Questions 75.1-15.10: excavation principles 463.............................
..............................(8) resistance of rock to breaking (fracture toughness, Klc, MPa-ml/*);
(9) in situ block sizes (mean of block size distribution, mean);
(10) fragility of rock mass (fractal dimension of rock block sizes, D);
(11) integrity of rock mass (ratio of field: lab P-wave velocities, R,,(12) fracture plane’s strength (cohesion, c, MPa and friction angle, 4,Compile a table of these twelve parameters in three columns with
headings Intact Rock Properties, Fracture Properties and Rock Mass
Properties, and then comment on how well the rock mass has been
characterized.
dimensionless);degrees).Q 15.8 In relation to rock tunnel boring machines (TBMs), two progress
rates can be defined:
Cutting rate, C: the rate at which a tunnel is constructed when the tunnel-
ling machine is operating; and
Advance rate, A: the rate at which a tunnel is constructed.
These two rates are not the same because the TBM is not continuously
cutting; there is a Utilization factor, U, which is the amount of time
(expressed as a proportion) that the machine is cutting, i.e. A = U x C.
Over a short period, say 20 min, A can be equal to C. Over longer
periods, of days, weeks or months, there can be a greater and greater
discrepancy between C and A, as indicated in the diagram below
(Barton, 1999).
Cutting rate, C m/hr
(log scale)
Advance rate, A
m/hr (log scale)Q15.9 When an underground excavation is made in a rock mass, there
are three primary effects, all three of which result from Newton’s 3rd law,
i.e. for every action, there is an equal and opposite reaction. Thus, when
new underground space is created, the resistance of the rock previously
occupying the space becomes zero, forces cannot be transmitted from the
rock into the space, and the water pressure is reduced to atmospheric
pressure.