The new 19km potable water pipeline reduces borehole abstraction at Speen WTW, safeguarding a new EA water abstraction licence

● A reduction in Thames Water’s abstraction licence from raw water supply boreholes was the driver for the pipeline which transfers potable supplies from Tilehurst Reservoir in Reading to Cold Ash Reservoir near Newbury.● Increasing flow into Cold Ash by up to 5ML/day will allow its former main supply source from Speen water treatment works to be shut down.● Speen will considerably reduce its raw water abstraction rates from local boreholes, located in the grass-covered wetland floodplain of rivers, where one of the world’s rarest snails, Desmoulin’s Whorl, has one of its main European homes.● Several rivers, the M4 motorway, attractive Berkshire villages and even contaminated land complicated the design challenge; while known habitats of badgers, great crested newts, Japanese knotweed and bats added to the no-go areas.

Pipe insertion was deployed using the old cast iron pipe as an outer sleeve for the first 1km long run of the new pipeline. The net cost saving over conventional trench construction was put at approximately £200,000.

● Collaborative thinking by the design team looked at different ways to plan the route to ease planning consent and reduce the scope of pumping required, despite adding slightly to the route length.

● Utilisation of site-won material, some 9400m3, led to faster construction, no imported shingle and hundreds of eliminated lorry journeys, totalling a cost saving of around £700,000.

The completion of a 19km pipeline, transferring potable water between two Thames Water Berkshire catchments, has not only safeguarded the new EA reduced raw water abstraction licence, but will also assist the survival of a rare snail.

Cold Ash Reservoir near Newbury is the major supply source of potable water to some 35,000 local households. This covered reservoir receives its main treated water from Speen WTW, 8km to the south west, via a 200mm diameter pipeline pumping in up to 5ML/day.

Built in the 1960s, Speen also supplies several other smaller reservoirs, but Cold Ash takes 90 per cent of the water that the treatment works abstracts from five nearby boreholes, including the River Kennet. This routing works well and would have needed no technical changes were it not a requirement for water companies to look at ways to reduce borehole abstraction across their water networks.

The Environment Agency has ordered a reduction in the current 13.6ML/day raw water abstraction licence at Speen, as part of a nationwide programme, and abstraction rates were to be lowered to an average 4ML/day and maximum 5 ML/day.

The new restriction still allows Thames Water to supply its smaller reservoirs from Speen, but it needed to find an alternative major water source for Cold Ash. The local Water Resource Zone supplying the Newbury area was fully committed and so Thames Water turned to the neighbouring eastern zone serving Reading. Tilehurst Reservoir, 3km west of Reading, was identified as having surplus supplies, triggering the water company’s decision to construct transfer mains between the two reservoirs.

In January 2011, Thames Water Utilities awarded a £10.2m design and build contract to Optimise – a joint venture consisting of J. Murphy & Sons Ltd, Clancy Docwra Ltd and Barhale plc, plus designers MWH. Construction was not scheduled to start until January 2013 offering the JV an unusually generous year-long design period, which it used well.

Thames Water’s award was, at the time, a new contract approach setting out only a very broad design brief and allowing Optimise to analyse a wide range of options.

We were asked to identify a ‘suitable supply source’ within the Tilehurst area and design a transfer pipeline between Reading and Newbury catchments routed somewhere within a 15km wide corridor.

Much of the landscape between the two reservoirs was rich in local and statutory wildlife sites, ancient woodland, scheduled monuments and even the remnants of an Iron Age fort.

Using hydraulic modelling, topographical and environmental surveys, the final route was only marginally longer than the 18.5km minimum length, and succeeded in avoiding the sensitive areas identified. The vast majority of the chosen alignment runs beneath privately owned land, with over 80 per cent classified as agricultural.

Tilehurst Reservoir is fed from nearby Fobney WTW through a 710mm medium density polyethylene (MDPE) pipe. This replaced the original parallel 60 year old 21 inch (533mm) cast iron mains, which was capped off but left in situ.

The team decided to connect into the live pipe about 1km from the reservoir, and insert its new 355mm pipeline into the original redundant cast iron mains lying alongside it.

The team also decided to run the gravity section of the pipeline a further 6km along the route, siting the new pump booster station within Thames Water’s secluded Bradfield Windmill WTW. The remaining 11.9km pumped section would then be laid direct to Cold Ash. Further advantages were won by also siting the transfer main’s motor control centre (MCC) at Bradfield.

The vast majority of the high density polyethylene (HDPE) pipeline was laid in a conventional open trench, some 2m deep. The alignment was unusually flat and, with three excavators plus a specialised trencher on site, the excavation was opened up in maximum 200m lengths.

The team’s best rate was 250m of completed pipeline a day, an achievement helped by a further value-engineered innovation.

Conventional pipe bedding, surround protection and 100mm cover material, would have been imported shingle with the trench then backfilled with the excavated soil.

But Optimise decided to reuse some of the as-dug gravel, chalk and sandy clay instead, converting it on site into a shingle substitute.

A riddler bucket, equipped with geared spinning wheels, was fitted to a standard excavator arm to cut and grind the mixed ground into shingle-sized rock and the replacement bedding material was used for over 70 per cent of the route.

JMH Directional Drilling routed the pipeline beneath the M4 near junction 12, and under the Rivers Pang and Bourne. A total 1km of the mains was directionally drilled, with the motorway crossing demanding 12m cover.

The booster station at Bradfield WTW, housing two 50kw high lift pumps, was a small 20m long glass reinforced plastic hut.

Alongside it, the MCC - installed by Nomenca in an adjacent treatment works building - will remotely record flow rates, pump output and reservoir levels at both ends of the pipeline. This is monitored and controlled remotely by Thames Water.

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Now fully commissioned, the pipeline is supplying a maximum 5ML/day to Cold Ash Reservoir, and the original feed mains from Speen WTW will be shut down.

An actuated plug valve, installed in the mains can, when needed, reverse the flow allowing the reservoir itself to boost Speen’s water supplies, and this too will further safeguard the new EA reduced raw water abstraction licence - and the Desmoulin’s Whorl snail.

© Faversham House Group Ltd 2020. WWT and WET News news articles may be copied or forwarded for individual use only. No other reproduction or distribution is permitted without prior written consent.

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