Comprehensive risk assessment gave May Gurney Geotechnical confidence to use CFA piling for a land bridge in Kent.
The 2.5km-long A21 Lamberhurst bypass in Kent is, says Highways Agency project sponsor Graham Link 'basically a simple, straightforward bypass.'
But it has been a long time coming. The scheme was drawn up in the 1980s, passed through public inquiry in 1992 and was part of a design build finance and operate package that was top of the list of projects about to start when the 1997 roads review scuttled the scheme.
Lamberhurst is situated in the River Teise valley within the High Weald, a designated Area of Outstanding Natural Beauty, but even so local support has always been overwhelmingly in favour of the project.
When travelling out of London on the A21 towards Hastings, Lamberhurst is the first village you encounter beyond the Sevenoaks to Tonbridge dualled section, which ends with the Pembury Bypass about 3km to the north.
Traffic bound for the south coast makes the A21 heavily congested in summer. The problem is particularly acute at Lamberhurst; slow moving traffic, resulting from the road's steep gradients and bad alignment creates a major blot on what is otherwise the quintessential Kentish village.
So there was much jubilation when the scheme finally got the go-ahead last year. The tender went out in October 2002 and May Gurney won the £11.9M contract at the end of March. URS is the consultant.
Because the scheme is a hangover from the 1980s - the route and detailed design had already been undertaken - it was not let as an Early Contractor Involvement project. Much of the scheme was already in place, which cuts down the potential for contractor-led innovation and value engineering. As Link saw it, 'there was little sense in reinventing the wheel'.
While the community is strongly in support, there was concern that the route cuts across the entrance to the National Trust's Scotney Castle - the garden is an outstanding example of early 19th century 'Picturesque' design and is grade 1 listed in English Heritage's register of parks and gardens.
As a result, the Highways Agency introduced a 40mwide 'land bridge' to carry the 3m wide access track to the property.
This is the main area where the scheme differs from the proposal approved at public inquiry 10 years ago.
'Thinking has moved on since then, ' says Link. 'We wanted to maintain the National Trust's access and to put in a land bridge to see if the theory of a wildlife corridor over a major road would work in practice.'
The idea is that the 20m-wide strips of land either side of the access road will be landscaped and planted with local trees and vegetation, so that in time it will maintain the character of the woodland either side of it (although for safety reasons, the Agency will not allow large trees to grow on the bridge). The hope is that drivers will be unaware they are driving over a dual carriageway. It also provides safe passage for wildlife crossing the bypass.
Ironically, the introduction of the land bridge was initially opposed by locals, because they were concerned it would delay the scheme. Villagers also argued that the £1M additional cost involved would be better spent on providing some kind of flood protection to the village - which was 2m underwater in December 2000.
Link's counter argument - based around an explanation of how budgets operate and the fact that roads and flooding projects are controlled by different agencies tapping into different pots of money - was met with some disappointment.
But he clearly has some sympathy for the principle of their argument; he made it clear that opposing the land bridge could introduce a longer delay than the additional design and construction time needed to implement it.
From a foundations point of view, the land bridge is the most interesting section of the project. May Gurney Geotechnical is using CFA piling to install 127 piles for the 8m high abutments and wing walls.
Abutment piles are 900mm in diameter spaced at 1m centres; wingwall piles are 600mm diameter. The project was tendered on the basis of rotary bored piles, because it was assumed the presence of siltstone and in particular iron-cemented sandstone beds would preclude CFA.
However, May Gurney has a powerful Llamada P100TT piling rig, and felt it could have a chance with CFA. 'It's a very powerful machine. In these conditions you need both high engine capacity and high torque and our Llamada, with nearly 30tm torque, is exceptionally good in these respects, ' says Steven Longdon, May Gurney Geotechnical southern area manager.
'We had a brain-storming session and undertook a detailed risk analysis, ' he adds.
'We also went back and looked at drill cores taken in Babtie's original site investigation and undertook some additional site investigation of our own.
'On this basis we were confident that we could drill it, which has introduced significant cost and programme benefits - around 25% in piling costs alone.'
The 83, 900mm diameter abutment piles (just over 40 in each wall) extend to 12.2m depth, whereas the wing wall piles drop back from 13.2m to 9m.
The main function of the piles is earth retention of the bridge abutments, since the Tunbridge Wells Sand is a good bearing material.
'Given the tough drilling conditions, we had to look carefully at the arrangement of the teeth on the auger, and we have made some modifications since the start of the job, ' says Longdon. Nevertheless May Gurney is now ahead of its programme, 'fully vindicating our decision to use CFA.' In fact Longdon expected to complete five piles a day, but May Gurney is now achieving eight using a single rig.
'The key thing on this job was the detailed risk assessment, which gave us the confidence to go ahead with a solution that on first appearances didn't look hopeful, ' says Longdon. As a contingency, May Gurney has a coring barrel on site which it could use if drilling conditions became to much for the CFA auger - but to date, it has remained locked up in the site compound.
Piles are also used on the River Teise crossing, in which a low-level bridge will cross a new sheet-pile lined river channel. Piles here are purely for load bearing as the sheet piles provide earth retention.
Ground conditions are alluvial silts overlying sands and gravel.
Here, May Gurney has already installed 22, 600mm CFA piles, each reinforced to full depth with 6T40s.
Other geotechnical issues include the possible use of surcharge loading to accelerate settlement of soft alluvium in the valley bottom and the stability of cuttings, particularly those that include the boundary of the Tunbridge Wells Sand and Wadhurst Clay.
Ancient tectonic movements during the formation of the Weald led to shearing at the boundary of these materials - and as a result of pre-existing shear surfaces even shallow cuttings can be unstable. The boundary also tends to be a natural spring line, so there is potential for a lot of water - a problem that came to light during construction of the nearby Pembury bypass in the mid 1980s.
According to May Gurney project manager Nigel Steer it will not be a problem this time round as the cuttings where the road crosses this geological boundary have been kept shallow and water seepages are being closely monitored.