Life-cycle oriented optimization of maintenance costs for drainage systems in traffic tunnels - by (further) developing a new calcification reduction method

Abstract

For quite some time, many operators of drained tunnels have been faced with the problem of cost-intensive maintenance measures to counteract the phenomenon of scaling in tunnel drainage systems. In view of the severe worsening of this problem in the numerous tunnels on its high-speed railway link between Hanover and Wurzburg, Deutsche Bahn AG, the German railway company, decided to conduct a research project together with the Institute for Construction Engineering and Management of the Swiss Federal Institute of Technology Zurich. On the one hand, the project aims to investigate the cause-and-effect relationships that explain the scaling phenomena in those tunnels that are influenced by descending ground water, since the tunnels on the high-speed routes only have a low depth of coverage on average. On the other hand, the research project was to be used to test and improve the application of a new preventive method for stabilizing water hardness. For a long time, the causes of scaling in tunnel drainage systems were unknown. Although there was clearly a connection with more recent tunneling methods and the materials used, the actual mechanisms remained unexplored. The findings of the first investigation phase of the research project showed that the enrichment of the descending seepage and ground water with carbon dioxide was a principal natural cause of the formation of scale, especially in low covered tunnels, and that, in conjunction with the materials used, this resulted in the - in part - considerable occurrences of scaling. And although such scaling as a result of natural causes cannot be prevented entirely, an intelligent choice of building materials and method of constructing the tunnel are possible and necessary to alleviate the effects. Thus measures must be taken in tunnel drainage systems to counter the adjustment to deeper chemical equilibriums and, in doing so, to reduce the formation of stable salt deposits. The water, enriched with lime and carbonic acid above atmospheric conditions must be drained out of the tunnel relatively uninfluenced and, as such, without any significant traces of scaling. This can only be achieved with an adequate design and construction of the entire drainage system. For example, the water falls - which are still to this day a common feature - where the pipes flow into the manholes need to be abolished since the drainage water is aerated and carbon dioxide degraded at this point, leading to reduced lime solubility and the formation of - in part - massive lime deposits. A further element of maintenance, which helps to reduce the formation of deposits on the one hand, whilst at the same time ensuring that the inevitable deposits are much softer than before, is the stabilization of water hardness. This is a process originally derived from nature whereby molecules that are rich in aspartic acid control the formation of inorganic skeletal material, for example in crustaceans. The active agent of hardness stabilization, polyaspartic acid, is a nature-related substance, which is therefore fully decomposed by micro-organisms and regarded as environmentally unproblematic. By adsorbing the hardness-forming ions in the drainage system, polyaspartic acid prevents, disrupts or delays the formation of hard scale. As an overall result, the right use of hardness stabilization in connection with constructional modifications can result in longer cleaning intervals and a substantial reduction in the scope of cleaning works in the pipes, which in turn generally leads to a considerable extension in the life cycle of the tunnel drainage system and lower direct maintenance costs together with increased availability of the traffic routes. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.