Page 428 - Rock Mechanics For Underground Mining

ARTIFICIALLY SUPPORTED MINING METHODS

loading of backfill occurs under drained conditions, where that term is understood in
its strict soil mechanics sense.

14.2 Backfill properties and placement

Techniques for preparation of suitable mine backfills, their placement in mined voids,
and for mining adjacent to fill masses, have been the subject of extensive investigation
for about 50 years. The evolution of fill technology and related mining practice can be
traced through reviews and proceedings of specialist symposia (Anon, 1973, Anon,
1978, Stephansson and Jones, 1981) and monographs (Thomas et al., 1979, Mitchell,
1983). A comprehensive review of current technology is provided by Landriault
(2001).
Materials used as mine backfill or components of a fill mass are of five types:
run-of-mill concentrator tailings, used with a cementing agent to form paste fill;
deslimed mill or concentrator tailings, or sandfill; natural sands; aggregates, devel-
opment mullock and similar coarse, cohesionless media; and cementing agents of
various types. Although sandfill was for many years the most common backfill mate-
rial, Landriault notes that more recently, paste fill has become widely used, because
of the more economical use of cementing agents, the disposal of a higher proportion
of mined waste in underground voids and the more homogeneous properties of the fill
mass.
Several types of backfill are placed using water as the transport medium, and
effective de-watering and long-term drainage (ensuring pore pressures are low) are
critical aspects of fill placement and long-term operation. Water is removed from
newly placed fill by two mechanisms. Following settlement and consolidation of fill
solids soon after placement, excess water collects on the fill surface. The provision of
vertical drainage conduits in the fill mass, such as perforated pipes, drainage towers
and timbered raises, allows decantation of surface water and its rapid removal at
the stope base. Alternatively, surface water may flow through the porous fill bed,
and be discharged at the base of the stope through horizontal drains in the bulkhead
in the backfilled drawpoint. Percolation of the excess water and prevention of the
development of high pore pressures in the fill are clearly dependent on adequate
permeability in the vertical direction in the backfill mass.

14.2.1 Sandfill
Sandfill is frequently called hydraulic fill, because the sand is transported by pipeline
or borehole and distributed in stopes as an hydraulic suspension. Typically the sand-
water slurry has a composition of about 70% solids by weight. A flow velocity
greater than about 1.5–2 m s −1 (depending on the fill size grading) is required to
suspend the sand and prevent settlement of sand from the sand-water slurry. Sub-
critical flow velocities result in plugging of the pipeline and borehole distribution
system.
Sandfillispreparedfromconcentratortailingsbyhydrocyclonetreatmenttoremove
the slimes, or clay-size fraction. Typical products of such a classification process, for
various mine fill preparation plants, are given in Table 14.1. It is seen that, in most
cases, the highest proportion of the fill product lies in the 40–150
m range of particle
sizes. In soil mechanics terms, this corresponds to a fairly narrow grading of coarse
410