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Galliford Try flushed with success

SITE REPORT

Dolphins, seals and greater crested newts, as well as members of the general public, have had to be accommodated during works on a new sewer network and sea outfall at Holyhead in Anglesey.

Adrian Greeman reports

LOOKING from London, the island of Anglesey, at the northern tip of Wales, has always seemed a little like the end of the world.And to the locals in the ferry port of Holyhead it had begun to feel like it too.

'It's a run-down urban area and development had stalled because of the poor infrastructure, ' says Chris Hastings from contractor Galliford Try.

Now a new sewer scheme that the firm is building is beginning to make a difference - small construction sites have sprung up all around the town as a 10-year development plan gets under way.

'There are retail parks, offices and housing going up all over, ' says Mr Hastings, project controller on the two-year long contract.

The scheme for Welsh Water stretches well outside the port boundaries, covering most of Holy Island to the north of Anglesey. It is a scenic spot - a flat, low extension of the Snowdonia National Park southwards and on the west side an important tourist area with pleasant sandy beaches.

The previous sewerage system included a large and extremely wellbuilt Victorian peripheral sewer around the town.Although it was good for its time, it had reached its sell-by date.The Victorian works and four other lines discharged untreated sewage direct to sea, the system was overloaded and caused flooding at peak times.

This was obviously not ideal for the local tourist industry and the area has to be cleaned up by the end of this year to meet European bathing water legislation.

The replacement sewer scheme comprises a biological treatment works just south of the town and a 23 km network of rural sewer lines with 17 dispersed pumping stations at various locations.New connections are also in place to divert the Victorian culvert flow to the treatment works.

The biggest part of the work is a 6 km-long pipeline and an 1,100 m sea outfall for the treated effluent.

Keeping a low profile in an area like this was one of the biggest constraints on the design and construction work, which under the contract are both co-ordinated by Galliford; so too were the locations of a number of ecologically important heathlands and coastal areas. Some of these are designated Sites of Special Scientific Interest.

Measures have included erecting 5,000 m of special 0.5 m-high tilted 'newt fences' to protect the greater crested newt, moving hedgerows, avoiding badger sets and installing protection for voles and bats.

'We also have Special Areas of Conservation, which are a grade above the SSSI, ' says Mr Hastings.He adds that even the sea areas were sensitive, housing populations of seals and dolphins.

He says the team had to counter a little of the 'not in my back yard' mentality from local residents.'During the design phase we had objections to a number of the proposals for the main treatment works.' Eventually they were sited at Penrhos, a couple of miles down the road.

Galliford developed the scheme in consultation with Welsh Water and design partner Bullen, Black & Veatch - the same designer Galliford works with for AMP3 projects with Welsh Water.

Pipeline routes were chosen to avoid the ecologically sensitive areas and also to skirt round the many archaeological sites nearby.Geology was also a consideration - the ancient Snowdonian geology extends northwards to the Anglesey island group and is formed mainly from very hard igneous rock.

In an effort to pinpoint the exact nature of the underlying strata, specialist contractor Fugro carried out extensive ground investigation and site surveys using radar and borehole techniques.

During the design phase, the team managed to shave some £6 million from the project's cost, bringing down the final price to £34.6 million.

Changes included a switch to shaft construction for most of the storm storage and pumping stations, rather than using conventional concrete boxes.

'That saved money because you don't need coffer dams, ' explains Mr Hastings.'And they are quicker and less disruptive, which is important in the tourist areas.'

Unusually, the contract also included obtaining specific permissions for various parts of the work from assorted bodies, such as the county council, the Environment Agency, English Heritage, the Countryside Commission for Wales and Welsh historic environment agency, Cadw.

'We approached them for their suggestions and worked in consultation with them to get around problems, rather than simply presenting proposals and getting a yes or no, ' says Mr Hastings.He says the same negotiating philosophy informed all of the work.

The biggest item was the outfall, which had to be constructed through an SAC, along cliff tops west of the town.No work was allowed that would disrupt the 100 m-wide area, even though a major pipeline had to run under the cliffs and out to sea.

The solution was directional drilling and then undersea trenching.

'We looked at drilling the entire 1,100 m-long outfall, but it was too risky, ' says Mr Hastings.The problem was the very hard and abrasive rock through which the line had to go.The rock is folded and faulted and can throw the drill off direction.

