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Under cover agents

One of the longest box jacking operations ever undertaken is under way in Kent, and compact excavators are crucial to the project’s success, as Margo Cole reports.

Observant passengers speeding along the high speed line to the east Kent coast may spot a new bridge being
constructed over the tracks as part of a major road construction scheme.

What they are unlikely to notice is that a few hundred metres down the line, one of the largest box jack operations ever undertaken is slowly but surely creating a 126m long underpass beneath the live railway.

The original plan for the Cliffsend Underpass - a key component of the £87M East Kent Access Road - was to construct it as a pair of full box sections (roof, walls and floor), each 126m long, 7m high and over 10m wide.

Jacking capacity

However, the jacking capacity required to push each of these two boxes would have been prohibitive, and keeping control of the alignment throughout the jacking operation would be extremely challenging. This design would also have created a large open face ahead of each box, and the larger the face, the more potential there is for settlement of the ground above.

“This model is very scarce. There are only about six of them in the UK and we’ve got four of them”

Nigel McBurney, McBurney


Instead the site team, consisting of design/build contracting JV VolkerFitzpatrick/Hochtief, its consultant Atkins and subcontractor Mammoet, developed a solution by which the tunnel would be jacked in six sections, each the full width, but made up of just the top half of the box - the roof and half the wall.

The new design allowed for intermediate jacks to be placed between each section to share the pushing duties, so that, by the time the last section is in place, a full complement of 35,000t of jacking force is applied, but spread throughout the length of the structure rather than all being applied from the back.

The first task was to drive two 126m long, 3.05m diameter pilot tunnels through the rail embankment on what would become the lines of the box walls. The tunnels were excavated over eight weeks using an earth pressure balance tunnel boring machine.

Working inside these tunnels the contractor then installed 530 bored cast insitu piles, each 450mm in diameter and up to 14m long. They were installed in pairs at 1m centres using Klemm KR 702 limited height rigs.

Pile tops

The top 8m of the piles are reinforced and topped with reinforced concrete pile caps, which effectively create a wall along each side of the underpass alignment that supports the slide track for the jacking operation. Mammoet designed the jacking system, which consists of vertical jacks supported on elastomeric bearings that have a PTFE layer that sits on the stainless steel slide track.

The box itself is made up of six units, each weighing 2,500t and containing 1,040m3 of concrete and 240t of reinforcement.

“We cast each deck unit in the same location using the same falsework,” explains VolkerFitzpatrick/Hochtief agent Santiago Daniele. “It’s very important that the boxes are as accurate as
possible to minimise friction.”

A steel mining shield and cutting edge was cast into the front of the first deck unit to provide support and protection at the excavation face. This shield is divided into eight cells, and plant and miners can access each cell at the same time.

Jacking process

The actual jacking process involves alternately excavating a section of the face, with miners and their plant protected by the shield. Jacks to push the box along the slide rail until the shield is flush with the new excavated face. Tunnelling subcontractor McBurney is using Takeuchi TB138FR mini excavators to dig out the embankment, excavating about 200mm ahead of the shield before each push.

“This model is very scarce,” explains McBurney managing director Nigel McBurney. “There are only about six in the UK and we’ve got four of them.”

The appeal of this particular model is that they are “zero swing” excavators with offset booms. Unlike conventional tail swing compact excavators, which have a rear counterweight that extends beyond the tracks when the body rotates, the body of a zero swing unit stays within the width of the tracks through the full rotation. This allows the operator to focus on digging, without worrying about causing an obstruction when the cab swings round.

They can be easily manoeuvred within the confined space of the tunnel, and set to work with their tracks perpendicular to the tunnel face and the cab facing forwards.

Offset booms

The Takeuchi machines are not only zero swing, but also have offset booms.

On a conventional compact excavator, the boom is fixed in one position on the front of the excavator body, and can be slewed from this position, typically digging in an arc shape, or requiring constant repositioning to excavate a square face.

The offset boom can be positioned anywhere on the front of the cab, making a square cut easy to achieve simply by lining up the cab with the face.

In an excavation like this, where a section of face is exposed before each push, a parallel face in essential to maintaining the integrity of the excavation.

The result of these two functions is that McBurney can position the excavators in alternate cells within the shield, and a loading shovel can work in the intermediate cells confident that the machine will not swing round and that the excavator tracks will not get in the way.

Low tech

McBurney admits that the solution may look a little low tech compared to other options - an army of compact equipment rather than roadheaders, for example - but his choice has paid off: the mining team is achieving an average rate of 1.5m of excavation per 12 hour shift and a maximum of 2m, and is ahead of the anticipated programme.

They are working on two shifts around the clock, stopping for three days each time a new 22m long deck section needs to be pushed into place at the head of the tunnel ready for the next stage of jacking.

Excavation itself is fairly straightforward, with the compact excavators easily coping with the lightly weathered chalk that makes up most of the rail embankment.

Once the entire box has been pushed into place the jacks will be removed and the box section will sit on top of the pile cap wall. The contractor will then dig out the lower section of the tunnel and cast a floor and internal non-load bearing walls.


East Kent Access

Cliffsend Underpass is a key structure on Phase 2 of the £87M East Kent Access Road.

The road sits in a 15m deep cutting before running beneath the London-Margate railway line (NCE 23 September 2010).An overbridge was ruled out because of the proximity of local properties and the geometry of the site, making a jacked box solution the most viable option.

Readers' comments (1)

  • One major omission to the description was that the jacked deck is a patented technique (James Thomson) and the design of the installation was undertaken by Jacked Structures Ltd.

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