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Monster Embankment for Bodmin's A30 Bypass


The A30 is the main road link in Cornwall to the rest of the UK and an 11.5km section from Bodmin to Indian Queens is now being upgraded and widened in a project worth £93m (£60m of which is construction costs).


Geotechnical Engineers

Site Challenge

With three planes and sweeping curves, the geometry of a new embankment for Bodmin's A30 Bypass required meticulous attention to detail.

It was realised early on that the A30 crosses a valley at Holywell, this realisation changed the design from a viaduct to a high embankment would save millions of pounds. To a certain extent this was because there were to be areas of cut at either end of the site that would provide sufficient stone fill for the embankment.

A reinforced soil slope was chosen to minimise land take and reduce fill volumes. Huesker supplied the project with 50,000m² of Fortrac Geogrid erosion mesh for the inside of the front faces. At 22m high this structure is without doubt the largest reinforced soil slope in the UK supporting a dual carriageway.

 


Construction was divided into two phases: the lower, 12m high tier will be seperated from the 10m high tier above by a bench on which the underpass will sit.

The first tier features a stream culvert and an underpass for wildlife, such as otters, badgers, lizards, snakes and some protected species.

The complex shape of the embankment meant that close attention to detail would be required in order to get the project right. The main carriageway orientation is east-west, then the culvert and animal tunnel go north-east to south-west. However, the valley's natural sides are north-west to south-east, all of these planes had to be accounted for during the construction and this made it difficult to tie in the primaries (primary geogrids). This was a three dimensional problem because normally we like geogrid reinforcement to go in horizontally.

The Bodmin embankment has been constructed with a primary layer (longer tail lengths and stronger Fortrac grades) for every 1m lift, with a secondary layer midway between each primary. This means site workers form the face with wraparound lifts of 500m, typically backfilled with 250mm deep layers of fill compacted to 95% of maximum dry density.

Geotechnical Engineering

 

 
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