Contractors widening the M25 motorway have this week successfully completed one of the most challenging sections.
Balfour Beatty and Skanska engineers have widened the 230m long Berry Lane viaduct near Chorleywood, the biggest single structure on the 36km section between junctions 16 to 23.
This portion occupies most of the north-western quadrant of the M25 between the M40 and the A1(M) junctions. It is being widened by the Connect Plus consortium of Balfour Beatty, Skanska, Atkins and Egis Projects.
As part of its £6.2bn concession it will also widen the M25 from three lanes to four between junctions 27 and 30 and refurbish the A1(M) Hatfield tunnel (see box below).
Berry Lane viaduct was built in 1978 as one of the formative parts of the M25 to carry three lanes of traffic with a full hard shoulder in each direction. The widening scheme increases the width by one full lane in each direction.
The extra lanes are accommodated by “stitching” a 4.75m wide extension onto each side of the existing reinforced concrete viaduct to form one integral structure.
“This is the largest structure on the M25 widening project and it was technically challenging,” said Balfour Beatty structures team leader Chris Till. It was a difficult project, with the seven-span structure crossing a London Underground railway line, a road and a natural valley. At its deepest point the viaduct rises 16.5m above the valley floor.
Work on site started in October 2008, with the clockwise side tackled first. This meant the viaduct could be kept open to traffic via a contraflow system.
At each pier, two additional concrete columns have been built either side of the existing bridge. The 1.2m diameter columns sit on a cast insitu concrete base consisting of 12 600mm diameter, 12m deep bored piles.
The columns themselves were built up insitu using 40/50 concrete pumped into specialist circular steel shuttering. The reinforcement cages were fabricated on site.
“You’re working with as-built drawings but they’re often different from what was actually built.”
Chris Till, Balfour Beatty
The crosshead, which forms a platform between the bridge deck and the columns, is constructed between the two columns. This was cast on site using RMD Kwikform’s Paraslim modular cantilever formwork system.
The crosshead was stitched into the existing structure by hydrodemolishing the existing concrete structure back to the steel reinforcement.
Reinforcement for the new crosshead was tied into the existing structure and then concrete poured. This was the most risky part of the project.
“You don’t know what you’ll find when you expose the original structure,” said Till. “You’re working with as-built drawings but they’re often different from what was actually built.”
Once the crossheads were in place, 24m long, 1.2m deep precast concrete beams were installed to span the gap. Three were needed for each span and two 160t cranes installed one per night in a tandem lift.
A reinforced concrete deck was installed on top of beams and engineers were this week installing 2.5m tall noise absorbent barriers.
M25: Hatfield tunnel refurbishment
The 1.2km long Hatfield tunnel on the A1(M) north of London is also undergoing a major refurbishment as part of Connect Plus’ £6.2bn DBF O deal. Refurbishment is required because the 25 year old tunnel’s mechanical and electrical systems are at the end of their design life.
Contractors will also install increased ventilation, passive fire resistance and improved communications. The refurbishment has taken place one bore at a time, with the traffic diverted into the other bore. Work is close to completion on the northbound bore.The cut and cover tunnel was constructed between 1984 and 1986.
“A lot of the components in the tunnel were coming to the end of their design life,” said Highways Agency senior project manager Eamonn Colgan. “Many of the components need to be upgraded to meet new safety regulations.”
For the works the tunnel has been stripped down to its concrete shell. The concrete was then inspected and any cracks over 2.5mm filled. “We filled about 450 cracks and attached over 250 concrete patches,” said Skanska Balfour Beatty project manager Duncan Thompson.
Passive fire protection has been installed at critical areas of the tunnel, such as where it runs beneath the Galleria retail outlet and a major gas pipeline.
It is made up of panel boards attached to the concrete shell and protects the concrete against temperatures of up to 300degC for up to three hours. The increased ventilation system is provided by seven sets of four fans.
Work on the southbound bore will begin next month, with the tunnel reopening in May 2011.