The facts
Client: Carillion
Location: Cumbria
Services: Contractor’s Designer,
Highways Design, Structures Design, Geotechnical Design,
Environmental Assessment, Public Inquiry Expert Witness
Sector: Highways
Contract Type: Highways Agency
ECI
Project Value: £130m
Start/Completion: 2003 - 2008
The project
The Mossband Embankment is
a new motorway bridge spanning the West Coast Main Line
railway near Guardsmill, Cumbria. The geotechnical design
involved piling, basal reinforced platforms, ground improvement and
reinforced soil slopes.
The M6 Extension project between
Carlisle and Guardsmill completes the M6 motorway to the southern
end of the M74 which runs northwards into Scotland.
This section of road was formerly dual
carriageway, (A74T), posing safety problems and conflicts between
local and agricultural vehicles and the high speed traffic,
especially at entry / exit points.
The 9km long project was essentially a
widening process to increase the running lanes of the A74T to
three in both directions. The new offline viaduct at Mossband was
constructed to carry the motorway across the West Coast Main Line,
to the west of an existing viaduct, which it replaced. Additionally
a new bridge crossing the River Esk, to the south of
Mossband, was constructed adjacent to an existing road
bridge.
The new river crossing carries only
southbound traffic, whilst the existing structure was
refurbished to carry northbound traffic and the separate All
Purpose Route (APR). The APR runs along the length of the motorway
to enable local, and non-motorway traffic to travel the same route
without having to enter the motorway.
The geotechnical challenges unique to
the Mossband Embankment where primarily the depth and weakness of
the alluvial deposits under the proposed location.
Typically the ground comprises a
desiccated crust some 0.5 to 1.2m thick overlying a soft, silty
alluvium to depths of up to 6m below which there is up to 4m of
dense sand and gravels overlying a silty clay down to the Sherwood
sandstone bedrock.
Where the embankment was greater than
4m high the design was for Vibro Concrete Columns (VCC) to be used,
founded at 6 to 9m depth in the sands and gravel.
The 450mm diameter VCC piles had a
design safe working capacity of 1000kN. In total approximately 4300
piles were used for the approach embankments. For the bridge
abutments a 500mm CFA pile was designed to found at 30m depth into
the sandstone bedrock.
On-site restrictions due to the
proximity of the railway line meant that a conventional 32m high
CFA piling rig could not be used. The contractor resolved this
issue by utilising the new technology of a Segmental Flight Auger
(SFA) piling system. This lower height rig could be used adjacent
to the railway line and utilised hollow sectional flight augers in
7.5 to 10m lengths. These individual auger elements could be
connected in turn as the auger was advanced.
In areas where the embankment was not
piled the design specified the use of band drains on a variable
triangular spacing. Approximately 18,600 drains were installed
through a Class 6C drainage layer /working platform down to the
sands and gravels thereby allow the dissipation of excess pore
water pressures in the alluvium to occur as the embankment fill was
placed.
Typically the embankment was raised in
3m lifts with interim hold periods to allow for pore water pressure
dissipation, consolidation and strength gain to occur.
Instrumentation and monitoring provided data to enable an
assessment of strength gain of the underlying alluvial deposits to
be made before the embankments were raised further.
Above the piles a low strain, high
strength (1200kN/m) geosynthetic reinforcement geogrid was used to
provide lateral restraint across the pile caps and transfer the
vertical loading to the piles themselves. The Basal Reinforced
Platforms (BRPs) were designed in accordance with BS8006, 1995,
‘Code of practice for reinforced soil and other fills’,
and the specified geogrid had BBA certification. The design of the
BRP and side slope reinforcement was strain limited to 3% to ensure
compatability between the lateral strain extension of the
reinforcement and the piles.
The same code of practice was used for
the design of the reinforced side slopes, with reference to HA
68/94. The side slopes were again reinforced with low strain, BBA
certified geogrids with strengths ranging from 110kN/m to 35kN/m.
The reinforcement was placed at 600mm vertical lifts with no wrap
around to the front face. A 3D geomat was detailed to retain a
veneer of topsoil over the face of the embankment to support the
subsequent vegetation of the slopes.
A Class 1A crushed rock fill was used
in the shoulders of the embankments, within the reinforced soil
block. In some places a river sand, won from elsewhere on the site,
was used to form the core of the approach embankments, thus
enhancing the sustainability credentials of the design as a
whole.