Page 24 - Rock Mechanics For Underground Mining

ROCK MECHANICS AND MINING ENGINEERING

The implications of the effect of groundwater on rock mass strength are consid-
erable for mining practice. Since rock behaviour may be determined by its geohy-
drological environment, it may be essential in some cases to maintain close control
of groundwater conditions in the mine area. Further, since backfill is an important
element in many mining operations, the lithologies considered for stope filling op-
erations must be considered carefully from the point of view of strength properties
under variable groundwater conditions.

1.2.5 Weathering
Weathering may be defined as the chemical or physical alteration of rock at its surface
by its reaction with atmospheric gas and aqueous solutions. The process is analogous
to corrosion effects on conventional materials. The engineering interest in weathering
arises because of its influence on the mechanical properties of the intact material,
as well as the potential for significant effect on the coefficient of friction of the
rock surface. It appears that whereas weathering causes a steady reduction in rock
properties, the coefficient of friction of a surface may suffer a step reduction (Boyd,
1975).
Although physical processes such as thermal cycling and insolation may be im-
portant in surface mining, underground weathering processes are chiefly chemical in
origin. These include dissolution and ion exchange phenomena, oxidation and hy-
dration. Some weathering actions are readily appreciated, such as the dissolution of
limestone in an altered groundwater environment, or softening of marl due to sulphate
removal. In others, such as the oxidation of pyrrhotite, the susceptibility of some forms
of the mineral to rapid chemical attack is not fully understood. A weathering problem
of particular concern is presented by basic rocks containing minerals such as olivine
and pyroxenes. A hydrolysis product is montmorillonite, which is a swelling clay
with especially intractable mechanical behaviour.
This discussion does not identify all of the unique issues to be considered in rock
mechanics. However, it is clear that the subject transcends the domain of traditional
applied mechanics, and must include a number of topics that are not of concern in
any other engineering discipline.

1.3 Underground mining

Ore extraction by an underground mining method involves the generation of different
types of openings, with a considerable range of functions. The schematic cross section
and longitudinal section through an operating mine, shown in Figure 1.3, illustrate
the different rˆoles of various excavations. The main shaft, level drives and cross cuts,
ore haulages, ventilation shafts and airways constitute the mine access and service
openings. Their duty life is comparable with, or exceeds, the mining life of the orebody
and they are usually developed in barren ground. Service and operating openings
directly associated with ore recovery consist of the access cross cuts, drill headings,
access raises, extraction headings and ore passes, from, or in which, various ore
production operations are undertaken. These openings are developed in the orebody,
or in country rock close to the orebody boundary, and their duty life is limited to the
duration of mining activity in their immediate vicinity. Many openings are eliminated
by the mining operation. The third type of excavation is the ore source. It may be a
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