UTILITIES | WATER & SEWER TUNNELS
REVITALISING WATER AND SEWER TUNNELS
Many and varied are the vital tasks to refurbish, maintain and upgrade both existing sewers and water tunnels, such as the networks across the London area, and beyond. Barhale’s Tunnelling Director, Ovi Frunza, discusses a selection of the interesting challenges
INTRODUCTION Maintaining water infrastructure is analogous to painting the Forth Rail Bridge. The moment you think you’ve finished, you have to start again. The reality is that much of the UK’s water infrastructure is quite old; is trying to cope with a population far beyond that it was originally designed for; and, is under assault from both the natural and built environment. And that’s before we start to deal with the wee beasties that have decided there’s nowhere better to live. Broadly speaking, when considering the maintenance
and rehabilitation of sewers and water tunnels the issues fall into two categories: first, there are matters relating to the condition of the fabric of the asset; and second, there is the question of whether obstructions are compromising how efficiently the asset is performing.
In this article, we will illustrate the different
challenges we face across the UK undertaking maintenance, refurbishment and upgrade programmes to keep sewers and water tunnels flowing freely.
WHAT CAUSES DETERIORATION IN THE FABRIC OR THE PERFORMANCE OF SEWERS AND WATER TUNNELS? Sewers rarely deal with only sewage. Across the country, everyday activities – washing dishes or clothes, bleaching toilets – add a potent cocktail of chemicals to the already generous quantities of waste and effluent to create two principal issues. First, hydrogen sulphide gas mixes with the water and
creates sulfuric acid. In slow flows, the acid is able to lay on the structure and accelerate the deterioration of the concrete fabric and steel reinforcement causing leaks and failures. Second, fats are poured down sinks during cooking
and washing up. These fats solidify into particles as they cool and the particles collect together. Over time, they gather together in such great numbers that the sewer flows are slowed allowing even more to gather until the sewer is blocked. For raw water tunnels, it is a different story. Here
the threat is not from fats but from silts. The silt is natural and comes from our rivers, suspended in the water naturally. When abstracted, most of the silt is allowed to settle out of the water in reservoirs but some will always manage to find its way into the tunnels. Unfortunately, alongside the penalty to water tunnels’ hydraulic performance, the silts bring with them another problem – invasive mussels. The most widespread culprit, the Zebra Mussel, finds
water tunnels the perfect habitat in which to live. They attach themselves to tunnel walls, often in such dense concentrations that they begin to act as a barrier to the flow of silt – which instead gets trapped around them. With more silt comes more mussels, and with more mussels there is more silt – creating a true vicious circle, at least from the point of view of the efficiency of the tunnels (the mussels may not agree). It is worth bearing in mind that these tunnels are often
Above: Zebra Mussel concentration, QEII Reservoir ALL IMAGES COURTESY OF BARHALE
20 | September 2024
running at great depth and at high pressure (the same as at the reservoir head) so the mussels and silt, like plaque in arteries reduce the efficiency of the system and encourages water to escape.
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