manufacturing Inc. of Wayland, Mass. The units have a nominal resistance of 5,000 ohms, an accuracy of 0.1%, and an incremental sensitivity of 0.0013 mm. A water tight sealed unit is also available if required. The electrical connections were made to the transducer in accordance with the manufacturer's instructions. In appearance, the transducer comprises an elongate substantially square section box, with a rod protruding from one end. The end of the rod is provided with a thread, and the other end of the box is provided with attachment means, and a cable outlet. Since response to axial and radial change will not necessarily be the same, calibration of the transducers before use might be desirable.

#In FIG. 1 the borehole, shown generally at 1 has inserted into it only a radial measurement device. This comprises the transducer 2 which has attached at one end a linkage device 3 to which is attached a daisy wheel anchor 4, which can be better seen in FIG. 2. The daisy wheel 4 anchors the transducer 2 in the borehole 1. The radial measuring device is attached to the other end of the transducer 2. It comprises a block 5 attached to the case of the transducer carrying two pairs of arms 6 which can bend about the pivots 7, and which are attached to the block 8 to make up a pantograph linkage. The blocks 5 and 8 include suitable pivots for the arms 6. The block 8 is also attached to the spring 9 and the transducer operating rod 10. The spring 9 is in tension, so that as it urges the block 8 toward the body 2 of the transducer, it urges the arms 6 outward to contact the borehole. This arrangement can also be seen in FIG. 2. A change in diameter of the borehole will result in movement of the arms 6, which will move the rod 10, and alter the value of the variable resistance in the transducer. This change is read by suitable electrical means (not shown) attached to the electrical circuit means 13. Adjacent the daisy wheel anchor 4 a retrieval pin 11 is also attached to the linkage device 3. The retrieval pin 11 has a male thread 12 which is a standard drill string rod thread.

#As can be seen in FIG. 2, the electrical lead is accommodated between the petals of the daisy wheel anchor. These petals serve to anchor the extensometer in place in the borehole.