Analysis, design and construction - Practical Engineering Geology - page 253

support requirements and allow a contractor to choose his method of
working and type of machine if a tunnel boring machine (TBM) option
is selected (Barton, 2003). The ground model will also be used for
hazard and risk analysis, as discussed later, and may sometimes be
used as the basis for Reference Ground Conditions in Geotechnical
Baseline Reports (Chapters 2 & 4) , against which any claims for
unexpected or differing ground conditions can be judged. As noted
in Chapter 2, however, RMCs may be too coarse to represent
geological conditions realistically. They may also be open to different
interpretations, so that disputes are dif

cult to resolve.

6.5.2 Options for construction
Up to about a century ago, all tunnels in soil or rock were excavated by
hand, using explosives where necessary to break up the rock in advance
of mucking out. Nowadays, many are excavated using powerful
machines. The main options generally adopted in modern tunnelling
and typical support measures are set out in Table 6.2. The method of
tunnelling will often be decided on factors including length of tunnel,
availability of TBM, local experience and expertise. In South Korea, for
example, most rock tunnels, including very long ones, have been con-
structed in preference by drill and blast rather than TBM. There is a
wide variety of tunnel boring machines designed for all kinds of condi-
tions from rock to soft soil. The engineering geologist needs to be able to
predict the ground conditions so that the tunnel designers and tendering
contractors can select the correct machine. It usually takes a long time to
manufacture and launch a TBM with a whole series of ancillary equip-
ment in the following train, and if the machine proves unsuitable, for
any reason, it can be a costly mistake. Some machines are designed to be
able to cope with mixed ground conditions but can still run into
dif culties. Nevertheless, many TBM tunnels proceed well and at

much faster rates than hand dug/drill and blast tunnels. The adoption
of hazard and risk analysis (BTS, 2003), as discussed at 6.5.8, will help
reduce incidents but will not necessarily eliminate hazards entirely.

Table 6.2 Options for tunnelling (after Muir Wood, 2000).

Ground type Excavation Support

Strong rock Drill and blast or TBM Nil or rockbolts

Weak rock TBM or roadheader Rockbolts, shotcrete, etc.

Squeezing rock Roadheader Variety depending on conditions

Overconsolidated clay Open-face shielded TBM or roadheader Segmental lining or shotcrete etc.

Weak clay, silty clay EPB closed-face machine Segmental lining

Sands, gravel Closed-face slurry machine Segmental lining