Experimental study on borehole size effect and prediction of breakout initiation stress

Borehole breakout initiation stress is critical for mining and geotechnical engineering as it indicates horizontal stress magnitudes and rock quality in a specific layer. This stress can be significantly intensified under laboratory conditions where the borehole size is small, although the specimen sizes in previous studies varied. This may result in inaccurate experimental results as the sample itself is affected by the scale effect. In this paper, two series of normal compression tests (constant borehole–specimen ratio and constant specimen size) were conducted on various borehole radii (6 mm–12.5 mm) to study the influence of specimen size as well as borehole size on breakout initiation stress. Experimental results revealed that specimen size does not have an obvious influence on the breakout initiation stress when specimen to hole ratio is over 10, suggesting that data collected from previous studies in the literature are also valid and can be used for future breakout analysis. A comparative analysis of the stress averaging concept, failure criteria considering pressure-dependency and an empirical model was conducted to predict breakout initiation stress. Results showed that the stress averaging concept was not reliable for experimental data, whereas the linear fracture initiation criterion produced the most accurate results, although its parameters are dubious and difficult to obtain. An empirical relationship, derived from literature data under normal compression tests, also showed promising results in estimating breakout initiation stress. Due to its simplicity and reliability, it may be used in future studies with more validation.