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

Abstract

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.

Paste backfill continuous pour: Red Lake operations case study

“…For a spike of pressure of 4 kPa, a volume of 5.4 m 3 would have had to be added to the backfill to increase the backfill height by 0.2 m. This displaced volume agrees with the typical flush size of 6 m 3 . The increase in vertical pressure between 14:00 and 15:15 is explained by a 'lava' type flow as previously captured by Grabinsky et al (2014), as shown in Figure 5a. The paste was building up in an area of the stope that did not influence the instrument.…”

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

“…Estimating plug strength requirements is such a case, where using c = 1 2 UCS would reduce the calculated strength (UCS) requirement by a factor of two. If the recently placed backfill remains close to saturated, as was determined from field tests by [21,22], then c = 1 4 UCS must be used. It is therefore recommended that the assumption c = 1 4 UCS continue to be used until it can be demonstrated that greater estimates of cohesion are warranted.…”

“…Although others have considered CPB strength increases due to self-weight consolidation during deposition [16][17][18][19][20], the slow start to effective stress development (Figure 2) compared to material cohesion development (Figure 3) suggests minimum potential for self-weight consolidation and associated strength gain at the studied mines. Indeed, [21,22] show that the as-placed bulk properties at Williams, Kidd, and Cayeli mines are the same as the as-mixed properties, except that occluded air was mixed into the paste during deposition which had the effect of slightly increasing void ratio and decreasing degree of saturation. Similar results are reported by [13] at Casa Berardi where cores from the CPB plug had void ratios similar to the as-mixed design.…”

“…Although others have considered CPB strength increases due to self-weight consol dation during deposition [16][17][18][19][20], the slow start to effective stress development (Figure compared to material cohesion development (Figure 3) suggests minimum potential fo self-weight consolidation and associated strength gain at the studied mines. Indee [21,22] show that the as-placed bulk properties at Williams, Kidd, and Cayeli mines ar the same as the as-mixed properties, except that occluded air was mixed into the pas during deposition which had the effect of slightly increasing void ratio and decreasin The Williams binder is a blend of 50 wt% Portland Type I (GU) cement and 50 wt% Type C fly ash and shows relatively continuous cohesion development with cure time. In contrast, the Kidd binder is a blend of 10 wt% cement and 90 wt% ground blast furnace slag, and the Cayeli binder is CEM V from a Turkish manufacturer which is equivalent to 60 wt% cement and 40 wt% extenders.…”

A numerical analysis of how permeability affects the development of pore water pressure in early age cemented paste backfill in a backfilled stope

“…The minimal change in void ratio, shown in Figure 2, is comparable to the difference observed between the void ratios of freshly prepared laboratory samples and the void ratios determined from in situ samples from the test stopes, given the same recipe. Furthermore, these in situ samples all had similar void ratios and did not show much relationship with change of depth (Grabinsky et al 2014). This indicates that there is a slight change in consolidation experienced between fresh paste and deposited paste, but that this change is not dependent on the amount of material above.…”