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Slopes: Cut and hold

5

Work has been underway for 10 months on the groundworks for Jersey Electricity’s new £17M St Helier West primary substation

Jersey Electricity identified a need to reinforce its power network to the north and west of St Helier, the Channel Island’s main town, as long ago as 2006.

Siting a new primary substation was critical to this work. It had to be to be located between two existing substations – Esplanade Primary and Queen’s Road Primary – and it had to integrate into the existing 11kV and 90kV networks with minimal cable lengths but maximum cable separation to ensure resilience. The site also had to be accessible for maintenance but far enough from residential areas to avoid causing a nuisance.

Finding such a site was not easy, and between 2009 and 2012, planners and Parish officials considered seven locations with Jersey Electricity before the utility found, considered and then acquired what seemed to be the best one at Westmount Gardens.

This, on the western outskirts of St Helier, is an area of landscaped gardens traversed by footpaths sited on ground that slopes from north east to south west towards the coast and St Aubin’s Bay.

Sounds idyllic. But this idyll hides an industrial past that introduces plenty of geotechnical challenges.

“The site was the former location of a quarry dating back well over 100 years,” explains Laurence Tomlin, senior associate at scheme designer Peter Brett Associates. The quarry extracted volcanic bedrock from the St Saviours Andesite Formation of Precambrian age, which was present within a

Synclinal structure at the quarry site.

Quarrying operations ended before 1867 and the site was restored to gardens with a masonry footbridge constructed across the former quarry entrance. In the early part of the 20th century the footbridge was taken down and a retaining wall constructed in its place, with the quarry behind the retaining wall then backfilled – with what turns out to have been less than ideal backfill.

Geotechnical investigations to assess the ground conditions at the site showed that the backfill material was incinerator ash. The ground investigation also uncovered the presence of stiff gravelly silts and clays with angular cobbles of weathered andesite overlying andesite bedrock.

This was going to have to be dealt with as, to build the substation, a level platform was needed at a level consistent with St Aubin Road that runs along the southern – and lowest – site boundary.

Achieving this was going to require an excavation in excess of 15m deep at the rear of the site – and that excavation would have to be stable in temporary and permanent states.

The small area of the site, and the fact that all the excavation works would have to be kept within the site boundary meant that options for forming the excavation were limited, explains Tomlin. Nevertheless, various options were considered at design concept stage including construction of a reinforced concrete retaining wall, construction of a contiguous piled retaining wall with ground anchorages, and a combination of soil nailing and contiguous piles.

By tender stage, details for the development involved a combination of soil nails to support a temporary cut slope in the upper part of the site, with anchored contiguous piles used to form the temporary cutting in the lower part of the excavation to building platform level.

The permanent solution for the retaining works at tender stage was a reinforced concrete wall that would also require compression and tension piles. In addition, the substation building structure was to be supported on piles drilled into the bedrock where the building footprint stepped from rock to the silts and clays.

Local civils contractor Jayen was keen to explore a different option, and brought in Peter Brett Associates to assist with its tender. It was specifically tasked with the temporary works and pile designs, which were to be contractor designed, and with considering alternative designs to value engineer the project.

During this process the consultant developed an alternative design that would incorporate the temporary works into the permanent design. “The final value-engineered design we developed included for the temporary cut slope to be formed at 60˚ to the horizontal, stabilised using soil nailing techniques,” explains Tomlin. “The soil nails would also then be used to provide the permanent retaining solution,” he adds.

This was to be a Reco precast panel wall and steel reinforcing strips connected to the soil nails – so that the whole system would effectively be a hybrid reinforced soil design with a concrete panel facing. This system was developed and designed in association with Reco and successfully tendered by Jayen, with work starting on site in February 2016.

This brought other challenges, explains Tomlin. “The small area of the site meant that there were considerable constraints with the programming of the works, in particular the excavation, which needed to be done in stages to allow for the soil nailing works to be installed as the excavation progressed,” he says.

There was an added issue during excavation, in that the ash and clinker backfill that had to be removed was identified to be a hazardous waste and had to be separated from the natural soils during the excavation. There was also a requirement to frequently test the materials to demonstrate the requirements for waste acceptance.

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Quarrying operations ended before 1867 and the site was restored to gardens

Sourcing suitable Class 6I fill to replace it was hampered by the limited availability of resources on Jersey. However, a material locally known as Hoggin – which comprises residual soils derived from the weathering of the granite bedrock – was blended with some recycled aggregate for a consistent sandy gravel conforming to Class 6I that also has a good effective angle of friction.

“This material works very well in relation to the construction of the Reco panels and it has also been possible to achieve well in excess of 95% compaction with relatively small compaction plant,” notes Tomlin.

More challenges emerged as excavation progressed.

“The soil nailed temporary cut slope not only had to be designed to stabilise the ash and clinker fill but had to be designed to stabilise poor quality soil,” explains Tomlin. “And as the excavation progressed, weathered rock was encountered and the soil nails had to then become rock bolts to provide the stability of the temporary cut face – and also act as anchorages for the Reco wall,” he adds.

The final temporary cut was just over 20m deep at the rear of the site with soil nails ranging in length from 5m to 8m and with rock bolts ranging in length from 3m to 5m.

Local subcontractor Albion Drilling Group carried out soil nailing and rock bolting works. Innovation came during the project, with a modification to the Reco wall system designed to speed installation, saving time and money.

“The wall was originally designed to include a positive connection between the steel strips and the soil nails,” explains Tomlin. “This connection was developed between Dywidag Systems

International and ourselves and included an arrangement that would allow for adjusting the lengths of the connections so there would be no play in the system.

“But as the design was developed it was determined that this may be an expensive system and installation would be slow and affect programme,” he continues. “So Reco developed a frictional connection.”

This frictional connection effectively comprised primary strips connected to the rear of the precast concrete panel units and secondary strips connected to the soil nails with an overlap between them that would work in friction with the soil fill – removing the requirement for any positive connection.

Wall construction is now well underway, but due to site constraints more issues are expected.

“There will be issues to overcome in respect of getting fill behind the wall as it progresses in height and Jayen is gearing up to crane this in to place,” explains Tomlin.

Once completed the wall will be clad in granite to match the finish of the substation building and, says Tomlin. It will form an impressive feature in what is a prominent location – making all the effort worthwhile.

“In all this has been a very interesting, and at times challenging, project to be involved in,” he concludes.

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