Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 203

5.2.4.2 Effects on groundwater quality
Drainage channels not only carry the groundwater but also the service water pumped from
the surface. This water is heavily contaminated by construction operations. The waste-
water contains mineral solids from grated or pulverised rock and its chemistry has been
altered. Increased pH values have to be expected with the production of shotcrete or in-situ
concrete. This is due to leached contents of cement or accelerator.
Bulletin A 115 from the wastewater technical association [171] prescribes the following guide-
line values for the discharging of water into public drainage systems (drains, treatment plants):
- Waste water temperature < 35 °C.
- pH value between 6.5 and 8.5.
- Minimisation of salting by the neutralisation reaction.
Preventive measures. Operational methods should be modified to attempt to avoid con-
tamination of tunnel water.
If it is not possible to avoid the softening of the unsupported invert by wheeled vehicles next
to open drainage channels at the sides, then it is advisable to pave the invert with ballast. The
ballast must either be delivered or a suitable excavated material can be used instead.
The potential to contaminate water should be considered in the storage and processing of
all construction materials (cement, accelerator, grout etc.). For example, it is advisable to
clear rebounded shotcrete before drilling the following round in order to avoid soluble
substances being washed out.
Reduction of the pH value. Hardened cement paste also contains contents, which can
be dissolved out of the cement matrix by any natural water. This applies particularly to
calcium hydroxide produced by the hydration of cement and potassium and sodium, which
are also mainly present as hydroxides and derive from the use of accelerators.
Dissolved calcium hydroxide is precipitated on contact with carbon dioxide as calcium
carbonate and this can cause sintering of the tunnel drainage. The much more soluble so-
dium and potassium hydroxides are responsible for the increased pH value of drain water.
The following measures are possible to counter these mechanisms [112], [149]:
- Reduction of water flow at the concrete surface (collection of the water within the rock
mass).
- Increasing the density of the internal structure of the concrete (optimised production,
use of Flugacal or microsilica).
- Reduction of contents susceptible to leaching (use of lime-free aggregates, shotcrete
without accelerator, alkali-free accelerators) (see also Section 5.4.2).
After-treatment to neutralise alkaline water can be achieved by adding CO 2, such as for
example gasification in a basin 1.5 to 2 m deep (Solvocarb-B process). The use of carbon
dioxide has many environmental advantages over the use of mineral acids (HCL, H 2 SO 4 ,
HNO 3 , H 3 PO 4 ). It does not increase the salt load in the water, no storage of acids is re-
quired and the use of CO 2 prevents the danger of over-acidification.
Waste water treatment. A certain contamination of the tunnel water cannot to be avoid-
ed. Depending on the discharge conditions into the relevant waterway, treatment measures
may be necessary. The facilities for this can also be installed underground (Table 5-12).