Understanding of tunnel drainage scale formation by in-situ monitoring

Drainage systems in (geo)technical environments are facing a common problem. So called scale deposits precipitate from the hugely diverse drained waters in form of carbonates, sulfates, silicates and – often associated with microbial activities – also Fe-(hydr)oxides. This is causing serious problems, like clogging, which eventually leads to the loss of structural integrity of e.g. tunnel linings, due to the increase of the hydro-static pressure.

To get a better understanding of the scaling processes, a drainage test-track was setup in an active construction site of an Austrian railway tunnel using a locally discharging highly mineralized groundwater to decode scaling related processes by monitoring the pH, the electric conductivity (EC), the redox potential (pe) and the water-temperature (T) at high temporal resolution. The discharge (Q) was measured point-wise and analyses of water and solid samples by ICP-OES, IC, XRD and SEM complemented the study.

The results indicate a coupled binary system, mainly controlled by the level of the discharge (Q), which can be monitored by T adapting to the atmosphere. With sufficient discharge (and therefore sufficient supply of aqueous Fe2+) microbially induced Fe-(hydr)oxide formation embedded in an organic matrix is promoted, which is strictly pe sensitive. At prevailing stagnant water-flow conditions, inorganically driven CaCO3 precipitation due to enhanced CO2 degassing is favored, which can be followed and assessed by an EC decrease and rising pH.

The high-resolution monitoring of drainage water by pH, EC, pe and T provides a promising tool kit for in-situ decoding and assessing of scaling related processes, which allows for proper drainage system design and well-targeted maintenance actions.