Fracture initiation pressure as a measure of cemented paste backfill strength

A laboratory-scale hydraulic fracturing method measuring fracture initiation pressure (FIP) is demonstrated as a rapid, high-resolution technique to directly assess the strength of cemented paste backfill (CPB). The approach correlates linearly with conventional CPB strength measurements and is suitable for direct, in situ assessment, empowering mine operators with more reliable and timely quality assurance for underground support. Background As underground mining advances to greater depths and processes larger orebodies, responsible tailings management and ground support grow in importance. Cemented paste backfill (CPB) created from mine tailings and cementitious binders, is now widely used to fill mined-out voids, providing ground stability and minimizing environmental impact. Reliable assessment of CPB strength, especially tensile strength, is critical for safe mine design and operational sequencing and to reduce risks of backfill or bulkhead failure.Current practices typically rely on uniaxial compressive strength (UCS) tests of laboratory-cast cylinders, which are poor proxies for in situ CPB strength due to discrepancies in placement, curing and testing conditions. In situ methods such as coring or penetration tests are time-consuming, challenging and often unsuccessful for low-strength materials. Electrical-conductivity-based monitoring provides indirect assessments but require prior instrumentation. This study addresses the industry’s need for a direct, deployable method to assess both early- and late-stage CPB strength with improved reliability. Methods A laboratory-scale hydraulic fracturing protocol was developed to determine the FIP of cemented past backfill. CPB samples were prepared from silica tailings and type I Portland cement at three binder dosages (4 percent, 8 percent and 12 percent) and tested after curing for 3, 7, 14 and 28 days. For each condition, standardized UCS, Brazilian tensile strength (BTS) and Mode-I fracture toughness (KIc) measurements were also performed for benchmarking. The experimental procedure followed for the work presented here is shown in Fig. 1. Fig. 1 Experimental plan followed in the research. To measure FIP, a 2 in. × 4 in. CPB cylinder was prepared with a central borehole (1/8-in. diameter), into which hydraulic oil was injected at a constant flow rate of 4 mL/min. The injection of the hydraulic oil resulted in increase in pressure until a maximum pressure was reached. At the maximum pressure — that is, the FIP — a tensile failure was initiated, thereby providing a direct measure of tensile strength. All tests were conducted in triplicate for reliability. Results Figure 2 demonstrates representative hydraulic injection pressure-time curves for CPB samples at varying binder contents and curing durations. In all cases, hydraulic pressure increases gradually under constant oil injection until the FIP is reached. The onset of FIP is consistently delayed and occurs at a lower pressure for low-binder or early-age samples, while higher binder content or extended curing yields markedly higher, earlier peaks. Fig. 2 Hydraulic injection pressures obtained during fracturing of CPB samplesat various binder contents and cure periods. Quantitative results for all test conditions are summarized in Table 1. Here the mean FIP values are presented alongside UCS, BTS and KIc. All strength measures exhibit system growth with increased binder and curing duration. Across the data set, FIP is strongly and linearly correlated with all benchmark strength metrics regardless of binder or cure period. On average, FIP is approximately 1.6 times the UCS, 6.6 times the BTS and 10.1 times the KIc. The higher resolution of FIP as a measure of strength compared to other measures is attributed to the borehole size dependence of FIP as a measure of strength, as described by the point stress mode, which is discussed later. Together, Fig. 2 and Table 1 illustrate that FIP is a robust, sensitive measure of CPB strength, with clear evolution across binder and maturity conditions and strong predictive value with respect to established mechanical benchmarks. Discussion Hydraulic fracturing, as validated in this study, offers a breakthrough for real-time, direct assessment of in situ CPB strength, especially for mine sites where sample extraction or prior instrumentation is infeasible. The technique’s sensitivity and high resolution at small borehole diameters enable early detection of weak backfill, supporting improved operational decisions, bulkhead management and enhanced mine safety.The point stress (PS) model explains why FIP can significantly exceed the BTS-derived tensile strength for small borehole diameters. Small borehole diameters decrease the likelihood of sampling a critical flaw, thereby artificially elevating the FIP beyond the tensile strength of the material. Thus, practitioners can leverage small boreholes for sensitivity strength assessment or opt for larger borehole diameters to directly measure the true tensile strength, providing flexibility in application. While the PS model shows strong agreement with laboratory data, its predictive accuracy may vary with different backfill compositions, binder types and site-specific conditions. Validating and calibrating the approach under true in situ stress environments and at field scale are important next steps to ensure robust operational performance. Additionally, further investigation into how microstructural variables, such as particle size distribution, binder chemistry and hydration mechanisms influence the characteristic distance in the PS model will help refine its applicability and support broader adoption across a range of underground mining scenarios. Conclusions Hydraulic fracturing (FIP measurement) is a validated, direct method for CPB strength assessment, correlating with established benchmarks. The size-dependent point stress model enables practical field use and interprets FIP-benchmark relations accurately. This method supports rapid, in situ strength verification, improving operational safety and efficiency in underground mining. References A list of all references is available in the full paper.

Fracture initiation pressure as a measure of cemented paste backfill strength

Authored by: James A. Frimpong, Basel Ahmad Shabab, Rohit Pandey, Snehamoy Chatterjee, Gabriel Walton and Alexander S. Brand

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