Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 242

5.4.4 Operation and maintenance of drainage systems to reduce sintering
Not only the actual flushing costs but also the development of the expenditure or the ten-
dency to sintering have to be considered in the analysis of costs resulting from sintering.
Ideally a comparison of water parameters over a period of years should be performed. Water
analysis results from the construction period can offer a basis for comparison. In order to
estimate the development of expenditure or the tendency to sintering, the personnel who
carry out the flushing should also be questioned. The data gained can then be used to perform
a cost-benefit estimate and make decisions whether constructional or operational improve-
ment measures are economically justified. The specific quantity of data required for such
an exercise is based on the parameters already listed under 5.4.3.1. The necessary rhythm
of data collection, and the decision when to perform general inspections of the tunnel, de-
pend on the extent of the sintering problem. [Ri-BWD-TU] and [RiLi Tunneldrainage] give
detailed information about the exact determination of the specific inspection requirement.
5.4.4.1 Concepts to reduce maintenance through improvements to systems
Germany. The results of operational inspections are used to analyse the sintering behav-
iour of the drainage system and investigate the possible bypassing of the water and the
resulting savings potential for maintenance work.
Bypassing keeps less exposed runs of the side drainage free of deposits caused by ground-
water percolating from above. In addition, the flow of less concentrated water from the
side drains into the invert drain should result in a reduction of the calcite saturation.
In order to implement the bypass concept, points must first be identified, at which bypass-
ing of water with a high tendency to precipitate calcite from the sides into the invert is
possible. The results of this investigation are then used to design suitable constructional
measures to modify the drainage system, the intention being a considerable reduction of
the maintenance work due to sintering at relatively little cost. The basic principle is the
bypass construction already described in Section 5.4.2.
Austria and Switzerland .
Maintenance works can have a preventive or correctional nature according to the Austrian
standard ÖNORM EN 13306. [ÖNORM EN 13306]:
The following preventative measures are recommended by [RiLi Tunneldrainage]:
- hardness stabilisation of the groundwater in the so-called secondary drainage (all non-
accessible areas such as filter packing or surface drainage),
- low-pressure flushing (for example with the tunnel water supply as part of functional tests),
- periodic high-pressure flushing at low cost.
Corrective maintenance measures should be carried out after the formation of hardened
deposits. The measures recommended by [RiLi Tunneldrainage] include:
- high-pressure flushing with large quantities of water and additional equipment,
- the targeted use of hardness stabilisers to avoid new formation of hard deposits and
possibly to loosen existing accretion.