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Piling: End of an era

Pembrokeshire’s coastline is beautiful in the sunshine but the exposed location makes piling work for St Davids’ new lifeboat station a challenge. 

With more than a decade of experience of working for the same client on similar contracts you would expect the final piece of the puzzle to be fairly straightforward. However, work to upgrade the lifeboat station at St Davids in Pembrokeshire is calling on all the experience gained and more to complete the most challenging scheme of the project.

St Davids

Major rock stabilisation had to be carried out ahead of the piling work

Main contractor Bam Nuttall is part way through the final phase of a nationwide project for the Royal National Lifeboat Institution (RNLI) that has involved difficult logistics, severe weather and environmental issues. Yet the work at St Davids is described by Nuttall as unique despite 10 years of experience on the RNLI framework contract to upgrade numerous lifeboat stations for the larger “all-weather” Tamar class boat.

The project reunites a team that have been delivering similar schemes around the UK’s coastline since the first project in Padstow, Cornwall in 2005 with Bam Ritchies undertaking ground engineering work for Nuttall and working with project manager Royal Haskoning and designer Opus.

“This project is unique compared to the others we have undertaken for the RNLI in that it is being built in a new location, rather than being a direct replacement, and the remote location and lack of infrastructure has made it the most challenging scheme within this framework,” explains Bam Nuttall contracts manager Richard Hayman.

The new station is in the next cove over from the existing 100-year-old station, which is a listed structure, and is built on top of the slipway from the original 1869 lifeboat house that still nestles behind on the beach.

In total 27 Tamar class lifeboats have been commissioned for service and the last one was delivered in 2013 but, until St Davids’ new lifeboat station is completed, not all of these new boats have had launching facilities to match the boats’ all weather capabilities. St Davids’ Tamar boat, named Norah Whortley, has been moored offshore on a pontoon since it was delivered in April 2013 as it, like many of the boathouses Nuttall has worked on, is too large for the existing boathouse.

“Through the framework we have undertaken around £80M of work that has involved construction of seven new lifeboat stations and upgrade of another five,” says Hayman. “Each time we have completed one of these projects we’ve said that it was one of the most complicated but this one really is the most challenging.

“The new builds have previously been built on top of the existing structure which has given a platform to work from but the new location here has called for a different approach.”

Completion next year

Work on the £9M upgrade project at St Davids started in 2014 and the contract has another year to run but interested locals and tourists tramping the Pembrokeshire Coast Path can now see the scheme taking shape. Bam Nuttall’s sister company Bam Ritchies is getting close to finishing the second phase of piling and work on the boathouse structure is beginning to take shape with the floor slab being cast insitu.

Few people approaching the peninsula beyond St Davids can miss the fact that a major construction project is underway – there can’t be many occasions when you can spot a tower crane on the Pembrokeshire coast. The remote location with only narrow lanes leading down to the cove at St Justinians has meant that logistics have been challenging. Many pieces of equipment and materials have had to be brought in by boat and lifted into position by the tower crane. Even the choice of the piling rigs used for the earlier land-based phase of the foundations work was limited by the size of crawler crane that could be brought in to position the rigs on the shoreline.

Before work on the foundations could get underway, Ritchies had to carry out major rock stabilisation work around the back of the cove. The solution involved both permanent mesh and rock bolting to provide long-term protection of the lifeboat station and temporary mesh that will be removed at the end of the work to ensure the safety of both Nuttall’s and Ritchies’ site teams.

“Work on the stabilisation started in August last year as rock mass assessments had identified the risk of slab failure within the steeply dipping mudstones and siltstones that form the cliffs of the cove and underlying ground conditions,” says Bam Ritchies project manager Phil Howard.

In total 469, 20mm and 25mm diameter stainless steel dowels up to 3m in length and 20 8m-long rock anchors were installed by rope access techniques. The stabilisation was carried out to a design undertaken by Devon-based consultancy Frederick Sherrell, which has also been a long-term partner on the RNLI framework. The bolted rock mass was then covered with 3,150m2 of permanent Maccaferri netting and a further 572m2 of Maccaferri’s Armatec to provide the temporary protection.

