252 Excavation principles
(b) Does the work associated with the thrust contribute greatly to
the specific energy required?
A153 (a) (i) As the cutting rate is 3 m/h, during a 1-m tunnel advance
the TBM operates for 1/3 h or 1200 s. The amount of energy used
over the 1-m advance is number of motors x volts x amps x time =
4 x 500 x 100 x 1200 W s = 2.4 x IO' W s. Since 1 J = 1 W s, we find that
the amount of energy used over the 1-m advance is then
2.4 x IO' J = 240 MJ.
The volume of rock cut by the TBM during the 1-m advance is
1 x IC x (5/2) = 19.63 m3. Thus, the specific energy = energy used/volume
removed = 240/19.63 = 12.22 MJ/m3.
(ii) The calculation of specific energy should also include the compon-
ent supplied by the TBM thrust, which we now include. At the new
cutting rate, the same calculation procedure as above gives the spe-
cific energy due to the cutting torque as 10.19 MJ/m3. The thrust of
2.7 MN provides an additional 2.7 MNm of energy when applied over
the 1-m advance. Since 1 N m = 1 J, this energy is 2.7 MJ. The extra spe-
cific energy related to the thrust is therefore 2.7/19.63 = 0.137 MJ/m3,
which gives a total specific energy when the thrust is included of
10.19 + 0.14 = 10.33 MJ/m3.
(b) From the figures in the answer above, the energy input via the
thrust is only 0.14/10.33 = 0.01, or 1%, of the total energy required.
It does not therefore contribute greatly to the specific energy required.
It is interesting, however, that the provision of significant thrust has
increased the cutting speed, which has had the effect of reducing the
overall specific energy. In practice, we find that for a given TBM and
rock condition there is an optimal balance between the torque and
thrust which minimizes both the energy requirements and the ma-
chine vibrations, helping to prolong the machine life. At thrust force
magnitudes greater or lower than the optimum, the cutters will, re-
spectively, stall or not operate efficiently as they skid over the rock
surface.
415.4 Comment on the magnitudes of the specific energy values
obtained in Q15.1, Q15.2 and Q15.3 for the different circumstances
of a laboratory compression test, blasting, and using a tunnel boring
machine?A154 Laborato y compression test: 0.20 MJ/rn3. This represents extreme
microstructure degradation, but with few energy losses in the test. Note
that a compressive strength of 200 MPa represents a strong rock.
Blasting: 1.52 MJ/m3. The object of blasting is to change the pre-existing
natural block size distribution to the required fragment distribution, but
it is difficult to blast so that the breakage is uniform: thus, the specific
energy is an average value. Also, there will be high energy losses in the
components of the stress waves and gas pressure effects not contributing
to rock breakage, although the blasting fracturing is far less dense
than in the laboratory test which requires less energy. The rock type