Articles & Case Studies

WessexWater Avoids Major Sewer Construction Project with Hydro Drop Shaft ‘Keyhole Surgery’

Posted: Friday 14th November 2014

A CLEVER idea to perform ‘keyhole surgery’ down a 50 metre sewer shaft using Hydro’s Vortex Drop™ Shaft technology has saved Wessex Water from major excavation and construction work that could have run into hundreds of thousands of pounds.

When work was needed to increase sewer and drainage capacity in the Whiteladies area of Bristol, Wessex could have constructed a new 50 metre shaft through limestone bedrock. Instead Hydro International came up with a novel ‘ship-in-a-bottle’ solution to construct a Hydro Vortex DropTM Shaft via an existing access shaft, avoiding major construction and minimising neighbourhood disruption

Wessex Water Project Engineer Andy Roberts explains: “The existing sewers within the area were struggling during heavy rain, so we added a new sewer along Woodlands Road and needed to make a connection onto the Northern Intercept.

“Excavating a new 50 metre shaft through the limestone bedrock to reach the sewer would have cost hundreds of thousands of pounds. Through the engineering contractors on the project, we discussed the concept of a drop pipe with Hydro. They suggested inserting a Hydro Vortex Drop™ Shaft through the access shaft and constructing the Energy Dissipation Unit at the bottom.”

All the components were flat packed and inserted down the hole to be re-assembled inside at the bottom; then the pipework lowered down from above. It worked perfectly.

The Northern Intercept Sewer was constructed in the 1960s as part of a programme of works intended to increase capacity to match the spread of the Bristol urban area. In addition, it gave the opportunity to provide some re-organisation of the drainage infrastructure that has grown organically since Victorian times. The sewers discharge to the Avonmouth Treatment Works, and once fully treated finally discharges into the Severn. There are still a large number of combined foul and surface water sewers within the area. Stormwater flow volumes can be significant in peak rainfall and the Hydro Vortex Drop™ Shaft handles a combination of foul and surface water.

Attenuating vertical fall

The new Drop™ Shaft is designed to handle up to 680l/s (the average weight of a Sumatran Rhinoceros) every second at maximum flow. Falling through a height of 50 metres to the Northern Intercept, the water will carry so much energy that severe damage could be caused at the bottom of the drop unless the energy is dissipated.

“The forces involved would tear apart a standard sewer construction quite quickly,” points out Keith Hutchings, Hydro International Group Product Manager. “The erosive wear, vibration and stress could be very high unless the energy is carefully managed and dissipated.

“Standard solutions to sudden vertical differences greater than a few metres include weirs, cascades or flumes. However they need a great deal of space and need to be carefully designed to slow the water sufficiently at peak flows.

“Other pipe work and vortex schemes often entrain significant quantities of air at maximum flow. The results include highly damaging cavitation and in extreme events ‘glugging’ or ‘water hammer’ pose danger to the pipework and the receiving sewer. Care must be taken to design conventional drop shaft solutions to ensure that air is always entrained at all states of flow.”

Small diameter, large effect

While conventional vortex drop shafts are sometimes considered, their need for an air core down the length of the vertical pipe means a large diameter pipe, and the design of the stilling basin at the bottom of the shaft also takes up a lot of space.

The Hydro Vortex Drop™ Shaft is designed to run either with controlled air entrainment for lower flows and at pipe full without entrained air and with complete stability. The innovative design and wear-resistant coating to the stainless steel of the Energy Dissipating Unit at the base of the shaft enable a very compact yet highly effective flow receiving chamber to be constructed at the bottom. The anticipated maintenance is minimal.

A key element of installing the Hydro Vortex Drop™ Shaft in this project is its compact dimensions. Compared to the main shaft diameter of 3.6 metres, the diameter of the Vortex Drop™ Shaft top chamber is only 450mm, and the diameter of the pipework is only 300mm, so it was relatively simple and quick to cut an access hole through the existing ladder platforms at every 15 metres or so on the way down.

Additionally, Hydro’s system could be dismantled like a flat-pack wardrobe and lowered through the shaft, to be re-assembled at the bottom’ like a model sailing ship in a bottle.

The Hydro Vortex Drop™ Shaft is suitable for drops of up to 100 metres, and flows up to 5000 l/sec, whether foul or stormwater. As the Hydro Vortex Drop™ Shaft is of a small enough diameter to be easily contained within existing shafts, it allows safe access to personnel without the need to create additional access either for construction or maintenance.

The Hydro Vortex Drop™ Shaft has been widely used on stormwater and foul water schemes across the UK and internationally, both retrofitted within existing structures and on new and external works.

As part of Wessex Water’s AMP5 flood alleviation schemes programme, the drainage in the Clifton area is being upgraded by Wessex Water. An expected eight month programme of work started in September 2013. Wessex Water is also upgrading foul and surface water systems throughout the region in a multi-million pound scheme.

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