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Shaft Story

After some delay a new sewerage scheme is under way for a picturesque harbour town on Scotland's North Sea coast.

The locals in the little town of Stonehaven in Scotland were not that receptive to the idea of cleaning up the sewerage system when it was proposed a few years ago. A sea outfall into the North Sea had never caused any great problems and the town is not very big. "So why change things?" they asked.

Stonehaven is a picturesque village with an old castle cliff top above its small bay, a small stone breakwater harbour. In recent decades it has become a haven for those who grew wealthy on the back of the offshore oil boom in Aberdeen some 25km north. They, and the local business people, did not relish a couple of years of construction work, particularly if it would leave behind an unsightly wastewater plant.

In fact, the scheme proposed by Scottish Water was just for pumping stations with simple screen systems to de-grit and clean the waste and then release it northwards. Flows close to the harbour and a smaller flow on the far side of the bay would go 16km by a new pipeline to treatment works at Nigg in Aberdeen.

A big new treatment facility has recently been finished at Nigg as part of an £80M private finance initiative scheme servicing the entire region around the city. The Aberdeen Environmental Services consortium led by contractor Balfour Beatty is operating the whole scheme for 30 years with water company Kelda and consultant Earth Tech Engineering.

Balfour Beatty carried out the £63.5M construction work with Earth Tech as its engineer.

"The Stonehaven element is an addition to that scheme," says Steve Scott, Scottish Water regional community manager.

Like the rest of the project, it is needed to comply with the new bathing water and fishery standards under European Union regulations.

Cost will be a total £16.5M with £10M for the civils work.

But the residents protested and the scheme planned to start in 2003 was delayed for a year while a planning inquiry went through. More time was then needed to amend the scheme to satisfy all parties.

The initial plan was for a pumping station on a small headland by the harbour and a connection to another sewage pipe a few kilometres round the bay at Cowie, where an existing screen treatment facility currently services a second small outfall. Both sites had been used originally because of the complexities of the local geography.

Changes were made to connect the harbour pump station to the Cowie plant.

"That meant the screening could all be done at Cowie where there was slightly more space," explains Scott. The site near the harbour was cramped and besides the residents did not want regular truck journeys through the little town to remove the screenings by road.

Balfour Beatty Civil Engineering is currently building the new pumping station on the harbour end, having finished an initial pipe connection.
Trenchwork for the connection was disruptive says Balfour Beatty project manager Ian Downey, but the contractor did its best to ease local feelings by making itself useful to local householders. They did this by putting wheely bins out for the weekly refuse collection when the work blocked off houses.

Central to the project is the main sump, a 12.5m diameter concrete lined circular shaft excavated into the relatively firm sandstone which underlies some 3m or so of made ground at the harbour end.

"It was a small reclaim area" explains Downey.

The shaft was broken out using a hydraulic hammer mounted on a 16t excavator. Drill and blasting would have been impossible, since it could have damaged old stone buildings nearby. Breaker work was also restricted to a few hours a day to avoid disruption. Even so, work went well and was done in eight weeks against a programmed twelve.

A precast concrete lining was installed just over 1m at a time as work progressed. Precast units weighing 3t were bolted in position and then grouted. The units were off-the-peg elements, brought in by truck through the town, with two trucks carrying each eight section ring. Balfour Beatty routed the trucks through the south end to try and avoid the town centre.

Walls are now being built on top of the 1.2m-deep plug at the base of the shaft. The walls will separate three sets of pumps.

These are all submersible Flygt pumps which Earth Tech design engineer Tom Connolly explains are preferable to dry mounted units, saving on civils work to create extra dry areas in a cramped site "and helping keep the flow constantly agitated which reduces unwanted grit settlement".

Two of the pumps will be constantly used to transfer the flow to the Cowie site for screening. The other sets are for storm conditions, one cutting in when flows rise, pumping to a long sea outfall. The is used as a last resort in near flood conditions to discharge to a short sea outfall.

"But by then the dilution should be pretty high anyway," says Connolly. Both of the outfalls are existing structures.

The whole facility will eventually be invisible with traffic running over the top and small fibre glass buildings housing control systems. Other work in this small site includes an underground space 9m by 3.5m which will be filled with volcanic pumice stone. This is a porous medium which sustains bacteriological growth.

"You keep water trickling over it," says Connolly. The bacteria remove organic matter from vent fumes blown in below, leaving the exhaust smell free he says. "It is the Rolls-Royce of odour control systems."


Two 50m-long pipejacking operations carried out by suncontractor Pipeline Drillers complete the main works, taking the pipeline underneath the main A90 coast road. The pipe will be in 600mm diameter steel sleeves "so that it can be repaired without disrupting the units were off-the-peg elements, brought in by truck through the town, with two trucks carrying each eight section ring. Balfour Beatty routed the trucks through the south end to try and avoid the town centre.

Walls are now being built on top of the 1.2m-deep plug at the base of the shaft. The walls will separate three sets of pumps.

These are all submersible Flygt pumps which Earth Tech design engineer Tom Connolly explains are preferable to dry mounted units, saving on civils work to create extra dry areas in a cramped site "and helping keep the flow constantly agitated which reduces unwanted grit settlement".

Two of the pumps will be constantly used to transfer the flow to the Cowie site for screening. The other sets are for storm conditions, one cutting in when flows rise, pumping to a long sea outfall. The is used as a last resort in near flood conditions to discharge to a short sea outfall.

"But by then the dilution should be pretty high anyway," says Connolly. Both of the outfalls are existing structures.

The whole facility will eventually be invisible with traffic running over the top and small fibre glass buildings housing control systems. Other work in this small site includes an underground space 9m by 3.5m which will be filled with volcanic pumice stone. This is a porous medium which sustains bacteriological growth.

"You keep water trickling over it," says Connolly. The bacteria remove organic matter from vent fumes blown in below, leaving the exhaust smell free he says. "It is the Rolls-Royce of odour control systems."

Two 50m-long pipejacking operations carried out by suncontractor Pipeline Drillers complete the main works, taking the pipeline underneath the main A90 coast road. The pipe will be in 600mm diameter steel sleeves "so that it can be repaired without disrupting the road above," says Downey.

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