Page 433 - Rock Mechanics For Underground Mining

BACKFILL PROPERTIES AND PLACEMENT

Table 14.3 In situ properties of composite backfills (from Gonano, 1977).

Fill type c (MPa) (deg) E (MPa)
8% cemented sandfill (CSF) 0.22 35 285
composite of 8% CSF and rockfill 0.60 35.4 280

simultaneous placement in a mine void of aggregate or similar dry rockfill and
cemented sandfill can reduce the unit cost of filling the void with cohesive fill. This is
achieved by reducing the total amount of cement addition, and extending the capacity
of a backfill preparation plant to meet mine demand. The composite fill is placed
by discharging cemented sandfill slurry and rockfill into the stope simultaneously.
The variety of mechanical processes accompanying rockfill placement, including
high-velocity impact and compaction, bouncing and rilling, lead to a highly hetero-
geneous fill mass. The structure of a composite of rockfill and cemented sandfill (with
a rockfill/sandfill ratio at placement between two and three) has been described by
Gonano (1975), and is illustrated schematically in Figure 14.2. The various zones, of
different constitution and texture, and various degrees of cemented infilling of rock-
fill interstices, have different mechanical properties. For example, the zone with a
porous, open structure is poorly cemented, with low cohesion. However, such a zone
ensures that the interior of the fill mass can drain adequately during fill placement.
The development of a highly heterogeneous fill mass, whose structure is controlled
by placement conditions, indicates that, in practice, careful attention must be paid to
locationofdischargepointsintothestopevoid.Forexample,thegenerationofapoorly
cemented zone near the surface of a pillar, which is to be recovered subsequently,
would represent a failure in the fill design procedure.
The varieties of compositions and structural domains in cemented rockfill make
laboratory determination of their representative properties difficult. Gonano (1975,
1977) described the procedures used for large-scale, in situ determination of a well-
cemented zone in a composite cemented sandfill/rockfill mass at the Mount Isa Mine,
Australia, and comparable tests on a cemented sandfill. The results from these tests
are given in Table 14.3. It is observed from these data that the main effect of the
rockfill inclusion in the sandfill medium is a significant increase in the cohesion of
the mass. Clearly, there could be significant mining advantages if the design of the
fill placement system could produce, preferentially, a composite fill of the type tested
at locations requiring a high-strength fill mass.
As summarised by Landriault (2001), the cement content of cemented rockfills
(CRF) typically lies in the range of 4%–8% by weight. Because of the cost of Portland
cement, a proportion of the cement in CRF may be replaced by finely ground furnace
slag and fly ash, provided extra time is available for curing. In examining the effect
of cement addition on rockfill strength, Swan (1985) found that the 28-day uniaxial
compressive strength ( c ) of CRF was related to volumetric cement content (C v )by
the expression
2.36
c = C (14.4)
v
where is a characteristic of the particular rock type.
Some typical strength and deformation properties of cast samples of cemented
rockfill, for both small- and large-scale specimens, were c in the range 1–11 MPa,
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