Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 234

Figure 5-31 Sintering of seepage slots. Figure 5-32 Sintering of a pipe invert.

Many new guidelines and regulations have been published in German-speaking countries
in recent years based on this and other investigations, and also experience from large pro-
jects where drainage was used such as the new Gotthard Base Tunnel. These are intended
to enable the design, construction and operation of low-maintenance drainage systems.
There is general agreement in Germany, Austria and Switzerland about the basic con-
struction principles of drainage system design - except for some anomalies. There are
noticeable differences of opinion regarding the maintenance and repair of systems and the
associated strategies to reduce maintenance costs.

After a brief introduction to the causes of sintering, the design, construction and mainte-
nance of drainage systems are dealt with separately and the central differences between
the two most prominent approaches - German and Austrian - are discussed.

5.4.1 The origin of sintering
The chemical and physical processes, which finally lead to deposits in drainage systems,
are complex. Sintering consists 95 % of calcium carbonate (CaCO 3 ) with small quantities
of magnesium compounds.
The carbonate is a form of the carbonic acid, which is produced by the reaction of water
with carbon dioxide (CO 2 ). Increased carbon dioxide concentration is often a characteris-
tic of groundwater.

The calcium in the compounds comes either from the rock mass encountered or from
the construction materials used such as shotcrete, and is dissolved and transported by
the groundwater on its way from the surface down into the tunnel drainage (Fig. 5-33).
Due to processes, which will be described in more detail below, the calcium dissolved in
the drainage water precipitates in the pipes as calcium carbonate and forms a crystalline
structure.
Calcium carbonate is thus a product of lime and a form of carbon dioxide and the chemical
process of sintering is thus mainly influenced by the lime-carbon dioxide balance.
As the rainwater percolates down from the surface to finally arrive in the tunnel drain-
age, numerous local conditions can affect the lime-carbon dioxide balance. Chemical and
physical conditions can be differentiated according to whether the water tends to dissolve
or precipitate lime.