Flexible paste solution
To assess the fill flow properties, rheological testing was carried out on alluvial soil samples from the different locations across the available borrow pit area. These results showed highly variable rheological characteristics, indicating that maintaining consistent rheological properties during operation would be challenging. Furthermore, flowability was shown to be highly sensitive to changes in solids content.
Based on the perceived difficulty in maintaining consistent rheological properties and the likelihood of significant coarse aggregate content, control of a hydraulic delivery system (positive displacement pumping) from a fixed plant location was considered unmanageable. The only sustainable solution for this project was a mobile paste plant that could be placed directly above each borehole when filling.
To enable drilling of the fill holes, a roadway grid was required directly above the workings. This would allow the mobile paste plant, a twin-trailer, to move around the grid safely and be quickly re-established at each borehole. The plant consisted of a front trailer which housed the tailings feed hopper, main conveyor system with weightometer, mixer and hopper; and second trailer containing a generator and cement silo. The mobile paste plant achieved production rates in excess of 150m3 per hour.
Comprehensive testwork programme - Mix design
Outotec implemented a comprehensive testwork programme to ensure a flexible, robust and cost effective fill mix design. A paper “Fill design and implementation with challenging material – Wambo fill project” Helinski and Revell (2014), presented at ACG’s MineFill 2014 in Perth, outlines this programme in detail. Testwork undertaken included direct shear and consolidated undrained triaxial testing – on both cemented and uncemented paste. The objective of this work was to:
1. Define the fill mix and mechanical properties to maintain rock mass stability.
2. Ensure that the fill itself did not pose a risk to underlying mining activities, specifically through “flow” liquefaction.
3. Ensure that the implemented design was sufficiently flexible to manage variability across the source material stockpile area.
In order to prevent sinkhole formation, the paste required sufficient stiffness and bearing capacity to support any roof failures. During the fill process, the fill would be exposed to a range of different stress paths. Should the fill material remain in a saturated state, it could potentially liquefy and remobilise into the underlying workings during this stage. Therefore, in addition to the rock support requirements, it was also necessary to ensure that the fill material could not remobilise.
Results of testwork showed that paste batched in excess of 81% solids strain-hardened upon undrained shearing. This showed that this material would not be prone to “flow” liquefaction. At this solids content the in situ fill material was shown to possess adequate strength and stiffness to prevent any sinkhole formation. Consequently, if placed at a solids content in excess of 81% solids, the paste could be deposited without any binder addition.
Below 81% solids the paste was shown to strain–soften upon undrained shearing, leading to “flow” liquefaction. Over the range of mix solids contents tested, the results showed that all paste with a UCS in excess of 30kPa strain-hardened upon undrained shearing. Paste of this strength was also shown to have sufficient bearing capacity and stiffness to prevent sinkhole formation.
Testwork showed a unique relationship between the binder addition (required to achieve this target strength and behaviour) and the mix solids content (for samples taken from across the alluvial soil stockpile area). The required binder addition increased as the mix solids content reduced.