SCT provide high confidence 3D in situ stress measurements capability for the mining and civil industries. SCT use the ANZI strain cell for overcoring and stress change monitoring because of the instrument’s operational advantages and high density of strain measurements. An ongoing focus on this stress measurement technology over 35 years has SCT well placed as an international leader in the overcoring method and in the understanding of complexities of in situ rock behaviour.
In recent years, SCT has developed the capability to measure in situ stress in deep exploration holes. The development in 2016 of a downhole logging system allows high confidence measurements to be obtained in only a few hours at depths in excess of 500m. A wireline deployed drilling system to prepare the borehole for overcoring eliminates the need to trip drill rods. The integrated data logger enables accurate strain readings at high frequency. This high quality data gives insight into rock behaviour during the de-stressing process not previously observed.
SCT has several different versions of the ANZI strain cell that can be used for different applications depending on borehole depth, borehole inclination and rock type. Routine overcoring from underground excavations uses a double cell arrangement that provides 3D in situ stress measurement results for geotechnical engineers and mine designers in Australia and around the world. Smaller diameter versions of the instrument are used for measurements in surface exploration boreholes and for near surface measurements where access may be limited or there are special site requirements.
Confidence in the accuracy of the stress measurement is fundamentally important and the ANZI strain cell provides a quantitative measure of the level of confidence that may be placed in each individual measurement. The ANZI strain cell contains 18 electrical resistance strain gauges in various orientations that are bonded directly to the rock surface of the borehole when the instrument is installed. With only six independent gauges required to provide a 3D stress measurement, the ANZI strain cell provides a high level of redundancy.
The ANZI strain cell is also deployed to measure stress changes over time with the assistance of a specialised data logging system connected at the borehole collar. These instruments have been used routinely to measure the stress changes in a range of mining and civil applications in both surface and underground operations. Routine installations include chain pillar abutment loading and horizontal stress redistribution about longwall panels in coal mines and overburden behaviour during goaf formation. More recently, these instrumentation have been used for stress change monitoring of high value surface infrastructure proximal to mining areas including bridges, dams and sensitive geological structures such as cliffs and river beds.
Click here to watch a more comprehensive animation of the deployment of an ANZI strain cell and the overcoring process.