Frimpong, J & Pandey, R 2026, 'Hydraulic fracturing as a novel technique for strength evaluation of cemented paste backfill:
influence of injection flow rate on fracture initiation pressure', in AB Fourie, M Horta, M Oliveira & S Wilson (eds), Paste 2026: Proceedings of the 28th International Conference on Paste, Thickened and Filtered Tailings, Australian Centre for Geomechanics, Perth, pp. 1-13, https://doi.org/10.36487/ACG_repo/2655_10
(https://papers.acg.uwa.edu.au/p/2655_10_Pandey/)
Abstract: Cemented paste backfill (CPB) is a critical ground control material in underground mining to provide support and stability to mined-out voids, and accurate strength assessment is essential for ensuring long-term stability and safety. Conventional strength evaluation methods, such as uniaxial compressive strength tests on surface-cured specimens, often fail to represent actual in situ conditions, leading to unreliable strength estimates. To overcome this limitation, a new laboratory technique based on hydraulic fracturing has been developed to directly evaluate CPB strength under in situ conditions. The method determines the fracture initiation pressure (FIP), the maximum pressure at which the backfill first fractures, which has been verified as a reliable indicator of CPB strength. This study focuses on investigating the role of injection flow rate in hydraulic fracturing tests as part of validating the technique. Cylindrical CPB samples were fractured using AW32 hydraulic oil at controlled flow rates ranging from 1 to 50 ml/min. Results show that flow rate influences fracture initiation at lower rates (1–8 ml/min). At these low rates, gradual fluid delivery causes slower pressurisation, increased fluid leak-off, and delayed fracture onset, resulting in reduced FIP values. As flow rate increases, FIP rises, reflecting more efficient pressurisation and reduced leak-off. However, beyond 8 ml/min, the influence of flow rate diminishes, and the inherent variability in sample strength becomes more significant than the variations in FIP caused by changes in flow rate. The findings highlight that flow rate is a critical parameter to consider when applying hydraulic fracturing for CPB testing. Practically, this study establishes a baseline recommendation: low flow rates are unsuitable for strength estimation. To obtain the most representative FIP-based strength index, higher injection flow rate should be used, where rate effects diminish and variability in FIP is dominated by material properties. These outcomes provide a strong foundation for advancing hydraulic fracturing as a reliable method for CPB strength assessment.

Keywords: cemented paste backfill, hydraulic fracturing, fracture initiation pressure, injection flow rate, strength assessment, ground control