Page 443 - Rock Mechanics For Underground Mining

BACKFILL APPLICATIONS IN OPEN AND BENCH STOPING

Figure 14.9 A method of pillar re- The ability of cemented backfill to undergo autogenous healing, during placement of
covery in open stoping, using a co- superincumbent layers of the fill mass, is also determined by the composition of the
hesive backfill to control stope-wall
cementitious materials, and the physical conditions existing during curing.
behaviour.
Pillar recovery adjacent to backfill usually involves detonation of explosive charges
in rock close to the fill–rock interface, and questions arise about the effect of the
associated dynamic loading on the integrity of the fill mass. Using the terminology
adopted in Chapter 10, the difference in the characteristic impedances of rock and
backfill is such that very little dynamic strain energy can be transmitted into the
fill medium. Nevertheless, even a low magnitude stress wave in a fill mass may be
sufficient to cause sufficient pore pressure increase to pose a risk of liquefaction,
particularly where the fill is saturated and close to the critical state, in soil mechanics
terms. In the case where a blast hole is located close to the interface, i.e. within about
10–15 blasthole diameters, the risk arises of detonation product gases acting directly
on the fill surface. There is then an obvious risk of superficial damage to the fill
structure.
A key issue in the effective use of backfill for artificial support during pillar mining
is complete integration of the initial, primary stoping phase with the subsequent phase,
mining of pillar ore as secondary stopes. This requires that the general mining strategy
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