| Contextual Infrastructure Planning and Design |
| Engineering and Aesthetics for the A5 Glyn
Bends Improvement in Wales |
| By Jeff Rowlands, Cardiff, Wales, 0144 2920 396045, rowlandsj@pbworld.com
|
| Construction of major highways
in environmentally sensitive upland areas requires very sensitive
design because the deep cuttings and high embankments that are required
have a major impact on the landscape. The Glyn Bends scheme presented
such a challenge, which was met by innovative techniques, particularly
in the rock cutting to mitigate the effects. |
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The Glyn Bends Improvement lies on the A5 London-Holyhead Trunk
Road in Wales, some 25 km (15 miles) west of Llangollen. It passes
through high quality landscape of exceptional scenic beauty between
Ty Nant and Dinmael and is of particular engineering and landscaping
interest because of the dramatic topography created by the adjacent
Ceirw Gorge.
In the 1970s, this portion of A5 was identified as a priority for
improvement because of its higher-than-national-average accident rate.
Our firm was commissioned by the National Assembly for Wales to carry
out a route study for 50 km (32 miles) of the A5 trunk road through
mainland North Wales. We were to identify the operational characteristics
of the route in terms of road width, visibility, accident rate, etc.,
and produce a ranking to prioritise sectional improvements.
Figure 1: Aerial view showing original road and new road under
construction |
Figure 2: Masonry arch bridge over Ceirw Gorge |
Figure 3: Rock cut showing anticline |
New Bypass of Ceirw Gorge Recommended
The original trunk road area meandered around the very tight bends
of the gorge, having been constructed on a ledge cut into the rock
face and supported by high masonry retaining walls built by Thomas
Telford in the 1800s. Through the gorge, the road passed some 35 m
(115 feet) above the bed level of the River Ceirw (Figure 1).
In the area of the project, the trunk road was narrow with no verges
(a soft, grassed strip outside the carriageway, typically 2 m (6 feet)
wide) and it had a horizontal alignment that was severely substandard
with limited forward visibility. In addition, up-slope rock falls
had occurred and there was a history of movement at the base of the
supporting retaining walls. These walls offered no guarantee of long-term
serviceability. The constraints imposed by the gorge precluded an
on-line improvement of the existing A5 route, so the decision was
taken to bypass the gorge to the north.
The bypass required a deep cutting through the adjacent hillside,
some 640 m (2,100 feet) long and 31 m (102 feet) wide at its deepest
point. The design and construction of the cutting, together with the
contiguous length of embankment of up to 18 m (59 feet) in height,
presented a considerable challenge to our team and TACP, our landscape
subconsultant.
The new road, which opened to traffic in mid-1996, generally consists
of s 7.3-m (24-foot) -wide single carriageway with 0.5-m (1.7-foot)
hard strips (shoulders) reducing to 0.3 m (1 foot) in the cutting.
The carriageway widens to 10 m (33 feet) where a climbing lane or
right turn lane is provided.
As lead design consultant on the bypass project, we were responsible
for carrying out all detailed design of the scheme, preparing contract
documents and supervising the construction. The construction contract
for the 2.4-km (1.4-mile) -long A5 Glyn Bends Improvement was awarded
to Miller Civil Engineering Ltd. by the National Assembly for Wales
in May 1994 at a tender value of £4.2m.
Engineering Details
The engineered cut was formed by pre-split and bulk blasting techniques
removing 255 000 m3 (more than 9 million cubic feet) of rock, which
was then deposited into a new embankment that runs parallel to and
south of the existing road. A sophisticated array of vibrograph and
electro-level measuring equipment was installed to monitor the blasting
operations because of possible effects on the walls supporting the
existing trunk road above the Ceirw Gorge and on a high single-span
masonry arch bridge that carries a minor road over the gorge. Figure
2 shows the bridge and provides a good indication of the dramatic
topography of the gorge.
In addition, visual inspection of the retaining walls and arch bridge
were considered necessary and access platforms were specified at three
levels on the retaining wall and the masonry arch bridge to facilitate
inspection during the 20 minute closure of the trunk road.
The cutting was excavated in the Upper Ordovician deposits, which
consist predominantly of fine grained siltstone. Low-grade metamorphism
has produced a rock of high strength and has imposed a spaced cleavage.
Faulting was widespread over the length of the cutting and both anticlinal
(a fold in which layered strata are inclined down) and synclinal (concave
upward) structures were present (Figure 3). The bedding dips into
the cut from the north 30º - 60º typically and the presence
of weak/soft volcanic ash on these bedding planes has had a significant
effect on slope stability.
In the light of these characteristics, we recommended the cross section
indicated in Figure 4 in our Interpretative Report. The scale of the
cutting in relationship with the gorge (Figure 5) clearly demonstrates
the engineering challenge of this unique scheme. Due to a very high
water table and the presence of ash bands, dewatering was specified
to improve short term stability until permanent measures were installed.
