1.Introduction

The Borehole Pressure Cell (BPC) is designed to monitor stress changes in rock materials, including hard rocks, coal, rock-salt, potash, etc. It consists of a flatjack made from two steel plates welded together at their edges with the intervening space filled with hydraulic oil. The cell has a "dog bone" cross section, allowing it to expand easily over a large range without splitting apart.
A 6 mm (0.24") diameter tube connects directly to the BPC and on its outer end is T-bar used to orient the BPC perpendicular to the applied rock stress. A further length of 3 mm (0.12") diameter high-pressure steel tubing connects the tubing on the BPC to a pressure gauge and/or to a pressure transducer (see Figure 1). Cells are provided already filled with de-aired oil. De-airing is done because the cell is more responsive the more rigid it is. Setting rods are provided to push the BPC into the borehole and correctly orient it.

1:    

Model_3200_Borehole_Pressure_Cell.png

Figure 1: Model 3200 Borehole Pressure Cell

The figure below shows two different types of pressure measurement devices.

2:    

3200-WBG_1.png

Figure 2: Two Pressure Measurement Devices

The figure below shows a pre-encapsulated BPC.

3:    

Pre-encapsulated_BPC.png

Figure 3: Pre-encapsulated BPC

In use the cell is grouted into a borehole drilled into the rock and when the grout has hardened the cell is connected to a hydraulic pump and pressurized to a pressure level equal to the estimated in-situ stress level, plus 10%. (A check valve prevents the cell pressure from being lost when the hydraulic pump is disconnected.) Stress changes within the host material are transmitted to the cell causing a proportionate change in the internal hydraulic pressure, which is recorded on the pressure gage and/or pressure transducer, (vibrating wire or semi-conductor).

BPCs react mainly to stress changes in the direction perpendicular to the plain of the flatjack. They are relatively insensitive, (about 6%) to stress changes in the direction parallel to the plain of the flatjack. If stress changes are required in two directions, then two or three BPCs can be installed at different orientations in the same borehole or in adjacent boreholes.