Page 334 - Rock Mechanics For Underground Mining

ROCK SUPPORT AND REINFORCEMENT

time at which support can be installed is after radial displacement of an amount OF
has occurred.
Support1isinstalledatFandreachesequilibriumwiththerockmassatpointB.This
support is too stiff for the purpose and attracts an excessive share of the redistributed
load. As a consequence, the support elements may fail causing catastrophic failure of
the rock surrounding the excavation.
Support 2, having a lower stiffness, is installed at F and reaches equilibrium with
the rock mass at C. Provided the corresponding displacement of the periphery of the
excavation is acceptable operationally, this system provides a good solution. The rock
mass carries a major portion of the redistributed load, and the support elements are
not stressed excessively. Note that if, as in the temporary/permanent support concept,
this support were to be removed after equilibrium had been reached, uncontrolled
displacement and collapse of the rock mass would almost certainly occur.
Support 3, having a much lower stiffness than support 2, is also installed at F
but reaches equilibrium with the rock mass at D where the rock mass has started to
loosen. Although this may provide an acceptable temporary solution, the situation is
a dangerous one because any extra load imposed, for example by a redistribution of
stress associated with nearby mining, will have to be carried by the support elements.
In general, support 3 is too flexible for this particular application.
Support 4, of the same type and stiffness as support 2, is not installed until a radial
displacement of the rock mass of OG has occurred. In this case, the support is installed
too late, excessive convergence of the excavation will occur, and the support elements
will probably become overstressed before equilibrium is reached.
In Figures 11.1 and 11.3, constant support stiffnesses are assumed. In practice, the
stiffnesses of support and reinforcing elements are usually non-linear. Figure 11.4 il-
lustrates some of the effects that may arise. There is often initial non-linear behaviour
because of poor or incomplete contact between the rock and the support system.

Figure 11.4 Non-linear support re-
action curves observed for some sup-
port types.

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