These consisted of horizontal drainage in the rock face with stability
measures such as rock anchoring, bolting, rock netting and masonry
facing. Rock dowels were also subsequently specified during construction.
Fill slopes were generally 1 in 2 but steepened locally to 1 in 1-1/2
at one location and flattened to a 1 in 3 profile where possible to
mould the embankment into the existing side long ground below the
old road. Where masonry revetment had been used, considerable care
was taken to produce a face that blends with the rock cut. It was
for this reason that the use of shotcrete was discouraged in the contract.
Figure 4: Typical cross section in rock cut |
Figure 5: Cross section showing relationship with gorge |
Figure 6: Rock cutting |
Figure 7: Design perspective of rock cut showing ledges and
buttresses |
Figure 8: Example of masonry around slope drainage outfall and
ledge |
Particular Attention Given to Aesthetic Considerations
In view of the sensitivity of the area, we paid particular attention
to the impact of the scheme on the environment and to the methods
by which environmental impacts could be avoided or mitigated as far
as possible. We closely followed the philosophy and principles adopted
in the Welsh Office Highway Directorate document entitled Roads
in Upland Areas: A Design Guide to achieve the desired objective
of producing a good quality safe trunk road designed in sympathy with
the landscape through which it passes.
One such important area has been identified as a refinement of the
pre-splitting of the rock face to ensure a more natural-looking profile
rather than the "hot knife through butter" effect that commonly
results from this process. This profile has been achieved by varying
the cut slope angle at designated locations in order to produce an
irregular face and to form buttresses. In addition, ledges have been
incorporated in the cut at appropriate locations and where possible,
natural geological features have been exposed and emphasised (Figure
6).
The final refinement and incorporation of all these features at the
design stage was an iterative process. Having decided how the natural
effect could best be achieved and having checked the stability of
the steeper faces, TACP provided sketch proposals for the locations
of ledges and variations in cut slopes. These were checked from an
engineering viewpoint before computer perspectives were produced (Figure
7).
Our design team discussed the results and, if necessary, other options
were prepared, including further perspectives, until the final design
was acceptable. A fundamental principle of the process was to attempt
to expose the natural rock features and emphasise them. For instance,
small wedge failures were anticipated at the berm level during construction
but as long as the berm width was not reduced significantly, it was
not proposed to carry out any remedial work, thus avoiding long lengths
of masonry infilling seen on most rock cuts at berm edges.
In constructing the rock cutting, recesses have been formed into the
face at rock anchor locations to reduce the visual impact of the anchor
heads. In addition, horizontal seepage pipes, which normally emerge
from the rock face in a regular grid of 5 m (16 feet), have been installed
in a fan arrangement and collected at single specific locations on
the rock face that have subsequently been masked by masonry facing
won from the cutting (Figure 8). TACP and the National Assembly for
Wales' Landscape Advisor have worked closely with us in the development
of integrated engineering and landscape details, both during the design
and in the construction stages.
Other areas of interest include mounding at the back of verges to
the embankment to reduce the visual impact of the road from long-range
views across the valley. This mounding together with an indigenous
planting scheme ensured successful integration into the existing landscape.
The proposed masonry walls, boundary fences and gates have also been
detailed and specified to reflect existing conditions. Three reinforced
concrete retaining walls and a concrete box culvert were constructed
in the fill area and clad with masonry produced from rock in the cutting.
Key features were:
- The degree to which the irregularities of
the geology were used to create (as far as practicable) a natural
looking rock face
- Wildflower and tree planting mixtures, which
have all been tailored to help re-establish the original site
flora
- Special measures that were incorporated with help from the
Countryside Council for Wales to enrich the ecological interest
of the scheme.
Looking ahead, ongoing management of the new landscape ensures that
the measures taken will mature effectively and will help to minimise
the long-term visual impact of the road in this scenic area.
Conclusions
Both the original and the new trunk road routes are of particular
engineering and landscaping interest because of the dramatic topography
created by the adjacent Ceirw Gorge, which was designated a "Site
of Special Scientific Interest." The final outcome is considered
to be a testimony to all those involved, all of whom have been extremely
proud to have been involved in the scheme. The key success is the
degree that the rock face has been varied and the irregularity of
the geology used to create a natural looking rock face.
A dangerous section of road has been improved and the twin objectives
of building a safe modern road in sympathy with the environment have
been achieved. At the same time, a very attractive part of the countryside
has been given back to people to enjoy once again. |
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Jeff Rowlands is a Technical Director with PB's
U.K. Infrastructure operation and the senior line manager for Civils
2 Infrastructure in the UK and Europe and is based in Cardiff, Wales.
After graduation, he spent a period in local government followed by
three years in consultancy in South Africa before joining PB in 1973.
Jeff has been the recipient of a number of design awards.
This article was based on a paper co-authored by Jeff Rowlands and
Ray Hooper, of the National Assembly for Wales and published in the
Institution of Highways & Transportation Magazine June 1995. |
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