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Powering through

While renewable energy sources are seen as the way forward, getting those supplies onto the National Grid also requires some major investment - in terms of cost and innovation. Claire Symes reports from Scotland.

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Foundations on 19 of the towers needed to be upgraded

How do you take six operatives, a drilling rig, an excavator and consumables, then only allow them to cross a river four times during construction of new piles to support an electricity pylon?

It sounds like a riddle out of a left over Christmas cracker, but this was the challenged faced recently by Bullivant as foundations contractor on a major power upgrade in Scotland.

“The route passes through several RSPB areas and also SSSIs. There are areas of protected moss and arcaeologically important areas too”

Hugh Sloan, Bullivant

The problem had to be overcome not just at one site, but at multiple locations in order to carry out foundation improvements on electricity pylons as part of an upgrade programme being carried out by main contractor Babcock for Scottish Hydro Electrics.

The project involves the upgrade of a power line built between Dounreay and Beauly in 1968. It will enable to connection to cope with the additional power coming onshore at Dounreay from the new wind farm developments that are starting to come on stream.

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The 153km route has 474 towers

When the 275kV power line was built to carry energy generated at the old nuclear power station at Dounreay, only the west side of the towers were strung. But demand is expected to grow, so now Scottish Hydro Electrics want to string the other side to double capacity. However, there were some concerns about whether the foundations of the existing towers could cope with the extra cable weight and the additional wind and ice loading that a second set of cables would bring.

In total there are 474 towers on the 153km pylon route and many have been assessed as having sufficient capacity to carry the extra weight, but Babcock is building six new towers are part of the upgrade.

Assessment showed that 19 of the mass concrete pyramid foundations needed to be upgraded to cope with the new loadings and Bullivant has the job of strengthening these. The number of towers that needed work belies the scale of the job though, because effectively these were 19 separate construction sites, spread over the route and some were several kilometres from forest roads, let alone main roads.

The foundation strengthening work was not going to be a simple task - not only did the power lines have to remain live while the work was carried out but many of the towers are a long way from any paved roads and located on environmentally-sensitive peat-rich ground. Add rutting stags, nesting seasons and shooting seasons, plus limited river crossings into the equation and the challenge increases.

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Bullivant used its excavator to lift materials and equipment across sensitive areas

“The route passes through several RSPB areas and also SSSIs,” says Bullivant regional manager Hugh Sloan. “There are areas of protected moss and also archaeologically important areas too. Some of the rivers are protected salmon spawning habitats as well and we are limited on the number of times we can physically cross these, so we have been using the excavator to lift equipment and material over.”

Bullivant overcame the access challenge by hiring a Hagglund tracked carrier from Norpower to get the rig, staff and consumables across the difficult terrain to each site. The excavator being used is fitted with low ground pressure tracks to ensure it can operate at all 19 locations.

When Babcock carried out its original investigations, it hired over 2,000 trackway panels from Eve Trackway to access each tower with panels laid in a double layer over the most challenging ground. However, the cost and time involved in laying such a track precluded this approach for Bullivant.

According to Sloan, the foundations contract is worth in the region of £800,000 but he says that around half of that cost is for just getting to the remote locations.

“Each of the tower legs is formed from 90º angle steel embedded into the mass concrete pyramid foundation,” explains Sloan.

“We have a spare rig on standby because of the remoteness of many of the sites”

Hugh Sloan, Bullivant

“We had to build a new pile cap supported on mini piles located around the mass concrete pyramid. This pile cap had to be located above the top of the mass concrete foundation formed below ground but below the last lattice point of the steel leg to provide additional support.

“Babcock supplied us with information about the corner loads to allow us to develop a solution, so the actual pile design varies not just from site to site but on each of the four corners. Because of the type of structure, and potential additional load from wind and ice, we had to consider vertical and lateral tension loading.

“The capacity of each pile varies but the loads that we were using in design were 400kN compressive and 350kN in tension from the tower, and another 18 to 20kN transverse load from environmental effects.”

The basic design is for 220mm diameter permanently cased piles with a 190mm diameter socket into bedrock constructed using superjaws to drill into the rock using an odex system. Each pile has a single rebar to full depth and a second 140mm diameter cylinder of 10mm thick steel to 6m below ground level.

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The power line was built in the 1960s but was only strung on one side

After work is complete, each tower leg will be supported by pile caps supported by three piles. “Rock was expected at a maximum of 4m below ground level at most sites but in one location - Tower 200 - there was sand to 20m below ground level,” says Sloan.

The bedrock is generally a very hard gneiss, but Sloan says that the site team has encountered sandstone at some locations. “We are looking at the need to test piles - using tension testing on a working pile - at every tower because the ground conditions are so variable,” he says.

When GE visited one of the construction sites, Bullivant was achieving one site per week but Sloan said that the rate was entirely dependent on overcoming the access at each site and also dealing with the ground conditions.

“We have encountered some large boulders during the work that could damage the rig,” he says. “We have a spare rig on standby because of the remoteness of many of the sites and we also have a full time fitter here to ensure delays are minimised.”

 

A lot of attention has been paid to the equipment being used for the work in order to minimise the weight. The cement grout for the piles is being mixed on site using a P11 grout mixer supplied with bagged cement and water carried to the site. Even the standard compressor for such work has been replaced by two connected smaller units to minimise the loading.

So far the solutions appear to have paid off and the construction is progressing well but Sloan was not keen to say that the work would finish on time due to the unpredictability of the elements, the locations, as well as the ground conditions.

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