Page 436 - Rock Mechanics For Underground Mining

ARTIFICIALLY SUPPORTED MINING METHODS

Figure 14.5 Confined block model
for static analysis of fill stability (after
Mitchell, 1983).

which may slide on the inclined base plane. If required, a surcharge load may also be
taken into account on the upper surface of the fill block. Assuming that the inclination
o
of the critical failure surface is given by = 45 + /2, the net weight W N of the

block is given by
W N = wH f ( L − 2c )

where H f = H − 1/2w tan
The Factor of Safety against plane failure is obtained from

F = tan / tan + 2c L/[H f ( L − 2c ) sin 2 ] (14.5)

The minimum value of for sandfill can be taken as 30 , for which tan /

◦

tan = 0.33. Thus, for F = 1.33, the required apparent cohesion is given by

c = LH f sin 2 /2(L + H f sin 2 ) (14.6)
which defines the cohesive strength requirement for the fill mass.
Obviously more general fill design analyses based on particular site conditions
could be undertaken using a similar approach to that above. According to Landriault
(2001), the design approach proposed by Mitchell has been applied successfully in
more than 100 North American mines over a period exceeding 20 years. Successful
application of the simple model and analysis provides a measure of confidence in
its more general application in fill exposure stability analysis, where a different set
loading conditions may be considered.
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