Stress Estimation From Borehole Scans For Prediction Of Excavation Overbreak In Brittle Rock

In the field of geomechanics, one of the most important considerations during design is the state of stress that exists at the location of a project. Despite this, stress is often the most poorly understood site characteristic, given the current challenges in accurately measuring it. This stems from the fact that stress can’t be directly measured, but must be inferred by disturbing the rockmass and recording its response. Although some methods do exist for the prediction of in situ stress, this only provides a point estimate and is often plagued with uncertain results and practical limitations in the field. This research proposes a methodology of continuously predicting stress along a borehole through the back analysis of borehole breakout and how this same approach could be employed to predict excavation overbreak.
\n\tKGHM’s Victoria Project in Sudbury, Canada, was the location of data collection, which firstly involved site characterization through common geotechnical core logging procedures and laboratory scale intact core testing. Testing comprised Brazilian tensile strength and unconfined compressive strength testing, which involved the characterization of crack accumulation in both cases. 
\n\tFrom two pilot holes, acoustic televiewer surveys were completed to characterize the occurrence and geometry of breakout. This was done to predict the orientation of major principal stresses in the horizontal axis, with the results being further validated by the geometry of stress-induced core disking. From the lab material properties and breakout geometries, a continuum based, back analysis of breakout was done through the creation of a generic database of stress dependent numerical models. When compared with the in situ breakout profiles, this created an estimate of stress as a function of depth along each hole. The consideration of the presence of borehole fluid on the estimate of stress was also made. This provided the upper-bound estimate of stress from this methodology. Given the generic nature of the numerical models, potential shaft overbreak was also assessed using this technique and from the previously described estimate of stress.

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