The facts
Client: Carillion
Location: Isle of Sheppey, Kent
Services: Highways Design, Geotechnical
Design, Environmental Assessment
Sector: Highways
Contract Type: HA DBFO
Project Value: £100m
Start/Completion: 2004 - 2006
The project
A wide range of geotechnical
challenges were overcome during the improvement and new
construction works for this 30-year Highways Agency DBFO
project.
The geotechnical design involved
piling, basal reinforced embankments, piled platforms, ground
improvement, reinforced soil slopes and temporary working platforms
to enable construction and launch of the bridge super structure
from either side of the estuary.
Capita Symonds worked intensively with
Carillion during the tender stage to put forward a unique series of
design solutions for the proposed upgrade works. A fundamental
reason for the success of the tender bid was its short construction
period. The geotechnical design solutions we proposed were a major
contributory factor in ensuring the overall program could be
met.
The main geotechnical challenge was
that the existing A249 route, originally built on low embankments,
required to be widened either side as it crossed extensive flood
plains mainly to the north of the new river crossing. The flood
plains typically comprised a desiccated crust some 0.75 to 1.2m
thick overlying a soft, silty alluvium to depths of up to 11m.
Below the alluvium there was up to 2m of dense sand and gravels in
turn overlying London Clay.
An additional geotechnical requirement
was the design of the two large caisson foundations which would
form the central supports for the new bridge crossing. The central
shipping channel required that a 24m clear span be formed and
demanded that the foundations be designed to allow for a ship
impact.
Rather than opt for a piled embankment
solution for the areas requiring widening it was determined that
the use of band drains and surcharging would achieve the necessary
long term settlement characteristics demanded during the 30-year
maintenance period. In addition to the band drains, basal
reinforcement in the form of a high strength woven geotextile was
used under the highest slopes to control lateral sliding and
bearing / extrusion failure during the consolidation process.
Embankments from 2.5m up to 6m in height had this ground
improvement approach used, in conjunction with instrumentation and
monitoring.
The instrumentation under these
surcharged areas of embankment comprised, inclinometers at the toe
of the embankments, piezometers within the alluvium to monitor the
rise and dissipation of the excess pore water pressures caused by
the surcharging operation. Surface monitoring points were used to
verify when the majority of the anticipated settlement had
occurred.
Where the embankment was greater than
2.5m high on the approaches to the new bridge the design proposal
was for a piled embankment with a geosynthetic reinforced Basal
Reinforced Platform (BRP) to transfer the embankment loads to the
piles. The embankments were formed from locally sourced London
Clay, a Class 2 material. The embankment side slopes were formed at
45 degrees and therefore had to be reinforced due to the inherent
weakness of the cohesive fill materials being used. The piles under
the embankment were Vibro Concrete Columns (VCC), founded at 16 to
23m depth in the London Clay layer. 900mm diameter bored piles were
used under the bridge abutments and went to depths of 25m founding
in the dense sand and gravels of the Lambeth beds.
Another major design element for the
project was the need for a temporary working platform on the
northern side of the estuary to enable not only the construction of
the bridge foundations and piers, but also the fabrication and
launch of the steel bridge deck structure.
The areas where the working platform
was required extended over the tidal mud flats and boggy grassland
either side of the existing flood bund. The initial contractor
designs for this element of the works indicated an un-reinforced
depth of granular material some 1.5 to 3m thick, depending on the
prevailing ground conditions; the mud flats had a design strength
of some 10 to 15Kpa. A revised design was developed which utilised
a composite design technique including high strength woven
geotextile reinforcement and lower strength bi-axial geogrids. This
design when coupled with edge set backs, and no-go zones for
certain plant, ensured that a stable and secure working platform
was produced.