Preston sewer’s record microtunnelling rates are helping to quell the memory of flooded shafts and a buried full-face tunnelling machine. Report by David Hayward.
Completion on schedule of twin state of the art pipe-jacked tunnels; the first of their type in the UK - plus a new British pipe-jacking record on route.”
So reads a recent site diary entry from engineers building Preston’s sewer improvement scheme - a £114M complex of interceptor sewers, stormwater storage tunnels and rising mains designed to upgrade the city’s aged sewerage system.
The entry continues: “base slabs now complete in two maximum 36m deep, diaphragm-walled shafts, excavated through unpredictable water-bearing ground, but with not a leak in sight”.
Not surprisingly the site team is in celebratory mood, for its engineers have achieved two ground-breaking, technically complex operations. But congratulation carries with it a significant feeling of relief; for the same site diary 22 months ago read very differently.
Its pages then were dominated by reports of a trapped, and now abandoned, full face tunnelling machine after water and silt had inundated the 70m length of sewer tunnel it had completed (NCE 19 January 2011). And engineers also wrote of those same twin shafts which had to be evacuated as they too rapidly filled with water following major leaks into the excavations.
“It’s been a very challenging year, but teamwork and innovation has helped us overcome two major problems,” says Andy Parker contracts manager for contractor KMI Water, a joint venture of Kier Infrastructure and Overseas, J Murphy & Sons and Interserve Construction.
“The solutions we came up with have included one of the most complex grouting and dewatering systems ever installed, plus a German pipe thrusting technique, new to the UK but which proved fast and cost effective.”
“Challenging” had long been the dominant description of Preston city centre sewer upgrade, even before any of its 6km of tunnelling or 13 access shafts had been started. The existing, century-old combined sewerage system regularly exceeded capacity, with frequent flood-induced overspills direct into the river.
“It’s been a very challenging year but teamwork and innovation has helped us overcome two major problems”
Andy Parker, KMI Water
Adoption by the UK government of several European Union water directives, aimed at cleaning up river and coastal waters identified seven problem outfalls releasing “unsatisfactory intermittent discharges” and gave birth to local water authority United Utilities’ ambitious clean up scheme.
Numerous town centre sewers will be linked by a new interceptor tunnel feeding into a 2.5km long stormwater storage tunnel. This 3m diameter tunnel, plus two large, up to 27m diameter, linked shafts will provide flood storage, controlling flows - via a new pump station and twin rising mains - to the town’s main treatment works.
KMI Water started work in February 2010 and all went well until the following August when the smaller of the two shafts suddenly sprang a leak. Two thirds of this 30m deep shaft had already been excavated when suddenly water and silt started flowing into its base.
Within 24 hours a large surface sink hole had appeared close to the shaft wall. Soon over 60t of silt had inundated the base and it was decided to carefully flood the shaft to stabilise the area.
Surrounding ground had been dewatered, but not the uppermost 15m of alluvial deposits which were considered too permeable. This left a perched water table at the interface of these superficial deposits and lower Sherwood sandstone.
“The surface sinkhole confirmed we were drawing down sands, gravel and water from this upper alluvial layer, which then flowed beneath the toe of the completed 21m deep diaphragm walling to percolate up into the excavated shaft,” explains KMI Water construction manager Adrian Owens. “But the diaphragm wall had been founded in 5m of competent Sherwood sandstone and the exact cause of this piping failure was - and remains - unclear.”
Tackling sewage discharges
Heavy rain in the Preston area can cause the town’s existing combined sewerage system to discharge untreated effluent from at least seven outfalls direct into the River Ribble.
The new £114M network will intercept these “unsatisfactory discharges”, diverting all flow 10km westward, via a new pumping station, to the town’s main treatment works at Clifton Marsh.
A 2.5km stormwater storage tunnel, and two linked shafts, will hold back up to 40,500m3 of stormwater, regulating its flow to the treatment works. This storage facility will also restrict maximum direct discharge into the river to an acceptable 10 spills per year.
The 2km town centre interceptor sewer, driven by Lovat tunnelling machine “Caroline”, is complete, as are all 13 access and storage shafts. The 5km twin rising main from the pump station has been laid in trench, with the 866m twin pipe western river crossing into the treatment works, driven by a Herrenknecht pipe-jack, complete this summer.
“Caroline” is currently driving the network’s final section, the 3m diameter storage tunnel.
No sooner had engineers started to brainstorm a solution when, two months later, the second larger 36m deep shaft, just 10m away, flooded in similar circumstances.
The two failures triggered an extensive array of additional site investigation, involving a dozen boreholes plus vertical and raking piezometers. Six months of detailed analysis has resulted in the just completed recovery solution.
Both shafts have been protected by a network of jet-grouted columns, forming an impenetrable full depth grout curtain, plus a further extensive three stage dewatering system.
Excavation restarted beneath each shaft’s diaphragm walling, closely monitored in 1.5m stages.
Exposed shaft sides were reinforced with sprayed concrete, before 500mm thick precast concrete segments extended the wall lining downward. With completion this Spring of the larger shaft’s 3m thick concrete base, engineers can put behind them 20 months of apprehension, as both excavations remain totally dry and sound (see box).
