Matthew Fowler, a site agent working for Balfour Beatty on the Hinkley Point C (HPC) Marine Works project, delivered a BTS virtual lecture in June on the offsite fabrication of tunnel segments and the head structures required for the critical systems within the nuclear power plant.

Since the completion of his MEng at Cardiff University, Fowler has spent his career working in large infrastructure projects, having worked the past nearly four years on the Hinkley project, mostly focused on the tunnel heads construction.

THE PROJECT

Having been recently featured in the two-part BBC documentary ‘Building Britain’s Biggest Nuclear Power Station’, Hinkley Point C in Somerset is the UK’s first new nuclear power station in over 20 years. It is a project of massive scale and complexity and when finally operational will power six million UK homes with low-carbon electricity.

Balfour Beatty is delivering the key infrastructure for the cooling water system. This entails the construction of three tunnels under the seabed of the Bristol Channel (two intake tunnels of around 3.5km each, and one tunnel for the 1.8km water outlet; the placement of intake and outfall heads on the sea bed; and subsequent offshore connection works.

Fowler focused on the construction of the segments required to build the offshore tunnels, and the construction of the head structures which takes place in the port of Avonmouth on the outskirts of Bristol, away from the main HPC site.

SEGMENT PRODUCTION

All the segments are reinforced conventionally, rather than being fibre reinforced. The 6m internal diameter intake tunnels have six segments per ring, while the outfall tunnel has a 7m internal diameter comprising eight segments per ring.

TRIALS

Located outside of Balfour Beatty’s main factory building is the trial ring assembly area where the physical trials were performed during the early stages of manufacture to demonstrate that segments would be suitable for use in the tunnels. In order to de-risk the activity and ensure compliance with the Construction (Design and Management) Regulations 2015, the physical trials were replaced after a number of successful rings by virtual rings created every 500 rings by 3D scanning of the completed segments and then reconstructing them digitally into a ring.

SEGMENT FACTORY

Inside the factory, the 66 segment moulds were designed and produced by CBE in France. Seven intake ring sets and three outfall ring sets formed part of a moving circular floor carousel which facilitated the production of all segment types.

The carousel system was digitised by Balfour Beatty’s BIM team which developed a 4D model, taking the viewer through the segment journey along the carousel system. The visualisations aided the implementation of additional safety measures related to Covid-19, and were used in inductions and briefings. This had the added benefit of overlaying text in multiple languages, so guaranteeing the multi-lingual workforce was engaged and had full understanding of the works.

With a team of around 75 people per shift covering production, lifting, logistics, quality and others, the factory ran on a schedule of 24h Monday to Friday with a 12h shift on Saturday. This allowed for one-and-a-half days of downtime for preventative maintenance which ultimately improved production outputs.

FACTORY OPERATIONS

The onsite batching plant provided a dedicated concrete supply for the production of segments. The concrete was delivered in a rail-bound wagon known as the ‘bullet’ which shuttled continuously back and forth between the batching plant and the depositing point above the moulds within the factory.

Just before the concreting, on designated work stations, the moulds are prepared, having the gaskets fitted and reinforcement cages installed, with checks made to ensure there is adequate concrete cover to reinforcement. Once out of the curing chamber, where 30MPa of concrete strength is achieved in just under eight hours, the segments are lifted with a gantry crane between different work stations to facilitate post concreting checks.

In 2020, the team completed around 130,000 safe lifts and performed more Class 1 nuclear grade pours than the rest of HPC combined.

CONSTRUCTING THE HEADS

Being one of the most critical components of the cooling water system, the intake and outfall heads are large, reinforced concrete structures manufactured offsite, just next to the segment factory in Avonmouth. The heads are to be placed on the seabed of the Bristol Channel, ready for the connection with the tunnels.

Each tunnel has two heads, therefore a total of six heads are required. The site layout arranges the four intake heads longitudinally along the site, with the two smaller outfall heads arranged transversally across one end. The site layout was developed based on the need to move these structures offshore, meaning they are set for an easy transition onto a barge at some future time.

