Page 429 - Rock Mechanics For Underground Mining

BACKFILL PROPERTIES AND PLACEMENT

Table 14.1 Size analyses of sandfills prepared from mill tailings (from Thomas et al. 1979).

Size
fraction
(
m) Weight %
Mine 1 Mine 2 Mine 3 Mine 7 Mine 8 Mine 9

+3340 0.0 0.00 0.0 0.00 0.07 0.00
2360–3340 0.0 0.00 0.0 0.00 1.09 0.00
1180–2360 0.0 0.00 0.0 0.00 9.01 0.00
850–1180 0.0 0.00 0.0 0.00 8.16 0.13
600–850 0.0 0.00 0.0 0.00 11.00 1.39
425–600 0.7 0.05 0.0 0.20 8.28 6.02
300–425 2.5 0.21 0.0 0.59 6.44 12.86
212–300 8.3 1.62 0.8 3.16 6.37 19.73
150–212 15.7 2.33 3.2 8.69 7.08 18.88
106–150 17.1 11.08 17.2 22.50 5.71 18.33
75–106 20.9 11.67 22.8 17.30 4.79 10.04
53–75 11.0 20.52 13.2 17.53 3.57 4.90
40–53 7.5 9.32 14.9 6.78 1.17 4.04
30–40 7.1 14.25 14.9 15.93 3.36 1.09
20–30 2.4 15.72 8.0 5.14 4.04 0.69
15–20 1.4 7.91 2.2 1.11 4.39 0.43
10–15 0.8 1.32 0.6 0.30 2.88 0.23
–10 4.8 4.01 2.2 0.79 12.60 1.25

silt and fine sand. On average, the proportion of – 10
m material is less than 4%, so
that, by inference, the clay-size fraction is very low. Natural sands which have been
used or considered for use as mine backfill are generally coarser than artificial sands.
Particle sizes are concentrated in the range 150–600
m, corresponding to a medium
sand.
The permeability of a fill mass determines the drainage condition it experiences
internally under imposed surface load. Investigations by McNay and Corson (1975)
−8
indicatethatsuccessfulsandfillshaveapermeabilitycoefficientintherange7 × 10 –
−1
7.8 × 10 −5 ms . These values correspond to soil gradings of medium slit to coarse
sand, which are consistent with the classifications defined by the particle size data.
Common mining practice is to specify fill drainage properties in terms of perco-
lation rate, using a standard percolation tube test. The percolation rate (measured in
−1
mm h ) is essentially equivalent to the permeability of the fill as measured under
a gravity gradient close to unity. Mitchell (1983) proposes that the percolation rate
(P) of fill in situ may be estimated from percolation tube test results and in situ and
measured void ratios of fill using the relation

P corrected = P measured (e in situ /e measured ) 2 (14.1)

A percolation rate of 25 mm hr −1 (equivalent to a permeability coefficient of 7 ×
−1
10 −6 ms ) is recommended to provide a free-draining fill, and to ensure that placed
fill will be free of ponded surface water a couple of shifts after placement.
At the base of a stope, where the water flow is choked through the filled drawpoint
and bulkhead, the permeability P d of the fill must be higher than in the fill mass in
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