Page 431 - Rock Mechanics For Underground Mining

BACKFILL PROPERTIES AND PLACEMENT

rapid curing Portland cement component of the mix. However, it should be noted that
the addition of any fine material to sandfill reduces the permeability of the medium.
The design of a fill mix should take due account of the consequences of this reduction.
Sandfill, or sandfill with cementitious additions, is transported underground as
a suspension at about 70% solids (i.e. weight of solids/total weight of mix). Flow
velocities exceeding about2ms −1 are required to maintain homogeneous dispersion
of the fill components in the slurry. The slurry is discharged into the stope at selected
points, chosen to achieve some specified distribution of fill in the mined void. As
described in detail by Barrett (1973), segregation occurs after discharge, with the
coarser particles settling close to the discharge point, and finer particles transported
in the low velocity surficial flow of the transport medium. This leads to cement-lean
and cement-rich zones in any horizontal plane through a fill mass. A further degree
of heterogeneity of the fill mass arises from the different local settling rates of coarse
and fine particles. The low settling rate for the finely ground cementitious additives to
sandfill results in the development of a sedimentary structure in the mass, with the top
of any fill bed having a high cement content, while the cement content of its base is low.
The purpose of including cementing agents in sandfill is to provide a true cohesion
component of shear strength, which is then exploited in applications such as free-
standing fill walls adjacent to working stopes. Knowledge of fill strength development
is essential in fill design and in scheduling stope extraction adjacent to such fill masses.
Fill strength is determined using standard soil mechanics procedures and principles.
Some typical results provided by Mitchell (1983) of triaxial tests on 28-day cured ce-
mented sandfill specimens are shown in Figure 14.3. The strength can be represented

in terms of c and or by or by a low stress bond strength (C b ) which is deter-
mined by unconfined compression test results. Fill deformation shows pronounced
brittle-plastic behaviour. At low cement content (<5% cement by dry weight) and
high confining stress, the stress-strain curve shows ductile behaviour, but brittle be-
haviour dominates at high cement content and low confining stress. Mitchell (1983)
recommends that preliminary fill designs can be based on uniaxial tests, but that more
comprehensivedesignsrequiretriaxialtestingtoobtainc and forthoroughanalysis.

He also notes that, although cement segregation occurs in fill placement, the average
strength of the mass is generally comparable with laboratory control test results.
Strength development in cemented backfill is important in scheduling the extraction
of adjacent stopes. Mitchell (1983) proposes that the uniaxial compressive strength,
c, of good quality cemented sandfill increases according to the relation

2
c = A + BC log t kPa (14.3)
where A and B are constants, C is the cement content (wt%) and t is the curing
time in days. For a particular fill, representative values of A and B were A = 30,
B = 5.
Consideration of backfill variation in sampling and testing suggested that it is
comparable with that of natural soils, and therefore that similar safety factors should
be acceptable in fill design.

14.2.3 Rockfill
When backfilling large stopes, the demand for fill material can exceed the avail-
able supply, which is limited by the mine production rate and mill capacity. The
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