Site investigation rigMott MacDonald
is responsible for the full design of improvements at A3 Hindhead
in south west Guildford. Due to its existing carriage-way, severe
congestion is being caused around the A3/A287 cross-roads. This
£371 million improvement project will complete the dual carriageway
link between London and Portsmouth resulting in smoother journeys
and a safer road.
The new Hindhead tunnel will be the longest non-estuarial tunnel in
the UK.
We’ve been appointed as designer to Balfour Beatty, the main
contractor undertaking the project for the Highways Agency.
Twin tube bored tunnelMajor aspects of
development include assessing and mitigating potential
environmental impacts associated with the road improvements. The
areas surrounding the A3 Hindhead are under special protection and
include National Trust land and sites of Special Scientific
Interest (SSSI). This presented a range of challenges requiring
consideration at all stages of development. Our foundations and
geotechnics team have provided specialist input to almost all of
the design facets including structural design, tunnel design,
earthworks, environmental issues and drainage design.
Tunnelling will be under the Devil's
PunchbowlThe proposed improvements consists of building
a 6.8km dualled bypass, running from Bramshott Common on the
Surrey-Hampshire border and continuing north past Hindhead and into
1.9km of twin bored tunnels before rejoining the A3 near Thursley.
Mott MacDonald has designed and managed three phases of ground
investigations with a contract value in excess of £1.2 million.
These investigations have investigated the variable nature of the
Hythe Beds in detail to inform the tunnel design process. As a
result, a project specific stratigraphy for the Hythe Beds has been
developed. Geomorphological mapping exercises have been conducted
to aid in the interpretation of slope stability risk.
The 1.9km tunnel consists of twin, 10m diameter bores and regular
pedestrian cross passages at 100m intervals. The tunnel elevation
has been deliberately chosen to optimise the better tunnelling
medium while remaining above the water table. The bores will
therefore run almost exclusively within the weak sandstone of the
Hythe Beds up to 65m below ground level. However, excavation at the
southern end will be within sands – a crown canopy tube support
system had been designed for this section. Excavation will be
carried out using the sprayed concrete lining method.
Techniques to minimise land take include the design of extensive
soil nailed cut slopes and reinforced soil embankments to provide
steepened slopes. The requirement for a vegetated slope appearance
resulted in a detailed study being undertaken of the vegetation
establishment on similar steepened slopes within the UK.
The minimisation of visual intrusion has been a major
consideration, which is to be realised by the replanting of local
species, a green footbridge, landscaping and green slope finishes.
The road at Boundless Copse, for example, will feature a 10m to 15m
high embankment with 6m high bunds designed to produce a false
cutting. The scheme has been designed to produce balanced cut/fill
volumes so that import and export of materials is minimised.
Earthworks on the scheme involve the handling of over 1Mm3 of
materials and reuse is a key aspect. Where spoil is wet of optimum
and unsuitable for construction, lime treatment is proposed. This
requires stringent environmental controls due to the natural
acidity of the ground in the surrounding wetlands.
This encompasses Mott MacDonald’s committed approach to sustainable
thinking at all stages of design and construction. The tunnel will
be open in 2011, completing the dual carriageway link from London
to Portsmouth.
Slope management and vegetation
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