“If we had known about the Herrenknecht system at the design stage we would have considered it as an option”
Andy Parker, KMI Water
Fingers have also been uncrossed 5km west of the shaft site where the new sewer route passes beneath the River Ribble. Just completed is the second of twin 1.4m diameter steel pipeline “tunnels”, thrust 7m beneath the river using a specialist German Herrenknecht microtunnelling system.
Into these pipelines will be placed the two 800mm diameter rising main sewers en route to the treatment works close to the river’s northern bank.
Although employed in mainland Europe for several years, Herrenknecht’s “direct pipe” microtunnelling technique was given its UK premiere on the Preston site.
The technique centres on a powerful pipe-thrusting machine anchored in a shallow pit at the start of the 866m under-river drives. Behind, stretching back across the site, lay the first 92m long string of welded pipes ready to be thrust forward through the river bed’s glacial sands, silts, gravels and occasional boulders.
Micro tunnel boring machine
Heading up the pipe string was a 15m long micro tunnel boring machine (TBM) fully computerised and operated remotely from a control room alongside the pipe thruster. With the TBM steering the pipe string to impressive 1mm accuracy, the pipeline is gripped by the thruster and forced through the ground by the machine’s pistons in 5m strokes (see box).
Drive progress averaged 20m a day and this Spring, engineers claimed a new UK pipe-jacking record of 72m in a single 12 hour shift.
Pipe jack premiere
To the German operator of Herrenknecht’s twin 1.4m diameter pipe drives beneath the river, the operation proved routine.
But to KMI Water site engineers, witnessing the technique’s first UK use, and completion this summer on the same site where the original tunnelling machine now lies entombed, the fast, trouble-free drives instilled pleasure and relief.
Cobbles and boulders in the drive’s glacial deposits were easily disposed of by a cone crusher mounted on the head of the microtunnelling machine. The resultant spoil was then mixed with a bentonite slurry to feed back, through slurry lines within the completed pipes, to a surface separation plant.
A back-up facility to pull out the entire pipe string in case of major obstructions was not needed.
“We hit cobbles but they did not slow us at all,” says KMI Water tunnel agent Chris Gannon.
Lubricating the 60mm annulus around the pipe with a bentonite and polymer mix allowed a maximum thrust force of only 450t to be applied, just 60% of the pipe thruster’s capacity.
Such success has helped quell still vivid memories of the first attempt two years ago to tunnel beneath the river. Originally the planned route of the twin rising mains was to be through a single, 2.8m diameter tunnel, driven by a full-face Lovat earth pressure balance machine (EPBM).
Named Helen, the machine began its drive in October 2010 with little difficulty through the same glacial deposits but 7.5m below the route later followed by the Herrenknecht pipes. In November 2010, just 70m into the drive, and with completed sections fully lined with concrete segments, water and silt suddenly rushed into the tunnel invert having breached the segmental lining.
The exact point of entry, 20m back from the tunnel face, was obscured by overlying steel plates, part of the Lovat’s tunnelling conveyor system. Two hours later, with water nearly 1m deep, all 10 men in the tunnel were safely evacuated. Uneven ground pressures on the lining subsequently led to segments cracking and, within 24 hours, engineers ordered that the tunnel and access shaft be carefully flooded to ensure the surrounding area remained stable.
Recovery options included sinking an intermediary rescue shaft, plus ground freezing or grouting the entire 70m of tunnel. But all possibilities were expensive and risk prone.
The chosen solution - abandoning the 70m of tunnel and the Lovat full facer - turned out, in hindsight, to be the most cost effective. Opting for the twin Herrenknecht pipes, plus reusing part of the original access shaft cost less than half any other option and was significantly quicker.
“If we had known about the Herrenknecht system at the design stage we would have seriously considered it as an option for the river tunnel,” Parker reflects. “With the original tunnel and shaft now grouted up, plus the steel plates preventing early examination of the leak area, we have been unable to determine the exact cause of failure.”
Engineers do though stress that the tunnelling machine had performed well and was not a contributing factor to the inundation. Nor was the river, some 14m above the tunnel crown. The Ribble is tidal at this point and water in the tunnel tested fresh rather than saline.
While responsibility for the river tunnel and shaft failures has yet to be established, engineers on site are concentrating on the final machine drive for the 2.5km stormwater tunnel. Caroline - sister Lovat machine to the ill-fated Helen - began driving the 3m diameter tunnel in March and is due to break through next January.
Leakage of water and silt, into the two large pump station shafts 22 months ago, triggered a detailed ground investigation and solution searching exercise.
Two original surveys - by United Utilities and KMI Water - had revealed nothing unusual in the alluvial superficial deposits, or in the Sherwood sandstone beneath into which the toe of each shaft’s diaphragm wall perimeter penetrates by 5m.
Even following the third much larger site investigation, together with excavation of the shafts, the precise cause of the failures remains unresolved.
Various shaft protection options were considered, including ground freezing, or an outer secant piled wall. But engineers opted for a combination of further dewatering, plus a grout curtain around both excavations, as this was claimed to be the quickest most economic solution.
The new three stage dewatering regime involves a network of over 50 close-centred pipes penetrating 50m deep, well below shaft bases.
An equally extensive 120 column grout curtain was installed before shaft excavation restarted, working downward in closely monitored layers.