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Shear weight of history


Piling to stabilise a massive slope failure on the M25 near Godstone has just been completed.

One night last Christmas a huge slope failure threatened to close the M25 motorway near junction 6 in Surrey.

It was 19 December in a winter that was to prove the wettest in parts of the South East since records began.

The 200m long failure, beneath and adjacent to Flower Lane Bridge in the north face of Flint Hall Farm Cutting near the A22 exit, extended from the hard shoulder 80m up the slope.

'It had already begun to raise the hard shoulder by 150mm, ' explains Mott MacDonald geotechnical engineer Jon Davies.

'Further small movements occurred as a result of high rainfall. Alarms and monitoring instrumentation were installed and a detailed contingency plan was quickly developed to control the situation and, if necessary, to divert traffic, ' he adds.

Because of the M25's strategic importance, the Highways Agency decided to 'fast-track' the investigation, design and remedial works so they would be finished by December 2001, when more wet weather would make further movement inevitable.

Consultant Mott MacDonald was responsible for the project as the Highways Agency's maintenance agent for Area 3.

The lower slopes of the North Downs chalk escarpment, where the failure occurred, have a long history of movement. During an extensive preliminary study, geological and geomorphological mapping highlighted numerous fossil landslide features in surrounding fields, including some locally active areas of instability.

As little was recorded about the failure, contractor Foundation Exploration Services was commissioned to carry out a phased site investigation.

Phase one aimed to identify the mechanism and depth of failure for a conceptual remedial design. The second phase refined the results and gathered site-specific information needed for the detailed remedial works design.

'The investigation showed that the failure was occurring at two levels, ' says Davies.'The principal movement was of a large soil wedge formed by the intersection of a steep backscarp and a near horizontal main shear surface, which was up to 10m below ground level.'

Composed of stiff fissured Gault Clay, the slope was overlain by poorly sorted Head Deposits. There was a second level shear surface at the base of the Head Deposits where some movement had also occurred.

'Remedial works were designed to give a factor of safety of at least 1.2 over a design life of 60 years, 'Davies explains.'This 20% increase in stability was achieved by using a combination of piling and drainage.'

The works comprise 16m long, 1050mm diameter piles installed at 2500mm centres through the main slope; 9m long, 310mm diameter piles installed at 750mm centres beneath Flower Lane Bridge; installation of a deep cut-off drainage trench upslope of the backscarp; drainage at the slope toe; and deep counterfort drains in the lower slope. Piling was by Stent Foundations.

Davies says: 'As both the analysis and on-site observation showed the slope was marginally stable and highly sensitive to groundwater levels, the works were carefully programmed so its stability was maintained.'

Once remedial works are complete, monitoring of the slope will continue.TRL has also instrumented four piles with inclinometers and vibrating wire strain gauges as part of research with the Highways Agency on the use of piles to stabilise slopes.

The management, design and construction was based on a partnership between the Highways Agency, Mott MacDonald, Area 3 term maintenance contractor Raynesway Construction Southern, Stent Foundations and Balfour Beatty Major Projects.

Mott MacDonald geotechnical manager for Area 3 and design team leader John Perry stressed that partnering and early contractor involvement were vital in an emergency situation like this.

'Without this arrangement it is unlikely that such a large project with its important strategic nature could be delivered within a programme target date of a year, ' he says.

'I would also stress the importance of understanding the problem well, the need for open and extensive communication, particularly during the planning stage, the delivery of good workmanship and the importance of the methods used for risk assessment and management, which really showed their worth in this emergency situation.'

Because of the fast-track nature of the project, the Engineering Construction Contract Option E (Cost Plus) was used.

David Patterson of the Highways Agency says:

'Although this was the first time the Agency had used this form of contract the project has gone well, with the piling completed ahead of programme and under budget.

Piling and most of the drainage work is now complete and the project is expected to be finished before Christmas.

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