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Earthworks: Digging deep at Hinkley

hpc site progress 24th aug 2017 025

Earthworks play a key part of work on Europe’s largest construction site at Hinkley Point C

The pros and cons of nuclear power and its place in the supply of electricity to the UK have long been debated. The anti-nuclear lobby highlights the cost of construction, safety and the issue of waste, while those with a more positive outlook underline the desperate need for clean electricity as other power stations reach the end of their productive lives.

At Hinkley Point on the Somerset coast, a workforce of thousands is concerned with pushing through the construction process to deliver the UK’s first EPR nuclear power station, which will generate 3.2GW of electricity – enough to power 6M homes.

The site itself is huge. Stretched across a section of coastline in the shadow of the existing Hinkley Point A and B plants it is a warren of worksites and access roads, people and plant.

In the middle of this site Kier Bam, the fully integrated joint venture between contractors Kier Infrastructure and Bam Nuttall, with its specialist geotechnical division Bam Ritchies, is quietly going about its work under the £460M OH2001 Earthworks package.

There are four specific sections to this slab of work for which Kier Bam will provide bulk earthworks, temporary and permanent roads and networks across the site, plus temporary structures including a 760m long sea wall and finally the “Deep Dig” – essentially the foundations for the two nuclear reactors.

“The Deep Dig is a tough project in itself,” says Tom Moore, who is Kier Bam Deep Dig construction manager. He is leading delivery of this section of the work.

nuclear island handover photo 22nd jan 2018 006

nuclear island handover photo 22nd jan 2018 006

The Deep Dig is essentially the foundations for the two reactors

“We are carrying out the excavation and slope protection for the ‘nuclear islands’ – the foundations for reactors one and two – obviously everything is being built to nuclear standard and the quality assurance procedures reflect that,” he adds.

But the scope of the Deep Dig work goes beyond the construction of the nuclear islands. As well as the slope protection work, the team is tasked with installing a dewatering system, constructing rock replacement foundations, installing an earthing system, vertical profile dowel installation as well as ground investigation work.

It is a complicated working programme. The dig itself has been spilt into separate zones – more than 50 – each of which has a different “platform” level that the team must construct to. This will become the formation level that Bylor, the joint venture contractor for the main works package, will build from.

“There are so many different levels,” says Moore. “The deepest zone is the ‘heat sink’ section at the northern end of the dig. There are 80m by 80m shafts here that are excavated to 21m below ground level.”

There are complicating factors that surround the dig too. Thanks to the nature of the bed rock around the site – mainly fissured mudstone interbedded with limestone – the team has sometimes been forced to break out well beyond formation before meeting sound material. The mudstone’s mineral content also means that vertical rock faces can only be left exposed for a maximum of two days before the rock starts to break down.

“Much of the mudstone is pyritic that starts to oxidise and degrade as soon as it is exposed. We need to get that rockface inspected and covered within 48 hours. It soon starts to turn back into soil,” says Kier Bam technical manager for the Deep Dig David Lindfield.

Some of the figures for covering those rockfaces are astonishing. In all some 32,000m3 of sprayed concrete will be applied alongside 100,000m of ground nails and a 45,000m3 of blinding and substitution concrete which is used to replace the over-excavated rock.

Across the site more than 100,000m2 of rock slopes must be formed. These slopes are supported through the installation of ground nails, which provide global slope stability coupled with sprayed concrete forming a weathering protection to the rock face that is supplied from two dedicated batching plants. In places, the Kier Bam team has re-engineered the original tender requirement for the installation of diaphragm walls by designing and installing vertical nailed and spray concreted slopes instead. The team has used its experience honed on projects through Nuttall’s ground engineering arm Ritchies to great effect.

“We recognised that we could replace the diaphragm wall in the original design of the heat sink with passive fully bonded ground nails and sprayed concrete,” says Lindfield. “We had done similar on other projects and it offered significant costs savings at Hinkley Point – around £20M at the time we developed that method in 2009.”