St Davids

Precision is vital but tricky due to the sloping rock bed and over water working

Tidal range challenge

Piling work undertaken by Ritchies has been split into two phases – land-based and marine-based working from a pontoon. The description of the first stage, which was undertaken between September last year and February this year, as land-based belies the challenge as much of the work was within the tidal ranges limiting the windows available for work.

The minipile solution for the boathouse in the cove and cliff top “abutment” piles for the access ramp involved installing 407mm diameter piles to 6.5m in the cove and to 11m on the clifftop.

“In total we installed 54 piles in this phase – 48 for the boathouse in clusters of three to form a pile cap and six for the abutment,” explains Howard. “The pile cage was grouted in place with 52.5N cement.”

According to Howard, the piles themselves were not challenging but some of the locations were tidally constrained and work was initially limited by the piling rig being used. “Nuttall constructed a temporary dam across the mouth of the cove using an old shipping container to reduce the impact of bad weather on the work,” he says. “They also created a refuge for equipment that enabled us to position it above the high water mark rather than having to lift it to the cliff top after each low tide.

“We started the project using a Hutte rig as that was the largest one we could get into position due to limitations in the size of crawler crane available, but the demands of the project were at the limit of the rig’s capacity. Fortunately we managed to source a 400t crane with a narrow wheelbase that could get onto site and lift a higher capacity Casagrande rig down into the cove.”

Although the higher capacity rig and dam helped to improve the efficiency of the work, Ritchies still had to plan ahead and keep an eye on the weather. “The tidal range here is up to 4.5m and there are also swells that can mean there is effectively no low tide,” explains Hayman. “The currents here are also strong, so safety is a priority.”

Experience gained in understanding the steeply dipped and weathered siltstones and mudstones ground conditions during the landbased work stood Ritchies’ team in good stead when they returned to site for the marine-based phase in May.

The slipway piling is being undertaken by a bespoke RH600 drill rig on a jack-up barge – both of which have been used on previous RNLI projects where Ritchies has collaborated with Nuttall.

The 762mm diameter piles are being installed 6 to 8m into an 864mm diameter rock socket. The sloping bedrock means that a conductor pile is being reamed through the weathered rock to create a seal before the socket is drilled using a Numa down the hole hammer (DTH).

“The tolerance for positioning the piles is 75mm but this is quite challenging given the marine nature of the work and the steeply sloping bedrock,” says Howard. “The rock is generally 8MPa in strength but has harder and softer bands and where necessary we have reamed the casing further using a tungsten cutting tool.”

St Davids

The original 1869 lifeboat house is a listed structure

10-hour shifts

Once the rock socket has reached the required depth, 40mm clean stone is placed around the annulus of the pile and it is then grouted into position. The rig is also powered by six 960 compressors to ensure efficiency and there is a space DTH in case of breakdown. Howard says that work so far is going well. There are 22 piles in total to install and, when GE visited site, five had been completed. “We have been completing 0.6 of a pile per day,” says Howard. “We’re out of the tide here so we are able to work full 10-hour shifts.”

Work on the pontoon is aided by a 90t crawler crane to help reposition equipment and lift materials from boats that are ferrying it to site from Pembroke Dock.

The slipway piling is expected to be completed by mid-July, clearing the way for Nuttall to start work on the 70m-long slipway which will be formed by precast concrete beams placed onto the piles and then built up in 6m steel bays. This work will be carried out in tandem with construction of the timber-framed boathouse structure that will fit onto the concrete floor slab that is currently being cast.

It was a beautifully sunny and calm day when GE visited site in June and Hayman and his team are keeping their fingers crossed for more days like that. “If the wind gets up above 38km/h then we have to stop using the tower crane, which limits what we can do but so far the weather has not been a significant problem,” he says.

Reflecting back on the last 10 years working on the RNLI contract, it is clear that Hayman is proud of the work the framework partners have delivered during the time as well as the experience and knowledge they have gained along the way. Although Nuttall still has another year of work to complete at St Davids, Hayman appears sad that this work is coming to an end. “The introduction of the Shannon class of boat will mean RNLI will need to undertake further investment at lifeboat stations but that class of boat is smaller so the work will not be of the same scale as that needed for the Tamar class,” he says.

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