Page 331 - Rock Mechanics For Underground Mining

SUPPORT AND REINFORCEMENT PRINCIPLES

Support or reinforcement may also be classified as being either active or pas-
sive. Active support imposes a predetermined load to the rock surface at the time of
installation. It can take the form of tensioned rock bolts or cables, hydraulic props, ex-
pandable segmented concrete linings or powered supports for longwall faces. Active
support is usually required when it is necessary to support the gravity loads imposed
by individual rock blocks or by a loosened zone of rock. Passive support or rein-
forcement is not installed with an applied loading, but rather, develops its loads as the
rock mass deforms. Passive support may be provided by steel arches, timbered sets
or composite packs, or by untensioned grouted rock bolts, reinforcing bars or cables.
Untensioned, grouted rock bolts, reinforcing bars and cables are often described as
dowels.
The term strata control is used to describe the support and reinforcement tech-
niques used in coal mining. The term is a good one because it evokes a concept of the
control or limitation of strata displacements rather than one of support. Nevertheless,
support in the strict sense is a major function of some strata control measures, most
notably of hydraulic props used immediately behind the face in longwall mining.
Because this book is concerned with all types of modern underground mining, the
terms support and reinforcement will be used in preference to strata control. In the
present chapter, emphasis will be placed on the principles and major techniques used
in good support and reinforcement practice for mining excavations having an extended
life and for large underground excavations generally. Techniques used in particular
types of mining, including the use of fill and longwall strata control measures, will
be discussed in subsequent chapters.

11.2 Support and reinforcement principles

Consider the example illustrated in Figure 11.1 in which a heading is being advanced
by conventional drill and blast methods. The pre-mining state of stress is assumed to
be hydrostatic and of magnitude p 0 . Blocked steel sets are installed after each drill and
blastcycle.Thefollowingdiscussionconcernsthedevelopmentofradialdisplacement
and radial support ‘pressure’ at a point on the excavation periphery at section X–X
as the heading progressively advances to and beyond X–X. In this discussion, the
term support will be used throughout although the process involved may be one of
support and reinforcement or reinforcement alone. Following customary usage, the
equivalent normal stress applied to the excavation periphery by the support system,
will be termed the support pressure.
In step 1, the heading has not yet reached X–X and the rock mass on the periphery
of the proposed profile is in equilibrium with an internal support pressure, p i , acting
equal and opposite to p 0 .
In step 2, the face has been advanced beyond X–X and the support pressure, p i ,
previously provided by the rock inside the excavation periphery, has been reduced
to zero. However, the apparently unsupported section of the heading between the
face and the last steel set installed, is constrained to some extent by the proximity
of the face. Figure 11.2 shows the development with distance from the face of radial
displacement at the periphery of a circular tunnel in an elastic material subject to
a hydrostatic in situ stress field. In this case, the zone of influence of the face may

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