Moore adds: “To deliver the project we have some very skilled and experienced staff that we can call on. For example many of the sprayed concrete teams honed their skills on London’s Crossrail project and have transferred directly over from there.”

The ground nails themselves are up to 14.5m long, 40mm diameter Dywidag steel bars which are being installed in a diamond grid pattern, with standard spacings of 2.5m dropping to 1m in poor or faulted areas.

The team has divided the slopes into “matrix” and “designed” slopes which reflect the level of surcharge loading and overall slope height. Designed slopes have a predetermined support solution based on conservative rock mass parameters that are validated in the field prior to installation. There is a large number of matrix slopes, so the team has developed a series of support classes which enables it to select the appropriate support class for a particular slope.

hpp site progress 19th april 2018 082

hpp site progress 19th april 2018 082

The team has divided the slopes into “matrix” and “designed” slopes which reflect the level of surcharge loading and overall slope height

Lindfield explains: “We excavate the slope which is then inspected and assessed by geologists. From that assessment we select a predesigned nail pattern from the matrix. It means that support for the matrix slopes can be optimised, based on actual encountered geology providing a more efficient support solution and allowing rapid reaction to change.”

In the heat sink area, where the slopes are vertical, the team has installed a system of dowel bars to help guide the team breaking out the rock. Drilled into position, they are installed at 1m centres and in two tiers across the heat sink at a maximum height of 18m for each tier.

“They have been installed to help profile the vertical slopes,” says Lindfield. “We were concerned about over break in the higher slopes and they are there primarily to provide a guide, but they also reduce the excavation disturbance to the rock face. Here in the heat sink, that zone is around 1m from the face. For the rest of the Deep Dig it is 3m. This has enabled the ground nail design to be further optimised in these areas.”

There is still plenty of work for the Kier Bam dig team to clear before it hands over to Bylor, but with the efficiencies it has developed during the delivery of Unit 1 nuclear island, that handover date is creeping closer.

Innovative installation

All engineers share the trait to automatically want to problem solve and this has been hugely beneficial at Hinkley. Confronted with a project the size of Hinkley Point C and the Deep Dig, even the slightest adjustment in working practice can make a huge impact on safety, efficiency and cost effectiveness.

When the Kier Bam team realised it was going to have to install over 100,000m of ground nails across the project, the efficiency and safety of its nail installation process came into focus.

“Normally when we install ground nails they are split into manageable lengths and then manually connected as they are being fed into the borehole,” says Lindfield. “We wanted to improve efficiency and eliminate or reduce manual handling so needed to develop a system of lifting the bar in one length and installing it.”

kbjv mechanical installation beam

kbjv mechanical installation beam

The Ground Nail Installation Beam is a nail handling device mounted on a standard excavator body

The result is the “Ground Nail Installation Beam”, a nail handling device mounted on a standard excavator body. This can pick up, deliver, and feed up to14.5m long ground nails into the boreholes safely, accurately and efficiently.

“The nail is fully enclosed by the beam which can be extended to handle a range of lengths as well as all diameters of bar,” explains Lindfield. “It can also install the grout tremie pipe at the same time which is critical for the quality of the nail installation.”

The initial design has been tailored during work on site, but it has proven to be a huge boost to the efficiency and safety of the installation work.

“It has eliminated any working at height and manual handling issues while improved our process efficiency. It has been a great success,” says Lindfield.

It’s not just in the installation of the ground nails that the Kier Bam team has pushed the boundaries – it has also taken a similar approach to its monitoring.

It has used a system from specialist company Dywidag Systems International to measure the force and stress in selected nails, helping the team assess their performance.

The Dyna Force measuring technique is based on the elasto-magnetic properties of ferromagnetic materials such as steel and is carried out using sensors that measure the change in magnetic permeability of a steel bar as it is loaded. By measuring that change, the amount of stress in the nail can be accurately determined.

“More traditional load cell monitoring systems can be affected by temperature changes or vibration. This Dyna Force system allows us to monitor the performance of the nails accurately and simply. There is less to go wrong with it and it is easier to install. Its simplicity is its major benefit,” says Lindfield.

 

 

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