The winter of 2000/2001 has been recorded as one of the wettest
on record. As a result of the additional rainfall landslides
occurred more so than normal, including one at Flint Hall Farm
adjacent to the M25.
View of unstable wedge adjacent to M25
carriagewayMott MacDonald, working with the Highways
Agency, carried out a fast track investigation of the Flint Hall
Farm landslide, designed remedial measures and supervised
construction of these measures, all within a 12 month period.
Investigation of failure
The wedge shaped landslide, triggered by intense rainfall,
mobilised 90,000m
3 of material which was threatening to
close one of the countries busiest motorways. To achieve a swift
and successful outcome, site investigation and, design and
procurement were undertaken almost simultaneously. Investigation
comprised emergency monitoring, advanced site investigation
including detailed fabric logging, aerial photograph interpretation
and geomorphological mapping. This allowed a detailed understanding
of the ground model to be followed through to design and
construction.
Design, construction and buildability
Mott MacDonald worked in partnership with the Highways Agency and
the term maintenance contractor, Balfour Beatty Major Civil
Engineering, to design a solution that would be effective and
achievable. Discrete bored piles were selected to provide the main
restoring force with additional drainage measures. Temporary works
were integrated with permanent works design and a continuing
presence through construction allowed the ground model to be
confirmed leading to greater understanding of the residual risks in
the area.
Innovative management approach
Early Contractor Involvement (ECI) ensured that the design was
practical and manageable and also helped to instil trust between
all parties. Without this approach it is unlikely that such a large
project with its important strategic nature could be delivered
within the programme target date of a year.
Construction of bored pilesTo enable
works to commence on site before the design could be finalised, the
contract was let under the ECC Option E (cost plus) model. Cost
plus enabled full use to be made of the knowledge gained during
construction activities with the opportunity to adjust the design
as necessary and without excessive financial penalty. This was the
first use of such a contract by the Highways Agency and it proved
to be both flexible and financially sound. At completion,
unrealised risks were not paid for and the project was delivered on
time for just over £2 million, approximately 30% below the original
contract estimate.
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Applied geomorphology