Engineering Rock Mechanics

Mechanical excavation 255

Tangential postsplit)

Figure 15.17 Smooth-wall (or 'post-split') blasting.

principal stress will be concentrated to become the major principal stress. So,
we use this principle-by blasting twice (as with pre-split blasting but in the
reverse order)-to produce the desired high-quality excavation boundary.
First, an opening is created 'roughly' which is close to the desired size
and shape, and hence the stress field is changed all around the opening.
Second, using similar blasting principles to those required for pre-split
blasting (i.e. closely spaced and lightly charged parallel holes, decoupled
charges and simultaneous detonation), a smooth-wall blast follows. The
fracturing will be perpendicular to the minor principal stress and reinforced
by the adjacent blastholes, thus forming a 'perfect' excavation geometry.
The techniques of, and distinction between, pre-splitting and post-
splitting illustrate clearly the application of rock mechanics principles to the
enhancment of rock engineering. They show how, through the application
of engineering principles, blasting can be a sophisticated excavation tech-
nique, rather than a series of uncontrolled explosions.

15.4 Mechanical excavation

When this book was written, the Channel Tunnel, providing a land link
between Britain and Continental Europe, was under construction using
many types of tunnel boring machines (TBMs). The project is the final
realization of a long-standing dream of civil engineers: indeed, in 1882 Col.
F. Beaumont, a director of the Submarine Continental Railway Company,
drove 1.5 km of a tunnel towards France using a steam-driven full-face
tunnel boring machine. In a similar way to blasting, the development of
mechanized excavation has an interesting history.
Mechanized excavation is considered first through an outline of the
machines involved and then there is a discussion of the rock mechanics
mechanisms involved.

75.4.7 Tunne/ boring machines

There are two basic types of machine for underground excavation: partial-

and full-face machines, as illustrated in Fig. 15.18. Partial-face machines