Engineering Rock Mechanics

Mechanical excavation 257

(a) cutting rate, C: the rate at which the tunnel is constructed when the

(b) advance rate, A: the rate at which the tunnel is constructed.

These are related through the utilization factor, U, which is the amount
of time (expressed as a proportion) that the machine is cutting rock, as
A= UxC.
Machine manufacturers may specify the cutting rate, whereas the actual
tunnel construction rate will depend on the utilization factor, which
will itself depend on site conditions. There have been examples of such
machines operating with U = 10% in bad ground conditions and even
U = 0%, i.e. the machine has become stuck. It is unlikely that machines will
ever run at U = 100% because of scheduled maintenance, but values around
90% are possible when the total tunnelling system has been carefully
designed. Factors contributing to low utilization rates are difficulties with
ground support and steering, the need to frequently replace cutters, blocked
chutes and conveyors, and a wide variety of other problems.
Over the years, while these machines have been developed, there has
been continuing discussion on the best philosophy of design. Should a
robust machine be developed that will overcome almost any ground
conditions encountered, or should the machine be tailored to the ground
conditions at a specific site? The advantage of the former is that 'off-the-
shelf' machines and components will be available; the disadvantage is that
considerable over-expenditure may be incurred through the provision of
unnecessary quality and/or components. The advantage of bespoke
machines is that they represent optimal ground engmeering; they are,
however, susceptible to unexpected ground conditions (there have been
examples of soft ground machines designed to excavate sand encounter-
ing granite boulders, and of hard rock machines encountering clay).
The two main factors that will stop tunnel boring machines are either
that the rock is too hard to cut or that the rock is too soft to sustain the
reaction necessary to push the machine forward. Tunnel boring machines
will operate within certain ranges of rock deformability and strength: the
more the machine is tailored to the ground, the narrower these ranges (or
suites of ranges) will be. All tunnelling engineers say that the one certain
thing about tunnelling is the uncertain ground conditions.

tunnelling machine is operating; and

75.4.2 The mechanics of rock cutting

One of the primary elements of the rock excavation system is the
mechanism of rock cutting at the front of the machine. The four main types
of device that are used as cutters, and their requirements in terms of applied
forces, are shown in Table 15.1. The symbols F, and F, represent the forces
acting normal and tangential to the rock face. In tunnelling terms, the F,
is related to the thrust and F, is related to the torque applied by the TBM.
The figures above are only intended as a guide, and indeed, on most
machines, there will be a combination of two or more cutter types. For
example, on a machine with discs, there may be button cutters on the rim
of the cutting head to ensure a smooth sidewall; also, water jets can be used
to assist all three of the directly mechanical devices. There are many exotic