Page 459 - Rock Mechanics For Underground Mining

LONGWALL COAL MINING

coal mining will be discussed in section 16.5. In many coal mining districts, mining
legislation as well as operational experience requires that multiple roadways are used
for each panel as illustrated in Figure 15.7 which is based on Australian practice.
These roadways provide access to the face for personnel, materials and ventilation
air and transportation routes for the mined coal (maingate) and a second means of
egress and return air (tailgate). The maingate for Longwall 1 becomes the tailgate
for Longwall 2. It will be apparent that the chain pillars associated with the entries
will be subjected to changing loads throughout their operational lives. The long-
wall retreat method typically employed in the UK uses a single gate at each side of
the panel with adjacent panels separated by wide protection pillars (Cassie et al.,
1999).
Because of the low strengths of the coal measures rocks, it is found that fractured or
yielded zones develop around openings made at depths greater than 100–200 m. This
influences the distribution of stress and displacement around the mining excavations,
and imposes stringent requirements on support or strata control measures. These
measures fall into four distinct categories, each with different objectives:
(a) Face supports are required to prevent the fall of ground, including detached
blocks,nearthefaceandsomaintainsafeworkingconditionsandofferprotection
to the face equipment. The face support system may also aid in controlling the
development of caving behind the face. The mechanics of caving in longwall
coal mining will be discussed in section 15.3.3.
(b) Roadways which provide access to the face must be formed, reinforced and sup-
ported in such a way that displacements are kept within operationally acceptable
limits throughout their operational lives which may be in the order of several
months or longer. The formation and support of roadways will be discussed in
section 15.3.5.
(c) Chain or rib pillars between panels may serve a number of purposes as outlined
aboveandsomustbedesignedtofitthesepurposes,sometimesasyieldingpillars.
The design of chain or rib pillars will be discussed in section 15.3.6.
(d) Support of the mined out void by packs or the stowing of solid material may
be introduced to limit the total displacement of the superincumbent strata and
the associated surface displacement. This topic has been reviewed by Hughson
et al. (1987) and will be referred to in context in section 16.5.6.

15.3.2 Distribution of stress around a longwall face
Figure 15.8 shows an interpretation of the distribution of vertical stress, zz , around
a single longwall face, developed for UK conditions by Whittaker (1974) and widely
accepted for many years. The vertical stress is zero at the face and the rib side. The
stress increases rapidly with distance into the yield zone in the unmined coal, reaching
a peak stress at a distance into the coal which varies with rock properties and the in
situ stresses. For UK conditions, it was postulated that the peak vertical stress was in
the order of four or five times the overburden stress, p = h, where h is the mining
depth and is the weighted average unit weight of the superincumbent strata. With
increasing distance into the unmined coal, the vertical stress reduces towards the
overburden stress.
Whittaker (1974) suggested that in the mined-out area, the effects of arching and
of face and roadway support are such that the vertical stresses are negligibly small
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