Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 74

Special measures. The appearance of karst features has to be expected at any time as the
excavation advances. This will normally mean karst fissures with an average width of
about 10 cm, although voids up to many metres wide have to be expected. Voids, which
intersect the tunnel alignment, have to be permanently filled with suitable material (for
example grout).
In zones of rock subject to heavy weathering, it may also be necessary to grout with ce-
ment suspension to consolidate the structure of the rock mass.
When a tunnel is driven into a karst void, large quantities of water have to be expected.
This applies particularly to areas where the tunnel section is at the level of the karst water
table or the formation water table.
In this case it is necessary to dewater the ground around the tunnel by drilling drainage
holes or installing hoses. These works can be carried out in advance of the tunnel, but also
subsequently (perhaps shortly before the installation of the waterproofing).

Soil filling flowing out of karst voids also has to be expected. For this reason, it can be
necessary to carry out additional support works at any time, such as the installation of lag-
ging sheets, spiles or a grouted canopy.
The localised use of the special measures described here does not in itself justify a change
of class. The extra costs should be included in the estimation of the prices for special
measures, including operating costs.

2.5 Special features for tunnelling machines

2.5.1 General
When tunnelling machines are used, additional information about the ground conditions is
required in order to plan the construction process using the machine, determine costs and
perform a risk analysis. This includes the boring process with wear and sticking problems,
machine stroke and support.
For example knowledge of the stratification, particularly transitions from rock to loose
ground and pronounced faults, is more important for TBM drives than with drill and blast
or the use of roadheaders. The following information is taken from the topic “Hardrock
Tunnel Boring Machines” [148].

2.5.2 Influences on the boring process
In Fig. 2-19 shows the factors influencing the penetration of the boring process.
The factors that have a great effect on the boring process are the strength properties of
the rock, the abrasiveness and the size and bonding of jointed bodies or blocks. The latter
factor determines whether a TBM with disc cutters mounted in detachable brackets, as
is common in the smaller diameters, can be used at all. Blocks breaking out can do great
damage to the cutterhead of a TBM. Modifications to the cutterhead during a TBM drive
are expensive and disc changing is time-consuming. Such cutterhead modifications had
to be carried out, for example, to the Jarva Mk 15 gripper TBM for the pressure tunnel of