Interpretation of as-placed cemented paste backfill properties from three mines

Over the last few years considerable attention has been paid to predicting the as-placed properties of cemented paste backfill (CPB) using laboratory techniques that load the sample in ways that are thought to reasonably simulate the effective stress paths experienced by the CPB during initial placement and subsequent curing while backfilling is ongoing. Most of these studies suggest that consolidation mechanisms prior to significant hydration-induced stiffness increases are responsible for denser CPB and correspondingly higher strengths. The field evidence for significant density increases during placement, however, is mixed with some studies indirectly suggesting increased density may occur and some studies with direct field measurements showing no significant density increases. This paper presents three case studies in which very careful and detailed field measurements were taken during backfilling, and for which considerable success was achieved in subsequent field sampling to directly measure densities of as-placed CPB. The three mines cover a range of mining conditions with some narrow Alimak stopes, some large and some small stopes, at three different mining operations in two countries (two operations in good quality Canadian Shield host rock, and one in relatively poor host rock conditions in Turkey). Even for this seemingly wide range of mining conditions, the field results were similar and compelling; very little water was lost between transported and cured samples, and therefore the density increased only very marginally. For all three mines the field measurements collected during filling suggests that effective stresses remain essentially zero for a prolonged period (many hours) during filling. Evidence for hydration during this zero effective stress stage includes changes in temperature and electrical conductivity, which can be correlated to laboratory measurements on similar samples. Therefore, the material essentially hydrates under a zero effective stress condition and gains sufficient stiffness to withstand subsequent loading under non-zero effective stresses with very little change in density. The question then to be asked is ‘why’ does this prolonged period of zero effective stress exist? Some insight can be gained from modified Gibson analytic solutions but the most compelling explanation is derived from numerical modelling based on a Gibson approach for fully coupled stress-deformation-fluid transport analysis. The boundary conditions for such modelling are non-trivial and are considered in detail in a companion paper by Veenstra et al. (2013), but the consistency of the numerical solutions with the extensive field data collected suggests that the modelling approach has merit. Finally, for mines where such conditions exist, it appears that very little can be done through altering the deposition technique in order to affect the consolidation processes to increase density. In such cases, the only solution may be to use water reducing admixtures and such an approach is briefly addressed.

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