Required Plug Strength for Continuously Poured Cemented Paste Backfill in Longhole Stopes

Abstract

Continuously poured paste backfill dramatically improves underground mining efficiency through reduced stope cycle time and simplified logistics. For longhole stopes, a backfill “plug” is poured to a few meters above the undercut brow and must gain sufficient strength to prevent failure through the plug when the “main” pour begins. A novel, rational engineering design approach that determines the required plug strength is developed. The potential failure mechanism during continuous pouring is identified and the theoretical solution and its numerical validation/calibration are discussed. Four field case histories are then used, three of them involving continuous pours, to demonstrate the theoretical solution’s validity in back-analysis. These case studies are unique in the extent and quality of total stress and water pressure measurements made throughout backfilling. Additionally, comprehensive laboratory data are available to characterize strength development during binder hydration in the first few days, which are critical to the back-analyses. Results indicate that continuous backfilling is feasible with reasonably attainable backfill strengths at most mines. However, mines must undertake comprehensive early strength laboratory testing, and must carry out field measurements during the pour to ensure the placed backfill behaviour is consistent with the analysis assumptions.

Cemented paste backfill strength profiles for continuous pouring and liquefaction resistance

“…Prior to the Grabinsky et al (2021) plug strength analysis and design approach, there was limited reference to plug strength within the literature. To our knowledge, the only guidance was contained in the Handbook on Minefill, which stated that the plug should be poured to "…approximately 1 m above the draw point brow and permitted to cure to approximately 150 kPa strength before filling the remainder of the stope" (Potvin et al 2005).…”

“…However, to our knowledge, a formal definition of plug strength that would be adequate to isolate the barricade was lacking in the published literature. In response, Grabinsky et al (2021) proposed a solution to determine the required strength for a backfill plug used in a continuously poured plug/main backfill filling strategy, where the plug is a higher binder content backfill poured to a few metres above the stope undercut's brow, as seen in Figure 1. ) received feedback from consultants and mining personnel that the suggested design method (which uses the plug strength solution) is too general and onerous to implement, especially for preliminary design purposes.…”

“…Many mines use higher binder content in the plug to accelerate curing time, supporting continuous pouring (i.e., no plug cure time before starting the main pour). An analytical solution to assess the required plug strength for continuous pours was recently proposed in a paper by Grabinsky et al (2021). Similarly, various heuristics have been published suggesting a minimum Unconfined Compressive Strength will be sufficient to prevent CPB liquefaction under even the most implausible extreme loading events.…”

Tailings Utilization and Zinc Extraction Based on Mechanochemical Activation

“…For example, Hua Na proved the possibility of using copper-nickel slag to form expanding filling mixtures with the addition of hexadecyltrimethyl ammonium bromide and Sodium silicate [ 14 ]. Moreover, the issues of the geotechnical and technological parameters of the filling mass are far from being improved [ 15 , 16 ]. Hence, the transition to rock formation recycling during metal extraction and the integrated use of technogenic raw materials from tailings remains a fundamental scientific problem.…”

Best practices in continuously (or not continuously) pouring paste backfill

“…The continuous pour strategy typically assumes the paste in the initial plug region has gained strength sufficiently quickly so that barricade pressures peak before the plug pour is completed, or where pressure thresholds at the barricade are not exceeded during subsequent backfilling. Grabinsky et al (2021Grabinsky et al ( , 2023 presented an analysis approach whereby an initial plug strength condition must be met and, subsequently, the strength gain in the plug must increase more rapidly than the incremental loading increase during the continued pour. Barricade pressures during continuous pours should be measured to ensure a safe loading range is maintained.…”