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Case Study – Use of surplus material and optimisation of road
alignment, Porth Relief Road, South Wales
Porth relief road is a major series of bypass roads alongside urban areas in the narrow
constrained valleys in the former coal-field areas in of the Rhondda area of South Wales. As
part of the complex development, ground engineering was an important part of the road’s
design, and in order to dispose of surplus material from land excavated for the road, the
contractor(s) obtained planning permission to deposit 130,000t of surplus materials to form
“development sites” or “development plateuxplateaux” to benefit the local communities as
sites for future employment/industrial developments.

Flat land sites in the narrow constrained valleys in the former South Wales coal-field areas
are rare, which prevent local employment opportunities.

The main contractor was Costain, and the designers were Glamorgan Engineering
Consultancy, Halcrow, Arup and Atkins. The client was the Rhondda Cynon Taff CBC. The
earthmoving sub-contractor was the Walters Group.

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Case Study – Porth Relief Road                                                                 2




Optimising road alignment                                                                          Formatted: Font: Bold
At Watts Town, the Porth Relief Road is close to an unstable area which is the result of           Formatted: Keep with next, Keep lines
earlier coal-mining, and secondly is close to the River Rhondda.                                   together


The original design was for a concrete river retaining channel, and river diversion, which was
expected to be very difficult to construct, as well as potentially environmentally intrusive.

The contractor’s team, with their designer, opted for a different solution which meant that the
river channel could remain as it was, but the road alignment was consequently changed.
This solution would have meant that a cutting slope into the valley side would be too steep,
and therefore the contractor opted for a reinforced earth design with concrete panels and soil
nailing.

This was cost-effective in enabling the valley side to be maintained whilst avoiding structural
work in the constrained site alongside the River Taff.

Changes in the road alignment involved up to 25m shift in horizontal position and up to 9m
maximum vertical shift from the original concept design. Value engineering reduced the
earthworks costs by up to 25 per cent (£18m on the original costed figure of £72m works
cost). This caused design re-work but was effective in demonstrating the close co-operation
between Contractor and his Designer teams, and with the earth-moving subcontractor.

The Porth Relief Road was unusual in being built alongside and through a densely populated
area, with only very limited routes available. In this case, the road alignment was based on
the trackbeds of former railways in the valleys concerned. This caused significant
engineering difficulties.

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Reducing the risk of differential settlement                                                       Formatted: Font: Bold
Porth Relief Road involved difficult ground conditions, including problems of stability of areas
affected by former coal-shale tips. The route crosses crossed at least one former land-slip
area.
Case Study – Porth Relief Road                                                               3




To reduce the risks of differential settlement, and to save time compared with any method
which might have included “surcharging” of the affected peaty ground, in one instance the
Contractor used lightweight polystyrene blocks as “fill” to form a critical length of road
foundation and to prevent differential settlement.

Client - Rhondda Cynon Taff CBC.                                                                 Formatted: Font: 10 pt
Contractor – Costain
Designers - Glamorgan Engineering Consultancy, Halcrow, Arup and Atkins.
Earthmoving sub-contractor - Walters Group.

Source: Paper given to IHT Leamington Seminar January 2009                                       Formatted: Font: 10 pt, Italic

								
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