Earth structure shortly after completion, showing
transition between unreinforced and reinforced slopes
A history of instability to the slopes below the A38 trunk road
through the Glynn Valley, east of Bodmin in Cornwall, required a
long-term stabilisation solution with an innovative approach. The
400m length of road is on sidelong ground, sandwiched between the
main London–Penzance railway line above and the River Fowey running
close to the base, 10m below the road. The project was designed and
supervised by Mott MacDonald for the Highways Agency, and
constructed by Dean & Dyball. The works will ensure long-term
stability in addition to an improved road alignment.
Owing to the proximity of the river along a 150m length, reinforced
slopes up to 7.5m high at an angle of 63° were constructed. These
slopes comprised layers of geogrid connected to 480 soil nails,
wrapped around high quality granular fill. Where space at the toe
was sufficient, unreinforced slopes were constructed, with
transition zones blending into the reinforced slope. Slopes are
founded on gabions where required, to provide protection in a
100-year flood scenario.
Base of reinforced slope showing gabion
foundations constructed a minimum distance of 2.5m from River
FoweyKey indicators of the success of the project
included compliance with stringent environmental controls, together
with an environmentally and economically sustainable
approach.
The River Fowey is an important fishery, and South West Water has a
main intake one mile downstream of the site. A 2.5m exclusion zone
was established between the river bank and excavations, where all
trees were fully retained; pH and turbidity were continuously
monitored both up- and downstream of the site; and all run-off from
the site was trapped and pumped to a temporary settlement
lagoon.
Highest part of reinforced slope, showing
proximity to River FoweyReinforced slopes were filled
with a high quality, locally sourced China Clay by-product. Rapid
vegetation establishment on these slopes was assured by the
inclusion of Hessian bags filled with a topsoil and seed mixture.
At the end of construction, all slopes were fully planted, and the
floodplain area was restored to its natural state.
The project was completed four weeks ahead of programme, on budget
and with all environmental targets fully met.
Drilling rig for permanent soil nails, with
temporary stabilisation measures visible in foreground
Remedial earthworks design
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