High standards in a tight spot
Work is under way on two old aqueducts which take water all the way from the Lake District to Manchester. Andrew Taylor, director of confined-space specialist PMP, discusses his company's inspection work on the projects
Attention has so far centred on the Thirlmere aqueduct, which was built in the late 19th century. This is the senior of the two by 60 years - and engineers consistently confirm that it is in remarkable shape for its age. The task is made all the more interesting by the variety of construction techniques the Victorians used and the terrain they had to traverse.
Making the logical link between one of England's wettest regions, the Lake District, and Manchester, probably the fastest growing industrial centre in the mid-19th century, the aqueduct was built on a falling gradient of little more than one in 3,000.
It is entirely a gravity supply, without requiring pumping at any point, and it provides a flow rate that can approach 10Ml/h. As a result of the meticulously engineered gradient, water drawn from Thirlmere Reservoir reaches Manchester just over a day later.
Structural inspections began in October 2005, as a precursor to the main programme of works. These concentrated on the northernmost section of the aqueduct, between Thirlmere and Watchgate Water Treatment Works, near Kendal. As Watchgate controls a cross connection between the Thirlmere and Haweswater aqueducts, and also manages pumped supplies from Windermere and Ullswater, the northern section above it could be shut down with relatively few complications.
The project manager at the time, Montgomery Watson, brought in confined-space access specialists PMP. The first 26km section of tunnel and conduit was important because it featured all of the construction methods employed on the aqueduct. A safe system of work was devised between PMP and the project manager, which has formed the model for all the inspections and repair work carried out since.
Where the aqueduct crosses high ground, as in the Lake District, the conduit follows the hillside contours as far as it was practical for the original builders. On these sections, it is buried to a depth typically not much more than 1m and described as "cut and cover". But, in order to minimise deviations from the direct route and gradient, the conduit also tunnels deep underground for a distance totalling nearly 23km.
The Victorians generally made the tunnels 4.6m2 in section, including the floor, sides and domed roof lined with 600mm of concrete. In some parts, though, where the tunnel passed through particularly solid ground, the rock walls and roof were left bare. Only the floor was concreted and it was found that these sections were in excellent condition, needing no repairs.
Understandably, shutting down the aqueduct demands that survey teams work quickly, to minimise the outage period. As a result, PMP's inspection technique has primarily been visual, with constant dimensional checks and hammer testing (for hollowness and deterioration) at suspect points. The location and condition of any significant cracks, bulges or seepage are logged and photographed, allowing more detailed inspection or repairs to be prioritised.
John Butcher manages the operational aspects of the aqueduct shutdowns and is a frequent United Utilities spokesman on all aspects of the aqueducts. He said: "The defects that crop up in the surveys are mainly allowing groundwater ingress into the aqueduct. This is not necessarily a bad thing, and in fact the system was never designed to be fully watertight. Leakage from the aqueduct is, however, a matter for high-priority remedial work.
"Repairs on the conduit sections consist mostly of replacing damaged concrete. It is very rare that we discover poor examples of construction but occasionally a cracked floor will reveal, through core sampling, or radar examination, that the concrete was laid too thinly. The Thirlmere Aqueduct was designed with tremendous vision, and I would say that for 99.9% of the way the workmanship is excellent. There is also a lot of attention to detail in the construction. For example, every valve house along its 153km is built in stone quarried from the same place, at Annan in Scotland."
Some of the foresight of the engineering design is evident in the siphon systems, which allow the aqueduct to span valleys in buried pipelines. These occur at 29 points along the route and account for 64km of the aqueduct.
Each valley has a north well, where the conduit terminates and connects to multiple siphon pipes, and a south well, where the conduit restarts. The north wells house shut-off valves, while the south wells valves automatically stop the siphoning, allowing safe working on shut-down sections.
Every well house was designed to accommodate five siphon pipes and their valves, yet the pipes were installed sequentially over 28 years. During that period, a step change in pipe technology removed the need for the fifth siphon. The first three pipes were cast iron, the first was 1m diameter, and the other 1.12m. But by 1925 lead-jointed steel pipe was available at 1.37m diameter, giving the systems ample capacity.
Perhaps inevitably it is the valve hardware that requires most attention. It is mainly of iron construction with as much as 111 years in service. PMP has surveyed all the wells, revealing that repairs are required to some of the valves.
Valve gear in the north wells is all undergoing replacement by modern, remote controlled penstocks. Some of the non-return flap valves in the south wells were found to have problems with seized hinges, particularly where cast-aluminium flaps had been used on the iron valve bodies. In addition, the hollow connecting rods controlling the plug valves were showing rusting in some instances, where contact between dissimilar metals induced galvanic corrosion.
Working more recently under Daniel Contractors, PMP has completed the inspection of the aqueduct and been making repairs to the pipeline sections. Frequently this has involved fitting AMEX-10 Seals at joints in the siphon pipes.
These are WRAS-approved, low-profile mechanical seals for repairing leaking pipe joints, or cracks, from the inside of the pipe. This avoids the need to excavate the ground above the joint, for a more traditional external repair, and does not rely on any form of caulking or sealant compound.
In 2007, the entire aqueduct was shut down, requiring the careful co-ordination of more than 120 contractors involved in repair work. John Butcher says: "Our top priority was to make sure we kept water on tap for all our customers - no mean feat when an aqueduct of this size is taken out of service. The process was something similar to getting the runners under starter's orders for a horse race.
"We instituted a traffic-light report, which finally gave us the green light to shut down only when the condition of our various reservoir stocks were right and all the repair teams were in place and ready. We also imposed a 12-hour evacuation clause that meant every contractor would be able to clear the aqueduct of all equipment within half a day, should the need arise to put the aqueduct back in to service.
"During the whole of the programme, it is also vital to keep customers aware of the changes that could arise in the water supply. Thirlmere provides very soft water indeed, and we notify manufacturing companies with sensitive processes that the characteristics can alter when the reservoir comes off-line."
PMP is due to begin repairing and refurbishing valves on the Thirlmere aqueduct in October. This is normally the optimum month for shutting down, in terms of balancing water supply and demand in the areas supplied by Thirlmere.
United Utilities' cleaning and maintenance programme is planned to continue for another ten years and has just moved into Manchester where the huge ring main will be cleaned and repaired over the next four years. Plans are also well under way to start internal inspections of the Haweswater aqueduct.