Inspection of the 20km long Blackdyke to Kennett tunnel on the Ely Ouse Essex water transfer scheme was never going to be easy. Main access during construction in the late 1960s was via 10 drive and reception shafts, with diameters varying from 4.5m to 7.3m and between 30m and 90m. Shortly after completion, four of these were backfilled, leaving only six for maintenance work between 2.5 and 10km apart.
Awarding the £700,000 ($980,000) contract to drain, clean and inspect the tunnel section, client, the Environment Agency had expected that a "drogue" would be chosen to remove silt without draining or manual cleaning. But Delta proposed a purpose built mechanical system for the work, awarded in May under the ECC conditions Option C (target cost with activity schedule).
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The 20km long tunnel passes through predominantly Gault clay, lined with precast concrete segments. Delta convinced the client that the machine’s speed and effectiveness would allow it to complete the 18 week contract, on time for its critical "refill" date on 30 November. Timing was tight because the 2.4m diameter tunnel is part of a strategically important river, pipeline and tunnel transfer scheme supplying some 23M.m³ of raw water in a typical year from Norfolk to Essex.
Drain down began on 3 August using existing pumps at Kennett pumping station. A 90kW Flygt drainage pump was lowered down an existing 1.2m diameter, 90m deep "borehole" at the lowest point of the tunnel, between shafts 8 and 10. Drainage was carried out over 14 days to avoid excessive pressures being imposed on the tunnel lining as the water was removed.
A complex ventilation system, consisting of six 600mm diameter 23kW Engart fans, was then installed at four of the six shafts to create a safe working environment for the cleaning and inspection teams.
Meanwhile Delta began building the mechanised cleaning system, which involved a combination of two 138bar jet sprays fixed to rotating arms with wire brushes connected to the end of each arm. This "propeller" device was attached to a Kubota ST30 tractor and successfully tested at Delta’s Highbridge plant yard before delivery to site on 17 August.
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By GlobalDataIt was lowered in sections down the 30m deep intake shaft at Blackdyke, reassembled and began its 20km journey to clean the tunnel. Water to supply the jet sprays was pumped from the offtake structure at Blackdyke down the tunnel invert. The water was scooped up from beneath the tractor and filtered several times before spraying. The high pressure jetting and rotation of the brushes easily removed silt and algae, restoring the tunnel lining almost to its original condition.
The silt was washed down the invert of the tunnel in front of the cleaning machine. The drainage pump then lifted the suspension to the surface and an SD4 centrifuge separated the solids into a skip. Clean water was discharged into the nearby Kennett brook by above ground pipeline.
A regular 24 hour routine was established from the beginning, using two, three-strong crews. To minimise downtime between shift changes, the crew travelled on mountain bikes from the nearest shaft.
Cleaning was completed in just under four weeks, with record progress of 2.3km in a single shift.
Inspection of the tunnel was carried out by the teams and a list of agreed repairs has been assembled by the Environment Agency’s consultant Charles Haswell & Partners. Tunnel refill began on 7 November and took one week completing the whole project two weeks ahead of schedule.
The final contract figure is expected to be in the order of £1M ($1.4M). And in the future, a series of total pressure cells and submersible joint meters in the tunnel lining will monitor hoop stresses and joint movements, simplifying maintenace. These instruments, installed by specialist Datum Geotechnical & Structural Monitoring, will transmit data to a computer terminal at Kennett pumping station allowing continuous monitoring during critical periods.
Environment Agency project manager Peter Cowie emphasised the very good working relationship between client, consultant and contractor. He added that: "It was also reassuring to have found that the tunnel was still in such good condition after nearly 30 years of operational use".
