Excavation principles
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15.1 Rock excavationThe objectives in excavating rock for civil, mining and petroleum pro-
jects are different, as indicated by the examples in Fig. 15.1.
In civil engineering, the objective is to support loads and to create
surface or underground space for a specific engineering purpose, e.g.
a foundation, a cutting for a road, a shaft for a hydroelectric scheme,
a railway tunnel, a repository for disposing of radioactive waste. In
mining engineering, the objective is to obtain the rock itself, either
in bulk or for the minerals it contains. For example, in the case of a
surface quarry or underground mine for limestone, most of the mined
rock can be used, whereas in the case of an open-pit mine or under-
ground mine for, say, copper or gold, the mined rock has to be crushed
to release the mineral grains, a process known as comminution. In
petroleum engineering, the objective is to transport the oil from the
petroleum reservoir to the ground surface through boreholes, and there
is a variety of associated rock mechanics problems such as enhancing
flow from the reservoir into the boreholes and storing oil and gas in rock
caverns.
Thus, for civil engineering, the rock only needs to be broken up
sufficiently for the underground space to be created and for the rock
to be removed. For mining engineering, the rock itself is required
and the process of excavation is the first stage of comminution. For
petroleum engineering, the main issues are drilling and maintaining
open boreholes, and maximizing oil flow into the wellbores.
This leads to different design approaches for civil and mining engin-
eering, and special problems for petroleum engineering in weak rocks
and at significant depths. The geometry of a civil engineering excavation
is usually clearly defined and subject to definite constraints. There has
to be a clear design with a project lifetime of, say, 120 years. If possible,
safety factors are established. All this significantly constrains the rock
engineering design options. In mining, whilst the overall geometry of
the mining will be determined by the orebody and mining leases, the