Evaluation of strainburst potential associated with excavation damage zone due to excavation method

CIM MEMO 2010
Sean Maloney, Peter K Kaiser,
Abstract A strainburst is defined as self-initiated rockburst occurring when the local stresses exceed the rock mass strength and failure occurs in an unstable or violent manner. Many published criteria for strainburst potential assessment are of a form relating the burst proneness to a ratio of field stress to the intact or rock mass strength. In brittle ground conditions, the empirical depth of failure is roughly proportional to the stress level ?Smax/Sc, and starts to propagate when the stress level exceeds 40% of intact rock strength from laboratory testing. Hence, it follows that spalling, with or without little energy release, should be anticipated to precede strainbursting. In a drill and blast excavation there is often an annulus of damaged rock in which both the strength and modulus are reduced relative to the virgin values, the reduction being dependent on the quality of blasting. Consequently, it is possible that the blast damage creates a more favourable state vis-à-vis bursting, particularly in comparison to the undamaged wall of a mechanized excavation.
In this paper, it is numerically investigated that the disturbance factor is degradation ratio of rock mass strength and modulus due to excavation method. Then, it is calculated that the kinetic energy is releasable or released in the model. It is normally about 0.6m in drill and blast tunnel excavations and the depth of disturbance between 0.5 and 1m. Since the strainburst criterion is based upon the intact rock strength, the change in rock mass strength is not directly incorporated and the stress reduction is directly translated into a reduction in strainburst potential. In reality, the lower strength may contribute to yielding in the rock mass and a further stress reduction. Consequently, all other factors aside, mechanically excavated openings will be at greater risk of strainbursting in comparison to drill and blast excavations. A mitigating factor to this increased burst potential may be the rate at which the stress change is imposed. Hence the steady, rather than dynamic, advance of mechanically excavated headings may be beneficial in terms of burst severity.
Keywords: Strainburst potential, Blast damage zone, Burst-prone ground, Kinetic energy, Excavation method
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