Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 200

Table 5-11 Type of obstruction to the advance due to groundwater [233].

Activity in the
round cycle

Type of obstruction

Drilling Difficulties for drilling works are caused by water spurting from the drilled holes,

by water emerging from areas of the face and by erosion occurring at the face. The
moving of holes can lead to their collapse. Localised washing-out of material from
joints can also occur. Delays in withdrawing the drill stem and the general worsen-
ing of working conditions can lead to a further reduction of drilling performance.

Loading charges The time taken to load the charges can be increased due to moved holes and the

worsened working conditions.

Mucking The softening of the invert and any extra excavation can increase the time taken to

clear the muck.

Support works The duration of shotcrete spraying increases due to the increased rebound. The

anchors require longer to install due to the reduced drilling performance and
increased anchor manipulation time.
During the performance of support works, extra dewatering work such as fitting
hoses, waterproofing areas and digging sumps have to be undertaken. Support
works are also slowed by the worsened working conditions.

The extent of obstruction due to groundwater on the individual activities of the round
cycle is shown graphically in the following illustrations, using the rock mass type defini-
tions from the Austrian standard ÖNorm B 2203 [170] as percentages of the normal dura-
tion of each activity. There is a basic difference between two rock mass types, B1 and C2.
Rock mass type B1 describes an unstable rock mass with rapidly decreasing deformation
and slight loosening of the crown and the upper bench area. Rock mass type C2 describes
a squeezing rock mass with pronounced deformation, fracture events and plastic zones,
in which systematic installation of support is required in the crown and bench (see Sec-
tion 2.4 “The ground and its classification”).

Figure 5-8 Extent of
obstruction by groundwa-
ter (rock mass type B1 ac-
cording to ÖNorm B 2203
[170]).

In the worse rock mass type (C2), the percentage increase of the duration of support works
is less than in the better rock mass type (B1), since the support work in the worse rock
mass is already extensive without groundwater. The support works require about 50 % of
the round cycle time in rock mass type C2.
Fig 5.10 shows the result of investigations into dependence on the quantity of groundwater
[120].