Page 434 - Rock Mechanics For Underground Mining

ARTIFICIALLY SUPPORTED MINING METHODS

and deformation modulus in the range 300–1000 MPa. In situ strength may be con-
siderably lower than laboratory strength values, due to effects such as segregation,
difference in porosity and variable distribution of cement. Barrett and Cowling (1980)
proposed that, as a general rule, the in situ strength of CRF was about 50% of the
strength measured in the laboratory.

14.2.4 Paste backfill
Paste backfill is a relatively recent development. It is frequently composed of the
complete solids content of de-watered run-of-mill tailings and cement. It has a high
fines content, typically at least 15% by weight passing 20
m, and has a water content
sufficient to form a high viscosity paste. The total solids content is typically in the
range 78–85%. Control of the water content is critical in ensuring that the fines fraction
exists as a colloidal suspension which forms the transport medium for the coarser
fraction of the fill. Because of the high fines fraction and the danger of liquefaction, a
cementing agent is an absolutely essential component of paste fill in ensuring its safe
performance underground. Cement additions are in the range of 1–5%, depending on
the functions of the fill.
The relatively high viscosity of paste fill compared to sandfill requires higher
pressure gradients to induce flow, and results in higher overall pipeline pressures. The
rheology of paste is not only a function of the fines content, but also of the chemistry
and mineralogy of the component materials. The formation of a colloidal boundary
layer of fine material during the plug flow of paste is important in maintaining stable
flow. The sensitivity of paste fill to water content means that particular attention must
be paid to drainage conditions in mine voids both during and after backfilling.
The lower water content of pre-placed paste fill permits the use of lower cement
content than in cemented sandfill, and strength is further improved by the lack of
segregation during paste fill placement. According to Landriault (2001), cement con-
sumption using paste backfill is generally about 40–70% of what would be used in
alternative backfills with comparable mechanical properties. Further, some mines
have found that a coarse material can be introduced in paste fill. The resultant
blended paste fill has a wider particle size distribution, leading to a lower poros-
ity and therefore lower water content in the fill mix. After placement, blended fill
results in a relatively high modulus fill mass, which can be important in rock support
in some mining applications where development of support pressures in the fill mass is
sought.

14.3 Design of mine backfill

Design of backfill for stoping operations must consider two sets of requirements.
First, the in situ fill mass must meet various criteria for self-support, such as specified
stable free-standing, vertical or horizontal (undercut) spans, to permit excavation of
adjacent rock. Second, its role as a functional structural element in the mine must be
considered. It is possible to design backfill to perform particular functions, such as
provision of support for stope boundaries or lateral confinement of pillars to improve
their post-peak deformation characteristics.
Design of backfill to assure stable performance under the operating conditions
imposed by extraction of adjacent rock involves an analysis of stress and displacement
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