Underground » Strata Control and Windblasts
The role of horizontal stress in affecting strata behaviour in underground coal mines has been well documented. In Australia, the nature and depth of the underground coal resources has resulted in high levels of horizontal stress, typically 2-3 times the vertical stress, and up to 9 times that expected by lithostatic burial. Horizontal stress impacts on all facets of strata behaviour, and is a fundamental input into the geotechnical design process.
Traditional stress measurement techniques are based around point measurement and are typically cost and time intensive. The use of naturally occurring borehole breakout to provide stress directions, and stress magnitudes, provides a readily available resource to quantify stress regimes on the basis of depth and spatial distribution.
Borehole breakout can occur where the stress concentration around a borehole exceeds the rock strength. The borehole will fail in shear (due to overstressing) with the breakout location oriented at 90 degrees from the maximum principal horizontal stress ( H).
As well as logging breakout orientation, by assessing the rock strengths in which breakout is occurring, and conducting an assessment of stress concentrations about the borehole, the stress magnitude can also be constrained.
The two primary objectives of Project C10009 have been to:
- Quantify stress magnitudes associated with borehole breakout through understanding strata failure mechanisms about the borehole.
- Optimise field characterisation of rock properties.
The technique utilises commercially available geophysical tools to collect the raw data. Imaging of the breakout is achieved using an acoustic scanner. Mechanical properties of the rockmass are derived from the standard wireline suite, including sonic, density and natural gamma.