OUTFALL HEADS

These two nuclear-classified structures each have a footprint of 16m by 16m, with an elevation of around 8m. They each contain around 245t of reinforcement, embedded in 1,100m3 of concrete. In addition, there are significant quantities of embedded high-grade stainless steel items which needed to be managed throughout the build process.

The structures were divided into 11 distinct pours and once completed, each head weighed around 2,700 tonnes.

The weight is of critical importance for the planning and management of the placement operation on the seabed. As a consequence, monitoring the weight of both outfall and intake heads throughout construction played a key role on design and building decisions, such as the selection of lightweight permanent formwork.

INTAKE HEADS

These larger structures, also nuclear classified, stand 8m high, 45m long and 17m wide. Each contains 1,540m3 of concrete with 772t of embedded reinforcement.

To construct the 4,500t structures, the concrete pour was broken down into nine main pours based on shape, complexity, encast elements and other items.

A specially-designed nuclear-classified concrete mix – which went through extensive and thorough approvals processes – was required for both the intake and outfall heads to combat the high density of reinforcemement and to resist the marine environment for almost 100 years.

3D MODELLING

The shape, size and complexity of these structures make them some of the most densely reinforced and geometrically complex in the UK. Due to this complexity, it was of paramount importance that the reinforcement detailing was completely clash free. To combat this, ADDA, a specialist 3D reinforcement detailer, was contracted to ensure any clashes were resolved prior to steel being placed on site.

Throughout the construction, the 3D models were available as a tool for the engineers and supervisors to evaluate the build. Given the positional tolerance of the reinforcement was +/-5mm, the models were critical to the success of the build.

With approximately 120,000 individual bars for each intake head, more than half of which were shear links, the productivity was of concern. The models allowed the works to be broken down in tonnage outputs which then fed the schedule exercises.

ENCAST ITEMS

Some of the key components of the intake heads included:

? 8no. super duplex lifting lugs weighing 5t per piece, critical for the placement offshore.

? 48no. copper-nickel baffle plates weighing 1t each, designed to control the water flow into the shaft of the structures.

? 186no. copper-nickel screen bars to stop large debris from entering the tunnels.

? Other items, such as edge protections, locking mechanisms for cover slabs, and hatches for future diver access in maintenance scenarios.

In total, 120t of both permanent and temporary encast items have contributed to the staggering weight of the intake heads.

EARLY STAGES OF CONSTRUCTION

During the first concrete pour of the intake heads – a 500mm-thick pour containing 25% of the required total reinforcement – all the focus was on achieving the critical tolerances for the fixing of rebar, so that each starter bar was perfectly placed for the next pour.

The second – and largest of the pours at 600m3 of concrete per head – saw embedded a total of 50% of the required reinforcement.

One of the biggest challenges with the largest pour was thermal management post-casting due to the significant amount of heat generated in the thick concrete sections. To ensure the temperature limits as detailed in the specifications were not exceeded, the pours were started at night and chilled water was used from the batching plant. Other measures were used, such as the insulation of formwork; covering the top of the pour with thermal blankets; and embedded monitoring sensors.

COVER SLABS

The cover slabs, the final components of the heads’ construction, are standalone concrete reinforced lids to cap-off the structures. They will be placed on the heads after the tunnels-to-shaft connections are completed.

LESSONS LEARNT

Fowler shares the lessons he took from across the two packages of works described:

? Collaboration at early design stage incorporating buildability input was key for the success of reinforcement fixing and concrete placement (e.g. the use of couplers to align with the concrete splits sequencing and to improve operators safety).

? Complexity of nuclear specifications must not be underestimated (e.g. a schedule impact of finishing and minor repair works must be taken account of).

? Requirement for smooth communication across the various operational functions to enable synergies.

? Development of project-specific processes and briefings to remove ambiguities and maximise production.

? Implement changes progressively and on a regular basis; take advantage of root-cause analysis and lessons learnt to ensure delivery success.