Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 235

Precipitation processes. The water in the drainage contains water containing either dis-
solved calcium hydroxide Ca(OH) 2 or dissolved calcium hydrogen carbonate CaH 2 (CO 3 ) 2 .
As the water enters the drainage, the conditions change and additional water with different
properties is sometimes mixed in. This causes the precipitation along the route from the
gully to the discharge point.

Figure 5-33 Route of
groundwater into the tunnel
drainage.

Drainage water containing calcium hydroxide. Two possible processes can lead to pre-
cipitation from soft drainage water with increased calcium hydroxide content.

The first possibility is that the drainage water comes into contact with air in the pipe and
absorbs carbon dioxide. The calcium hydroxide and the carbon dioxide react to form cal-
cium carbonate and water.
Ca(OH) 2 + CO 2 → CaCO 3 + H 2 O
When carbonated water mixes with water containing calcium hydroxide in the drainpipe,
the calcium hydroxide reacts with the carbonic acid to form calcium carbonate and water.
Ca(OH) 2 + H 2 CO 3 → CaCO 3 + 2H 2 O
Drainage water containing calcium hydrogen carbonate. Calcium hydrogen carbonate
requires stabilising carbon dioxide in the environment in order to be stable. The solubil-
ity of CO 2 in water and thus the carbonic acid content is thus dependent on the physical
conditions of pressure and temperature.
The dependence of the water solubility of carbon dioxide on temperature and pressure is
illustrated in Fig. 5-34. The effect is that an over-proportionate amount of carbon dioxide
remains dissolved in the water at low temperatures. The relationship of pressure to CO 2
content, on the other hand, is proportional at constant temperature.

With the entry of the water into the drainage system, its temperature normally increases
and the pressure reduces. The consequence is that CO 2 gases off, and the calcium hydro-
gen carbonate breaks down into calcium carbonate, carbon dioxide and water.
Ca(HCO 3 ) 2 → CaCO 3 + CO 2 + H 2 O