M4 Junction 13 showing the underpass for the A34 and
the new sliproadsWorking with design and build
contractor Costain Civil Engineering Mott MacDonald was appointed
as designer for the A34 Chieveley improvement scheme – developed to
alleviate major congestion and so improve safety at the interchange
of the M4 and A34 Junction 13.
Geotechnical design was required for ten bridges, retaining
walls, earthworks, pavement and drainage.
The landmark structure carries the M4 (with two slip roads) over
the new A34 dual carriageway. The bridges were constructed on large
diameter contiguous bored piles with top down sequencing to
maintain three lanes of live traffic, each way, at all times on the
M4. Six of the other bridges have bored piled foundations while the
other structure has contiguous bored pile retaining wall
abutments.
Solution feature in bored pile wall, earthworks were designed and
constructed with a split slope profile due to the presence of an
interface between soil and chalk typically occurring within the
main cutting. A reinforced soil bund was also constructed to
provide environmental screening for several houses adjacent to the
road.
In order to minimise the amount of material imported to the site,
stabilised capping was constructed using lime and cement or cement
only. In addition, maximum re-use was made of site won road
planning in pavement layers to further reduce the requirement to
import material.
Underpass during constructionThe
presence of solution features (randomly distributed pockets of weak
soil) was an ever present risk to construction. The presence of
unexpected features had the potential to impact on all elements of
the scheme with potentially serious effects, particularly for the
main underpass structure. This was managed by a full time site
engineering geologist who used field observations and a ‘toolkit’
of pre-designed solutions to mitigate risks for each element of
construction.
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