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Under the sea

Near Edinburgh, piling specialist BAM Ritchies is playing a vital role in the delivery of a brace of huge 65,000t aircraft carriers.

There cannot be too many more impressive or fearsome sights, when flexing military muscle, than that of a fully laden aircraft carrier, steaming at full tilt through the ocean waters. The Royal Navy has recognised this and in a bid to boost its fleet has commissioned the construction of two new aircraft carriers. HMS Queen Elizabeth
and her sister ship the HMS Prince of Wales are set to weigh 65,000t and measure 280m long, 70m wide and 56m high.

The two will be built in sections at dockyards around the country but assembled at the Babcock-owned Rosyth dockyard on the Firth of Forth. But the dockyard’s existing facilities are not quite big enough for the modern shaped, flat-bottomed, slab-sided super ships and contractor BAM Nuttall is carrying out alterations to Number 1 dock in a £35M deal to enable it to accommodate them.

Its sister company, piling and ground engineering specialist BAM Ritchies has been commissioned in a £2.5M deal to take on the ground anchor installation work on the concrete dock walls and install a forest of piles that will help provide the foundations for the huge new 120m-span 1,000t capacity Goliath gantry crane that is required alongside the dock.

"It’s a big crane," says BAM Ritchies senior geotechnical engineer Ron Reilly. "She will be lifting some pretty hefty sections when they are in the dock." Cranes of that sort of magnitude require some solid piling work below ground to resist the huge forces being exerted as it trundles along set tracks.

Some 292 raking piles are being installed along with another three vertical piles. All of them are 600mm in diameter and are being bored using what BAM Ritchies claims is the largest reverse circulation Numa Super Jaws system in the UK with a 530mm T590RC coupled with a C210RC reverse circulation hammer. "Most of the piles are raked on a 1 in 4 incline, about 140 from vertical," says Reilly, "but there is an access culvert in one area which is why we will be installing the three vertical piles. It is purely a move to strength around that culvert," he adds.

The boring system is backed up by a Zeppelin piling rig alongside a bespoke BAM Ritchies rig mounted on a 25t tracked excavator and will be used to bore each of the reinforced concrete piles which are filled with a C32/40 concrete mix.

The BAM Ritchies team is installing permanent 610mm diameter steel casings, 10mm thick, advancing them through superficial soils in a series of nominal 2m long threaded sections down to underlying rock head.

The Numa 210 reverse circulation overburden drilling system features a 210mm down-the-hole hammer and a 632mm Super Jaw button bit at the business end. It is this section that will smash its way through the ground. "The Super Jaw system has a set of wings which open up to 632mm diameter as soon as weight is applied to the drill. These basically under ream the casing and, once we get to the depth required, close when we drop the pressure on the system. This allows us to draw the head back up through the casings," Reilly explains.

Each section of casing is threaded together enabling the system to be advanced by a top drive unit. The system allows the rod to be connected to the head while continually holding onto the casing. Once the rod has been connected the casing can be offered up and threaded into the drive head.

It is a system which makes installation more efficient while bringing benefits to surrounding structures. "This method means there is a reduction in air loss to the ground which greatly reduces disturbance to the foundations of surrounding structures. It also allows for more control over the discharge of the drill arisings. It acts like a big vacuum cleaner and all the loose material is sucked up through the drill string itself which draws more air away," explains Reilly.

On reaching the rock head the casing will be hung at the base of the pit on a series of welded flanges attached to it. These will enable the drive head to be disconnected from the casing while the hammer and Super Jaws bit are further advanced to drill the rock sockets which will measure either 3.2m or 7.8m.

Once the sockets have been drilled to the specified depth into the mudstone and sandstone bedrock the bit will be retracted through the casing enabling the hole to be flushed, the drill string removed and the borehole checked by CCTV. As soon as the steel reinforcing cage has been dropped into position the concrete will be placed through.

Groundwater issuing from the borehole as concrete is placed will be pumped to a settlement tank, to be filtered then returned to the dock. It is a working method that Reilly and the team will become well versed in, with pile installation due for completion by the end of summer 2010.


Scheme: Rosyth CVF infrastructure upgrade

Client: Babcock Marine

Civil engineering contractor: BAM Nuttall

Drilling and piling subcontractor: BAM Ritchies

Designer: Halcrow

Ground anchoring

The pile installation work is not the only part of the project that the BAM Ritchies team is carrying out.

The team has recently finished installing 150 ground anchors through the side of the concrete dock wall at Number 1 dock into the surrounding bedrock.

These seven strand anchors stretch between 15.5m and 20.5m into the rock. Their purpose is to help secure the sides of the dock following work to expand it. This will involve removing granite blocks that bench out from the bottom of the dock creating a V-shaped cross section.

Advances in marine architecture have meant the shape of the latest warships has become more flat bottomed and slab-sided, forcing the cutting and removal of these granite blocks.

At each anchor location a 225mm diameter and 350mm deep core was taken into the concrete at an angle of 450 on the west side and 300 on the east while the team used a drill mast mounted on a work platform suspended from a 3600 excavator to drill the anchor holes themselves. The two-man drill crew then began drilling from the base of the pre-cored hole using a 203mm diameter bit attached to an Atlas Copco 150mm down-the-hole pneumatically powered hammer.

The team prepared the anchors on the dockside with plastic lantern centralisers fitted at no more than 2m centres over the fixed length of the anchor. The plastic strand sheathings are trimmed back to allow the lifting head to be fitted. This grips onto two anchor strands and the top wedge retention plate is secured in place with two bolts.

A crane then lifts the anchor and positions and lowers it down the hole until it is fully installed. Each anchor is grouted into position using chem1 42.5 grade cement mixed at a water/cement ratio of 0.4:1.

The team installed 500mm by 300mm2 head plates bedded on 60N/mm2 mortar before the final bearing plates were added. The anchors were then stressed using a Dywidag 2,000kN hollow ram stressing jack.

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