Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 227

Soft PVC waterproofing membranes have become predominant in recent years. In order
to ensure adequate resistance to damage during installation and in the completed state, the
thickness should be 3 . 0 mm. Thicknesses of more than 3 mm are not recommended due to
the increasing stiffness of the material. The roll or sheet material is fixed at points, which
are either the same as those already used for the fixing of the fleece as shown in Fig. 5-21
or newly shot for the purpose, but always at the deep points of uneven tunnel vaults. The
number of fixing points per square metre should be at least 0 to 0.5 in the invert, 1 at the
sides and 2 to 3 in the crown.
The individual strips or pre-fabricated sheets should be overlapped at their edges by at
least 5 cm. The seams are welded as double seams with two 15 mm wide single seams
suitable for testing and are normally welded by an automatic welder. Groundwater emerg-
ing from the rock mass and building up behind the waterproofing must be prevented from
sagging the membrane during the construction phase. The danger of breaking the skin of
the membrane is at the greatest before the inner lining can support it (Fig. 5-22).
Figure 5-22 Plastic
waterproofing membrane.
Concreting imposes the first significant loading on the waterproofing. When the space
behind the formwork is filled, the weight of the pumped concrete acts fully on the water-
proofing and causes tension loading, which can separate the membrane from the fixing
elements. This separation means a localised defect in the waterproofing membrane, which
cannot be allowed to lead to tearing of the material.
The waterproofing should preferably be laid from travelling scaffolding, often equipped with
mechanical laying aids. This enables the installation of prefabricated sheets with a width of
up to 6 m. The length of the sheet corresponds to the developed length of the tunnel vault,
or at least half. Such large sheets cannot be moved by hand due to their weight, above all
overhead. Nonetheless, they are to be preferred to strips of 1 to about 2 m width since this re-
duces the number of seams, which have to be welded on site, often under difficult conditions.
Laying machines are mounted on a tracked scaffolding, which is adapted to the tunnel pro-
file (Fig. 5-23) . Considering the uneven tunnel sides, it is important that the already laid
membrane is not damaged by forwards or sideways travel of the laying machine. The pro-
vision of rubber-wheeled spacing rollers mounted on springs is strongly recommended.