Typically, the 3D stress hollow inclusion (HI) cell is used in the mining industry, particularly in high stress precious metal mines that continually need to modify their operations according to the changes in stress as tunnels are bored through the rock. But rock stress is not all the HI cell is designed to monitor – it is also used to monitor stress in concrete, often in road and rail tunnels, and in dams where there can be variable stress on the structures.
The HI cell is always installed in existing, fully cured concrete. A borehole is drilled and the cell inserted, and its strain gauges are then bonded to the borehole by the sensor’s piston pushing out glue from inside, leaving a hollow chamber allowing the strain gauges to flex as surrounding stress changes. See the video below for a cutaway demonstration of the process.
Of great benefit to monitoring concrete arch or gravity dams, and also useful in concrete faced rock fill dams, the HI cell can be installed at one or more points throughout the structure to measure long term stress changes. Up to four digitial HI cells can be connected to a dedicated serial logger, or each cell can be surveyed manually from time to time using a tablet and power/interface adaptor.
The modern digital HI cell has made 3D stress measurement a simple process as there is no more fiddling with strain bridge meters and manual data recording – it literally is now a plug-and-play solution.