Page 430 - Rock Mechanics For Underground Mining

ARTIFICIALLY SUPPORTED MINING METHODS

the stope. Free drainage will be maintained if

P d > P corrected (A s /A d ) (14.2)

where A s and A d are the cross-sectional areas of the stope and drawpoint, respectively.
Sandfill is a cohesionless material, with a purely frictional resistance to deforma-
tion. The apparent angle of friction is dependent on the angularity of the particles
and the packing density of the medium. Hydraulic placement of a sandfill results in a
loose fill structure, with a void ratio of about 0.70. This corresponds to an in situ dry
−3
unit weight, d , of about 15.7kN m , or a dry mass density of placed fill of about
−3
1.6tm . In this state, the peak angle of friction of many artificial sandfills is about
37 . In practice, sandfill at low water content also displays an apparent cohesion, due
◦
to suction developed in the pores of the dilatant medium when subjected to a change in
boundary load or confinement. This may allow free-standing vertical walls of sandfill,
of limited height (perhaps 3–4 m) to be maintained temporarily under some mining
conditions.

14.2.2 Cemented sandfill
The lack of true cohesion restricts the scope for mining application of sandfill. This
is overcome in practice by the addition of various cementing agents to the sand mass.
The obvious choice is Portland cement which, although an expensive commodity, can
provide a significant cohesive component of strength at a relatively low proportional
addition to the medium. The results given in Table 14.2 indicate the cohesion attained
inPortlandcement–sandfillmixturesaftercuringtimesof7and28days.Therelatively
low uniaxial compressive strength determined from these figures (e.g. 5.75 MPa for
a 16% Portland cement mix at 28 days) is partly the result of the excess water used
in preparing and transporting a cemented sandfill mix. The water content of such a
mix is always far in excess of that required for hydration of the Portland cement.
The expense of Portland cement as a fill additive has led to its total or partial re-
placement by other cementing agents. Thomas and Cowling (1978) reported on the
pozzolanic properties of such materials as quenched and finely ground copper rever-
beratory furnace slags. Other materials such as fly ash and iron blast furnace slags are
also known to be pozzolanic, and suitable for incorporation in a fill mass to augment
the cohesion conferred by Portland cement. In fact, quenched slags, ground to a fine-
2
ness of 300 m kg −1 or greater, may present real advantages as pozzolans. Their slow
reactivitymayservetohealanydamagecausedinthefillmassduetodisturbanceofthe

Table 14.2 Some typical strength parameters for cemented sandfill (from Thomas el al., 1979).

Cement content Curing time Specimens Cohesion c Friction angle,
(wt %) (days) tested (MPa) (deg)
4 7 22 0.13 30
28 23 0.15
8 7 24 0.24 33
28 24 0.31
16 7 24 1.02 36
28 24 1.46
0 (fines added) 205 11 0.03 32
4 207 12 0.21 37

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