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Rock Mechanics.pdf - Mining and Blasting

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BACKFILL PROPERTIES AND PLACEMENT<br />

rapid curing Portl<strong>and</strong> cement component of the mix. However, it should be noted that<br />

the addition of any fine material to s<strong>and</strong>fill reduces the permeability of the medium.<br />

The design of a fill mix should take due account of the consequences of this reduction.<br />

S<strong>and</strong>fill, or s<strong>and</strong>fill with cementitious additions, is transported underground as<br />

a suspension at about 70% solids (i.e. weight of solids/total weight of mix). Flow<br />

velocities exceeding about2ms −1 are required to maintain homogeneous dispersion<br />

of the fill components in the slurry. The slurry is discharged into the stope at selected<br />

points, chosen to achieve some specified distribution of fill in the mined void. As<br />

described in detail by Barrett (1973), segregation occurs after discharge, with the<br />

coarser particles settling close to the discharge point, <strong>and</strong> finer particles transported<br />

in the low velocity surficial flow of the transport medium. This leads to cement-lean<br />

<strong>and</strong> cement-rich zones in any horizontal plane through a fill mass. A further degree<br />

of heterogeneity of the fill mass arises from the different local settling rates of coarse<br />

<strong>and</strong> fine particles. The low settling rate for the finely ground cementitious additives to<br />

s<strong>and</strong>fill results in the development of a sedimentary structure in the mass, with the top<br />

of any fill bed having a high cement content, while the cement content of its base is low.<br />

The purpose of including cementing agents in s<strong>and</strong>fill is to provide a true cohesion<br />

component of shear strength, which is then exploited in applications such as freest<strong>and</strong>ing<br />

fill walls adjacent to working stopes. Knowledge of fill strength development<br />

is essential in fill design <strong>and</strong> in scheduling stope extraction adjacent to such fill masses.<br />

Fill strength is determined using st<strong>and</strong>ard soil mechanics procedures <strong>and</strong> principles.<br />

Some typical results provided by Mitchell (1983) of triaxial tests on 28-day cured cemented<br />

s<strong>and</strong>fill specimens are shown in Figure 14.3. The strength can be represented<br />

in terms of c ′ <strong>and</strong> ′ or by or by a low stress bond strength (Cb) which is determined<br />

by unconfined compression test results. Fill deformation shows pronounced<br />

brittle-plastic behaviour. At low cement content (

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