Experience of Overcoring the ANZI Strain Cell in HQ Exploration Boreholes to Determine the Three Dimensional In Situ Stresses A Depths Approaching 1KM

This paper describes recent use of the ANZI (Australia, New Zealand Inflatable) strain cell and the overcoring method of stress relief in an HQ exploration borehole at depths approaching 1km to determine the three dimensional in situ stress field in a one-day operation. The stages of a routine overcoring operation are presented to illustrate each step of the process. The results from a European metalliferous mining site are presented to illustrate the process of characterising the three dimensional in situ stress environment when multiple high confidence measurements are achieved.

The ANZI strain cell is an instrument system that uses the overcoring method of stress relief to determine the three dimensional in situ stresses in rock. The instrument has been used successfully for over three decades in numerous underground mining and civil projects but technical advances over the last decade have allowed the system to be deployed routinely in surface exploration boreholes. Recent development of a downhole electronic data logger, a wireline enabled drilling system and an instrument deployment system has simplified the process of obtaining three dimensional overcore measurements at previously inaccessible depths remote to any underground excavation.

The capability to deploy ANZI strain cells from surface exploration boreholes represents a significant breakthrough for the design of underground civil structures. High confidence characterisation of the in situ stresses at design stage provides the opportunity to design key infrastructure to take advantage of the in situ stress field from the outset before any excavation and construction activity even begins. Understanding the three dimensional in situ stress field not only provides a measure of the magnitude and direction of loads acting within the rock mass, it is also provided insight into the mechanics of the all the various processes driving ground deformations.