Stress Change Cells

Sigra can manufacture to special order, stress change cells that are devices grouted into a rock mass to measure stress change within it. These are all tubular devices which are fitted into a trammie pipe (cementing tube). Grout is pumped through them, into the hole annulus and around the transducer. The cementitious grout locks the transducer in place into the borehole by post initial set expansion. This actively loads the transducer via the grout to the borehole wall. The forms of stress change cells that can be manufactured are:

  • Axial strain cell using vibrating wire strain gauges
  • Axial and biaxial cell with vibrating wire strain gauges
  • Fully triaxial cell with resistance strain gauges

stress_change_cell_installation_1Sigra undertakes stress change monitoring in rocks. This may be undertaken for reservoir monitoring or mining purposes. In combination with stress_change_cell_installation_1fluid pressure monitoring, it is a powerful tool to determine what is happening in the ground.

Sigra’s installation process involves grouting a stress change cell into a borehole.

Where only the vertical stress change is required, the cell is fitted with axial vibrating wire strain gauges and requires no orientation. Its installation is relatively low cost.

Where full three dimensional stress changes are required the stress change cell is hollow and strain gauged. It has on-board electronics which sample from the gauges and built-in magnetometers and accelerometers. The tool communicates via an umbilical wire. The grout that must be used with this tool is of a special design that pre-loads the cell, thus the device can be used to monitor stress increases or decrease. In this case the cells are costly and great care must be exercised to install them correctly.

An intermediate cost device may be used to give biaxial and uniaxial stress change information. This comprises a cell with four transverse vibrating wire strain gauges and an axial vibrating wire strain gauge. It may be orientated through dropping a survey tool on to a mule shoe arrangement in the back of the grout tube prior to grouting.

For all the cells, excessive ground movement will lead to shearing of the connecting cable and loss of data. They are, therefore, unsuitable for long term monitoring where goafs may form and associated large settlements occur.