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Miner's Toolbox |
| Mine Backfill Engineering |
| Cemented Rockfill (CRF) | ||
Cemented rockfill (CRF) is generally described as any material greater than sand size that is used to backfill underground mines. It is typically used in large blasthole stopes, drift and fill, or undercut and fill stopes where stable vertical or horizontal fill exposures are required. In blasthole stopes, a cemented rockfill is typically required for two main purposes:
A strong fill is required to reduce dilution from the fill as the secondary stope is recovered. Dilution can com either from blasting impact or from planar or circular type failure within the fill mass.
In the ground support role, the stiffness of the fill (Young's modulus) is the key design property of interest. In the pillar recovery role, the shear strength is of greatest importance. In practice, the uniaxial compressive strength (UCS) is used as the design parameter for cemented rockfill since the test is relatively inexpensive and can be incorporated into routine quality control programs at the mine.
Fill exposures are generally of three types (Figure 1)
Stone (1993) has developed design charts for each of the three cases above using pseudo-3D formulations of Mitchel and Roettger (1989).
Binder Content The strength of CRF is a function of water content, binder content, temperature, aggregate type, gradation, void ratio, and other factors. Yu, (1989) provides the following relationship between the cement content and 28 -day UCS for 15 cm diameter samples taken at the Kidd Creek Mine:
It is important to note however that great heterogeneity exists within CRF particularly if segregation occurs during the placement process. Yu also reported that tests on 30 cm cylinders yielded strengths of about 90% of the in-situ strength and tests on 15 cm cylinder strengths were about 65% of the in-situ strength.
Water Cement Ratio A water:cement ratio of 1.2:1 (by weight) is optimum for CRF. A higher water content can result in a slurry the percolates too quickly through the rock. Lower water contents will result in a fill mass that is too dry resulting in incomplete cement hydration and segregation problems.
Aggregate Type Like normal concrete, the strength of CRF is greatly influence by the strength of individual aggregate particles. Generally, a rock with a UCS of greater than 70 MPA and a Los Angeles Abrasion values of less than 30 will perform well in cemented rockfill.
Particle Size Distribution In discussions about rockfill, the coarse size fraction generally refers to the material from greater than 10 mm to the top size of 100-150 mm. The fine aggregate is considered to be the material less than 10 mm in size. The fine aggregate should make up about one quarter to one-third of the total aggregate weight.
The ideal grading that results from minimizing the void space is given by
It is unlikely that an operation could afford to crush and screen the rockfill to achieve the ideal gradation. Rather, aggregate from a number of different sources should be blended to try to approach the ideal gradation as best as possible.
Void Ratio Unconfined compressive strength of CRF is directly proportional to the bulk density. Backfill having a particle specific gravity of 2.75 will have an in-situ bulk density as rockfill in the order of 2.05 tonnes/m3 when placed by end dumping or through fill raises. Material that is pneumatically placed or compacted with rammers will have a density in the range of 2.25 tonnes/m3. Areas of low density within the fill mass can occur if segregation occurs during placement.
Cemented Rockfill Preparation and Placement
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