Project Summary
Significant traffic congestion at the Oxford bypass bottleneck, and the need for
improved access to the Sainsbury’s Supermarket site, led to the commissioning of
an important project – a massive new roundabout. Independent transport
consultancy Mayer Brown won the tender for this project, and elected to use 12d
Model as their design software.
The Challenge
The proposed roundabout entailed the re-alignment of the existing roundabout and
adjoining ‘A’ Roads, with the A4074 alignment being restricted by the existing
railway overpass bridge to the South of the Oxfordshire site.
Traffic analysis and Site Surveys were carried out prior to design work being
started. This initial work was critical in assessing how Mayer Brown could
increase efficiency while reducing design development costs.
The new proposal entailed additional lanes being ‘cut’ through the centre of the
Heyford Hill roundabout to ease congestion, thereby providing a ‘through route’
for the A4142 onto the A4074 (Southern Bypass Road). The roundabout was
re-aligned to cater for additional lanes, as well as the incorporation of
signalised junctions to assist with traffic movements.
The Solution
12d Model was used to design the layout so that the proposal could be
constructed accurately. The need to adhere to the UK’s ‘DMRB’ K-values meant
that the vertical design became ‘somewhat challenging’, as they had to tie in
with existing roads, drainage, and crossfalls, while maintaining a design path
that complied with the standards, without compromising the efficiency of the
layout.
As with most design projects…‘for every action, there is an equal and opposite
reaction’. Despite changing the roundabout dynamics, the existing drainage
network had to be retained. This meant that cambers had to be designed to fit
with both gullies and vehicle paths without adversely affecting the rideability
around the roundabout.
The solutions evolved from utilising superalignments, modifiers, and additional
‘forced break’ superalignments – all within 12d Model. While conventional design
tries to utilise the application of templates from centrelines, Mayer Brown
found that the optimal approach varied depending on which junction they were
at.
In some cases, the split carriageways assisted with deciding on template use,
while others were best suited to superalignments being used as kerb alignments.
The Result
Forced contours were used to enhance the final surface finish.The implementation
of a specific design strategy, incorporating chains was critical to the success
of delivery. Due to late changes, the ‘chain setups’ allowed for a practical
resolution and efficient delivery of product This was of further
assistance in delivery of the visualisation at the conclusion of design works.
--As featured in the Dirt Digger Newsletter - Issue 30, December 2012
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