Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 198

Drainage drilling ahead of the tunnel should be as deep as possible so that tunnelling
works are not interrupted too often by the drilling of new holes. Holed or slotted hard
PVC pipes or perforated plastic drainage pipes with glass wool filters are often inserted
into these 35 to 100 mm diameter holes when the hydrogeological investigations suggest a
risk of retrogressive erosion. The pipes are normally extended with a flexible plastic hose
at the exposed end of the hole to drain the water into the side drainage channels or into a
temporary drainage pipe hung from the wall.
Drainage tunnel. The provision of a drainage tunnel or heading is practical when water
inflow is heavy or above-all one-sided, and it may also be useful for other purposes such
as ventilation, investigation or as a pilot heading.
As the tunnel is excavated, the drainage heading can be used to improve water drainage.
Further drainage holes 2 to 5 m long can be drilled from the heading. These holes should
penetrate as many aquiferous joints as possible and drain the rock mass.
As an example of this, the measures undertaken during the construction of the 7864 m
long Karawanken Tunnel (built 1987 to 1991) can be mentioned [140]. The dewatering
measures during the construction period were based on thorough draining of the rock
mass, which was also intended to relieve any high pressure. Systematic advance drilling
was used both for geological investigation and to drain the rock mass. During the tunnel
drive, a collapse occurred at the face of the top heading, with large quantities of water
and about 4000 m 3 of material pouring into the tunnel. In order to overcome this collapse,
drilling was carried out from a drainage heading and from the main tunnel to drain water
and to grout a zone wider than the presumed fault zone (Fig. 5-7). In addition, the top
heading area of the main tunnel was drained through further holes drilled from the bypass
tunnel.
Figure 5-7 Measures to
overcome a collapsed area
in the new Karawanken
Tunnel (1987) [140].