CSIRO Hollow Inclusion Cell Rock Stress Sensor

How it works

Highly sensitive strain gauges are bonded to the rock using temperature-specific glue packs in a mechanical process that lets the gauges become part of the rock very rapidly.

The gauge can be left in-situ for long-term stress-change measurement, or over-cored to determine a stress profile of that section of rock.

Deployment

The HI Cell requires a strain bridge monitor to read values. Alternatively, the digital version (the HID Cell) can be logged using a breakout box and tablet, a PC, or a serial data logger, with software available for data analysis.

Installation

The HI Cell is installed into a 38 mm hole by filling the hollow body of the cell with a pre-formulated epoxy cement. Cement is extruded by the piston when the cell reaches target depth, and multiple rubber seals confine the grout flow to around the cell. Once the grout has cured, the strain gauges are fully bonded 1.5 to 2.0 mm from the borehole wall, for which allowance is made during data reduction.

Strain response can be monitored during overcoring via the data cable, which runs through the drilling rods and a modified water swivel to a logger, tablet or PC — indicating conditions such as the onset of core cracking, cell de-bonding or inelastic response of the rock. The retrieved overcored section of rock containing the cell may be restressed in a biaxial pressure chamber to derive the actual rock properties.

Data analysis with Stress 201m

The Stress 201m programme determines stresses and relative stresses from raw data output from a CSIRO HI cell or HID Cell. Run from Excel, it converts raw voltage to strain, plots monitoring data, imports data from a logger, and saves and opens monitoring files. The programme is for genuine ESS Earth Sciences manufactured CSIRO HI and HID cells, with preset values for the alpha and beta angles of the strain gauges.