Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 239

ing potential and the resulting maintenance costs. Hardness stabilisers are chemical agents,
which can be added to the drainage water either solid in the form of depot stones which the
groundwater flows through, or liquid in large dosage systems. The released or added agents
are intended to condition the water to reduce the tendency to sintering and thus reduce main-
tenance costs. The following examples can be given of the use of depot stones:
- Placing of hardness stabilisation stones with a dissolving lifetime of two years, prefer-
ably in the percolation packing or at the pipe inlets, in order to moderate the typical
initial sintering potential.
- Laying of solid hardness stabilisation agents in manholes and cleaning shafts.
- Installation of hardness stabilisation plants with liquid hardness stabilisation agents at
critical locations in the drainage with constantly high water flow and uniform chemistry
(pH value).
- This type of measures can also be provided at the design stage of the drainage system if
aggressive groundwater is found to be present and integrated into the tender documents.
5.4.3 Construction of tunnel drainage to reduce sintering
In order to be able to ensure functional drainage in the long term, recording of data about
the drainage system and the conditions that affect it must already start in the construction
phase. This can include hydrogeological data recording, investigations of and in the drain-
age and camera surveying to ensure that a defect-free system is supplemented by quality
assurance measures.
Hydrogeological data recording during the construction phase. In order to provide a basis
for all further investigations and measures, the hydrogeological investigations from the
design phase should be updated with complete documentation of water ingress occurring
during the construction phase. The following parameters should be measured and docu-
mented as part of the geological engineering documentation:
- precise location of ingress of groundwater,
- quantities of groundwater entering the tunnel,
- concrete aggressiveness of the groundwater according to DIN 4030 [56],
- temperature, pH value and electrical conductivity of the groundwater.
The exact location and the incoming quantity should be documented for each ingress loca-
tion; the concrete aggressiveness and all other values should be measured every 50 m. The
calcium content and the total hardness only need to be determined in cases of increased
sintering potential. The documentation should be updated again before the installation of
the waterproofing and drainage system. When taking samples, care should be taken that
no contamination from outside materials leads to false results. In order to determine the
concrete aggressiveness according to DIN 4030 [56], pre-prepared reagent sets, called
compact laboratories, can be used. Temperature, pH value and electrical conductivity can
be determined immediately with measuring devices at the sampling location. When us-
ing the equipment for measuring the pH value and the electrical conductivity, it should
be ensured that these can compensate the temperature. The calcium content and the total
hardness can be determined, for example, with prepared titration testing sets.
The sintering behaviour of drainage systems should be documented during the construc-
tion period with water analyses, inspections of the manholes and camera surveys. This can