'This is big stuff, ' emphasises Mr Hastings, 'not the soft ground work for utilities in cities.'Technology developed for the oil industry was used by subcontractor LMR Drilling, with a Berco 100-tonne drilling rig used to carve through the 700 mm-diameter hole needed to carry the 500 mmdiameter outfall and its protective sleeve.A 170-mm pilot was bored to create the 300 m line through the cliffs and out to sea, which was then backreamed twice to bring it to the full diameter.

Out at sea, a joint venture between Land & Marine and Westminster Dredging prepared the trench.

Mr Hastings says: 'They used a three-legged platform with 28 m-long hydraulically controlled 'spud' legs.A huge, long-reach excavator sits at one end, using a maximum 21 m arm reach to form a 2 m-deep trench, storing the material on the sides for later backfilling.

'It was a fairly high-tech machine, using a series of video panels for the driver to locate his bucket - like a video game - because obviously he was working blind under water.'

Drivers alternated at 45-minute intervals, such were the levels of concentration needed.

Mr Hastings adds that 'cool' technology also helped with the weather forecasting, a crucial element in open sea, especially in an area with exceptionally fast tides racing up to 7-8 m a second.

Windsurfing is one of the major tourist attractions for Anglesey and Mr Hastings and the team used a windsurfing website to help them forecast the weather.'When it said windsurfing conditions were good, we stood down, ' he says.

Computer technology helped too with the local fauna.The very hard 200 Mpa rock formed many of the seabed areas without any sand cover and had to be blasted to loosen it. Before this was done, a sonar type pinging was created with underwater speakers to frighten off seals and dolphins.And, just to make sure, they let off a small mock blast each time as well.

Meanwhile, an outfall pipe was made in Norway. Short lengths of the pipe were welded there into three sections of 300 m, 400 m and 400 m.

These were hauled by tug across the North Sea.

Once the trench was formed, the first pipeline string was hauled through the hole, while the others were joined and then sunk into the trench, with concrete collars to kill the buoyancy. Finally, the trench was backfilled.

All this was scheduled for early on in the contract, says Mr Hastings, and finished last September.He says: 'To allow for the risk, we wanted to make sure we had a second summer if anything should go wrong.'

The directional drill found other work too, as the outfall pipeline had to cross the dual carriageway A55 and the railway corridor alongside.

A 150 m-long link was made underneath. A second similar link was needed for an incoming gravity sewer from part of the rural catchment.

More blasting work has been done to form some of the shafts for pumping stations.One in particular involved blasting extremely close to local housing, forming a 11.5 m shaft, 12 m deep in the island's hard schists and quartzites.

'With EJ Kelly, who did many of the shafts and Rock Blasting Engineering, we worked out a controlled explosion system which detonates 140 very small 0.1 kg charges in a 9 second sequence, ' explains Mr Hastings. Lifts of about 1 m were done in two stages, with an inner ring of rock blasted and removed before the outer ring was done.

'Even though we were just 25 m away, this prevented any damage or disruption, ' adds Mr Hastings.There were three blasts a week until it was done.

Other shafts have been sunk by a caisson method, using a 2 m-wide concrete reaction ring cast into the ground to support hydraulic jacks pushing 1 m-deep ring sections into the ground.

A 12 m-deep shaft close to the Irish Sea was done this way, with a 400 cu m concrete plug poured under the seawater filling the excavation.

Galliford itself has built the treatment works, using a system from US firm ITT Sanitaire that uses air blown through deep tanks to rapidly process the sewage.The small footprint of the system is an important feature, says Mr Hastings, and it is also odourless, which means covers are not needed.

'An ionised air blanket is used in the sludge-drying house, which is otherwise smelly, ' he says. Processed sludge will be tankered to another facility for final disposal.

While all this work proceeded, three subcontractors were busying themselves with the bread-and-butter trenching and pipework.Galliford selected three local north Wales firms for this, as part of a deliberate local sourcing approach, with a tender for initial rates and then a negotiated target cost agreed with each, splitting the work into three separate areas.

'Pulling in resources like that, which could work simultaneously, was a procurement strategy that has paid off, because we have pulled our completion date forward by three months, ' says Mr Hastings. Everything should wind up in July.