Adding an extra lane to a roundabout may sound simple but the M25 Junction 2 improvement scheme is calling for some complex piling work.
Motorists travelling over junction 2 on the M25 in Kent are probably oblivious of the ground engineering work underway just metres below them but drivers passing the work will be eagerly awaiting its completion. Careful planning means that the work to deliver an extra lane that will ease traffic congestion is not encroaching on the busy roundabout that connects the A2 to the M25 so is not adding to the problem.
The roundabout is a regular bottleneck for traffic moving from the northbound M25 onto the A2 and for eastbound A2 traffic connecting with the M25 with congestion common at peak times. The M25 Junction 2 improvement scheme will add a new lane to the northern side of the roundabout with a twin piled retaining wall cutting through the bridge abutment of the 1970s structure that carries the M25 overhead to make space.
The work is being delivered by main contractor Bam Nuttall under the call off framework agreement with Connect Plus Services, which undertakes maintenance and upgrade work on the M25 for Highways England.
Piling work for the scheme is currently being undertaken by Bam Ritchies but, while the ground conditions with piling in Chalk are good, the location of the work is potentially problematic.
Ritchies is using low headroom techniques to overcome the restricted headroom challenge but, despite being surrounded by motorways and A roads, the access issues are less easy to resolve.
“Logistics is a challenge as the work is in the middle of a busy motorway junction with access only from the eastbound A2 and the low headroom bridge effectively splitting the site into two halves,” says Bam Ritchies assistant geotechnical engineer Hollie Colville.
“When the low headroom rig is operating under the bridge, it is only possible to move from one part of the site to the other on foot.”
Work on site started in early August with the first full height Tescar piling rig brought into site on a night shift due to the traffic management needed to get it into position. The low headroom Tescar rig was brought in a few days later.
The design, which was undertaken by Atkins, uses a twin row of 600mm diameter piles to 11.2m depth that will extended 0.9m above piling platform level to form the retaining wall below the bridge. The restricted 3.8m headroom meant that a single row of large diameter piles was not possible.
These 109 piles are spaced at 750mm centres and will be tied together with capping beam later.
The cage length on these piles is 12.1m.
The wing walls are formed by 450mm diameter piles with 22 on the eastern side and 32 on the west. The depth of these piles varies from 4.6m to 8.9m depending on the distance away from the underbridge retaining wall.
Ritchies has also installed piles for the new bases for three overhead gantries that will need to be widened as part of the work. Each of these bases called for a pair of 750mm diameter piles to 8m or 9m and work is now underway on casting the pile caps for the new gantry supports.
Colville said: “One of the biggest challenges for the piling under the bridge is the headroom. We are using a 0.5m long auger with a telescopic Kelly bar for the piles under the bridge.
“Verticality is critical for the closely spaced piles in the twin wall though, so we are having to take the boring of these piles slowly to ensure this is maintained.”
The piles are being bored through the Seaford Beds within the Chalk and work has encountered a number of large flints which have the potential to push the auger out of verticality.
Bam Ritchies site foreman Steve Smith adds: “The slower augering helps to ensure a clean, straight bore.”
Placing the cages is also tricky due to the lack of space. Ritchies is using cages formed by Dywidag Gewi bars which are prefabricated offsite in four sections.
According to Bam Ritchies project manager Andy Morris, this design came about through some early contractor involvement following the ground investigation which Ritchies undertook in two stages in December 2017 and March this year.
“They are more expensive than a conventional cage but there is a real productivity benefit in the low headroom environment and the couplings help to minimise the safety issues from splicing the sections together,” he says. “I think this is the first time cages of this type have been used in the south of England.”
The cages for the wing walls are more conventional and made by Midland Steel in two sections with a mechanical zip tie.
The work on the abutment is being carried out in bays with Bam Nuttall cutting into the slope of the abutment in sections to create space for the twin piled wall in stages. The bay approach may ensure stability of the M25 overhead but it complicates positioning of the rig.
The piling platform was also designed by Ritchies with input from Campbell Reith and features lego-like precast concrete retaining blocks on the outer edge to allow the work to be undertaken without restricting the traffic lanes on the roundabout.
The congestion the project is aiming to relieve also creates challenges for deliveries to site.
Communication is key when it comes to concrete deliveries, according to Morris. The C32/40 mix is being supplied to the site by Hanson’s depot in Greenwich, which is 20km away but when the traffic is congested there is huge potential for delays.
“We are using open bores so we cannot leave them standing open overnight, so we need to be careful about sequencing,” says Colville.
Smith adds: “We have just come off a similar job for Network Rail in Streatham so we have been able to bring the lessons we learnt about sequencing to use on this project.”
One of the other challenges on the project is noise – not from the rigs making too much noise but from the noise of the traffic on the M25 overhead, the A2 below and on the roundabout itself. The site team is about to start a two week trial of Eave ear defenders to allow Bluetooth connection to communication systems.
Despite the challenges, work on site is progressing well and ground engineering work is expected to finish at the end of September. The overall scheme is scheduled for completion in March next year.