Anyone who has tried to drive through London during rush hour will know that some of the most troublesome bottlenecks are caused by roads narrowing to fit under-railway bridges. But replacing these structures causes untold disruption to road traffic and rail services, so many bottlenecks remain.
The London Borough of Barnet has bravely commissioned two new bridge structures to carry the Brighton to Bedford and the Midland Main Line routes over Aerodrome Road.
The £12M project is vital to Barnet's ambitious growth plans and will open up the deprived area of Colindale to public transport and pave the way for the comprehensive redevelopment of Grahame Park.
The new structures will be manoeuvred into place over two weekends in December and January, with significant advance piling and foundation works needed to prepare the site for the bridge replacement.
The piling is being managed by Simplex and the Westpile, division of Bachy Soletanche, in a £2m joint venture for main contractor, Norwest Holst.
"Both companies tendered competitively on an individual basis for this project, however it quickly became clear that to meet the needs of resourcing, logistics and timing issues, a collaboration between the two companies would be best," says Westpile project manager Julian Gatward.
Gaining access to the site, which is hemmed in by the two rail lines and the M1 motorway, involved building twin working platforms to support cranes and piling rigs. These were constructed up to the rail track level using a system of retaining walls formed from gabion baskets, with access ramps leading up from road level.
Timing constraints centred on the requirement to install the eight 2.1m main bridge foundation piles over a series of four consecutive weekend railway possessions. Two piles could be installed using two rigs by four gangs during this time. The gangs worked around the clock in shifts to achieve this while during the weekday night shifts, smaller piles for the bridge wall wings were installed.
All 2.1m-diameter piles had to be permanently cased, with all four piles on the southern elevation being painted with a smooth gloss finish to prevent load transfer to a Thames Water sewer nearby. The paint also provides corrosion protection. Permanent casings were manufactured by specialist subcontractor Deepdale and painted by Surface Technik. The permanent casings extend to 12m depth below rail level.
To place a casing of that length, it was necessary to first bore at 2.4m diameter and case at this size for the first 6m. The 2.1m diameter casings were then placed inside and below the permanent 2.4m-diameter casing.
With the inner casing placed into the underlying London Clay, the bore is continued toa 24m depth, through 8m of Lambeth clay terminating just above the Lambeth Sands.
Reinforcement cages for these piles weighed in at 24t at full length and were constructed off site by Romtech in three sections. The design by Atkins required the cages to be formed using 28 pairs of reinforcement bars measuring 50mm in diameter. Each cage was spliced together using the Romtech Safe Splice system.
"These cages are the largest that I have ever seen in 20 years of piling. They had to be safe to lift and easy to join together," says Gatward.
Before the final pair of 2.1m piles was installed, temporary track crossings are constructed to enable the piling rigs to reach the narrow work platforms sandwiched between the M1 and the rail track.
Since there was little space here for the usual service cranes supporting each rig, a single 160t-capacity crane was required to service both rigs from the opposite side of the track.
The bridge replacement scheme will see Aerodrome Road widened to three lanes. The carriageway will also lower to allow double-decker buses to use the route for the first time. New footways and improved lighting will make pedestrians feel safer as they pass under the bridges. "Much improved public transport links will benefit the population, and residents who move into the area in future," says Matthew Offord, Barnet Council's Cabinet Member for Environment and Transport.