Evaluation of different dynamic reinforcement elements using the rate of change of energy and strain during in situ dynamic testing

Authors: Gebremedhin, B; Mikula, PA; Sarmadivaleh, M; Darlington, B Download Paper Open access courtesy of:

DOI https://doi.org/10.36487/ACG_repo/2465_31

Cite As:
Gebremedhin, B, Mikula, PA, Sarmadivaleh, M & Darlington, B 2024, 'Evaluation of different dynamic reinforcement elements using the rate of change of energy and strain during in situ dynamic testing', in P Andrieux & D Cumming-Potvin (eds), Deep Mining 2024: Proceedings of the 10th International Conference on Deep and High Stress Mining, Australian Centre for Geomechanics, Perth, pp. 529-546, https://doi.org/10.36487/ACG_repo/2465_31



Abstract:
During dynamic ground disturbances the rock mass undergoes rapid change with time, resulting in various demands imposed on installed dynamic ground support. In dynamic ground conditions, every dynamic pulse is unique, as it is the result of different source mechanisms with different transient times. The time dependent parameters of power and strain rate were investigated in this paper as additional parameters to better characterise the performance of dynamic reinforcement elements. Sandvik Mining and Construction provided data from in situ axial tests conducted by their dynamic test rig, for a total of 52 MD, 65 MDX, 11 Kinloc and 13 collar-loaded Posimix reinforcement elements, dynamically tested across eight mine sites. The test data was statistically checked to identify patterns in the data and validate the combination of data from different mines into larger datasets. Then the performance of the reinforcement elements in the dynamic tests was studied with respect to the duration of the test. The paper examines the response of the element to the test impact, via two introduced parameters: average response power, i.e. the response energy or absorbed energy per unit time, and average strain rate, i.e. the strain of the element per unit time. Clear differences were found between the different reinforcement element types. Regarding response power, the MD and Kinloc displayed similar patterns, but the MD responded with up to more than twice the power level of the Kinloc. The MDX showed a tight cluster of power values. Although the dataset is small, there is indication that the collar-loaded Posimix may be unable to survive at response power levels higher than 400 kW. Regarding strain rates, failed Kinloc elements had high strain rates, indicating the high speed with which they were pulled from the borehole once the anchor was compromised. One failed MD bolt had similarly high strain rate, but the three other failed MD bolts did not. Failed and survived MDX and collar-loaded Posimix had similar strain rates. MDX and collar-loaded Posimix reinforcement elements operated at low strain rates. This probably reflects the nature of yield of a steel bar, where there is an upper limit to strain rate that can be survived. Overall, it is found that power and strain rate analysis add additional value to describe the performance and capacity of reinforcement elements. Also, there is suggestion that it is not possible to force a particular element to accept a higher power level than it can sustain according to its design.

Keywords: absorbed energy, power, strain rate, in situ dynamic testing, ground support

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