Engineering Rock Mechanics

Questions and answers: excavation principles 2 53

is a marble, and so it is probable that this will be a medium strength
rock.
Tunnel boring machine: 10.33 MJ/m3. More fracturing is required to
chip and remove the rock with a TBM than with blasting, and there
is potential for large energy losses during rotation of the head through
friction and vibrations. Limestone is usually a medium strength rock.
Overall comments. In the uniaxial compression test there are few energy
losses. So, although a high microcrack density has been generated at the
completion of the test, the specific energy is lowest because of the high
energy transfer efficiency. In blasting and cutting by TBM, there are large
energy losses: in both cases, the majority of the energy does not go into
breaking the rock, i.e. in creating new fracture surfaces, but is lost in
heat, sound, stress waves, kinetic energy, etc. This is why these specific
energies are higher than for the laboratory test. The blasting is more
efficient in terms of specific energy than the TBM because, inter alia, less
fracturing of the rock takes place.
We would not have expected the specific energy for the three cases to
be the same because different rock types and different systems of fracture
development are involved. Thus, the term 'specific energy' should be
suitably qualified to indicate the configuration and scale under which
it is determined (which is similar to the tensile strength discussed
in A6.9).

415.5 The objective of 'pre-splitting' when blasting is to create a
continuous new fracture plane through the rock mass (which will
become the final rock surface) before bulk blasting removes the
rock up to the pre-split plane.

This rock removed up

to the pre-split plane

(a) List the most important factors that need to be controlled to
ensure that the pre-split blasting techniques will indeed create a
continuous pre-split fracture.
(b) The three photographs below show different rock slopes that
have been pre-split. The pre-split plane has been created first;
then bulk blasting has removed the rock up to the pre-split plane,
as indicated in the sketch above. Thus, the photographs show one
side of the pre-split 'plane' which is the final slope. Each of the
photographs illustrates one of the following:
an acceptable pre-split rock face;
a pre-split face where the fractures have adversely affected the

a pre-split rock face where one or more of the necessary factors

Which photograph is which case?

pre-splitting; and

intimated in (a) above has not been properly controlled.