21/07/2008

On the southern fringes of Scotland’s Cairngorms, the A93 Perth to Braemar road runs through the twisting and steeply wooded Craighall Gorge, a mile or so north of Blairgowrie.

Here, the single carriageway road follows the path of the River Ericht and clings to the valley side with precipitous drops to catch the unwary driver.

 

The need to carry out improvement works to the section of the A93 between Craighall Bridge and Middle Mause Farm was first identified in 1985 when the collapse of a section of retaining wall occurred. The road was closed for a period of two weeks until a Bailey type bridge was erected to allow the road to be reopened.

This Bailey bridge, originally installed as a temporary structure, still carries over 3000 vehicle movements per day, to and from the Highlands.

 

More recently, engineering assessment of the gorge area around the bridge concluded that the slopes were not sufficiently stable to support new carriageway construction and the site was also too restricted to accommodate the works whilst keeping the existing road open to traffic.

 

The Environment Services of Perth and Kinross therefore proposed that a completely new road be built to by-pass the bridge and on their instruction, Consulting Engineers Mouchel of Dundee, designed a 2.4km long single carriageway road running to the east of the gorge and broadly parallel to the existing route but at a higher level.

 

At the southern end of the works where the low-level old and higher level new carriageways join up, Mouchel conceived a 130m long reinforced earth wall which would create a smooth transition from old to new.

 

The design of the reinforced earth structure was put out tender by main contractor, I & H Brown of nearby Perth and eventually awarded to retaining wall specialists Maccaferri who also have offices in Perth.

 

Maccaferri submitted a design comprising a hybrid reinforced earth geogrid wrap-around structure, in conjunction with a hard, stone- faced Gabion Terramesh wall, 2.5 – 7.0m high. Maccaferri Paragrid 80/15 uniaxial geogrid was chosen as the principal reinforcing element, sandwiched at 500mm and 1000mm vertical increments between layers of primarily site won, Class 6I compacted back fill. The Gabion Terramesh system creates a rigid outer face and provides additional mass gravity reinforcement to the structure.

 

The principal components of the Maccaferri solution; Gabion Terramesh and Paragrid Uniaxial Geogrid reinforcement have BBA Roads and Bridges Certification. The structure therefore satisfies the Scottish Executive requirement of a 120 year design life.

 

Explaining the challenges involved in the design of the wall, Maccaferri Area Engineer Dr David Cheer said, “One of the main issues was that the existing road had to stay open throughout the construction process. Where the roads join, the footprint of old and new roads were very close together but with a big difference in vertical alignment in places. To get around the problem we were asked to design a relatively steep wall which closely followed the edge of the existing carriageway and allowed the traffic to continue using single lane working”

 

Dr Cheer continued, “The reinforced earth wall is built to a gentle external curve and the wire Terramesh units, filled with local stone give it a crisp face that fits in well with the surrounding landscape”

 

The reinforced earth structure was constructed by JML Contractors of Auchterarder. The work involved 3000 cu m of excavation behind a 150m length of the eastern gorge embankment to accommodate the geogrid reinforcement, up to 7m long in places.

 

Firstly, Gabion Terramesh Units (2x1m with integral 4m geogrid reinforced tails) were placed to define the wall face with their tails laid to the back of the wall to coincide with the geogrid reinforcement. Subsequent courses were placed with a 175mm step-back to create an 80 degree slope.

 

Gabion Terramesh Units differ from conventional gabions in that they have a 4m or 6m long double twist PVC coated and galvanised steel wire tail, integral to the construction of the unit, extending from their bases. This tail allows them to be used as unitary components to form a gabion-faced reinforced soil structure.

 

According to the manufacturers, having an integral geogrid tail as part of the system, removes any need for forming a connection between separate reinforcement and fascia elements on the jobsite. This simplifies and speeds up construction as well as providing a guaranteed connection strength offering reassurance for all parties.

 

 

Gabion Terramesh structures can also be built with vertical faces, if required and have been constructed to heights of over 22m. When used in high structures or for structures that are supporting heavy loads, the tails of the units, or the individual lifts of compacted structural fill, can be reinforced with additional layers of geogrid, where required.

 

Subsequently, pre-determined lengths of Maccaferri’s Paragrid 80/15 uniaxial geogrid were placed between layers of compacted back fill. The geogrid was wrapped up the back face of the gabions and over the backfill to form a “wrapped face” structure. A 0.5 m thick layer of Class 6H, granular fill was placed behind the gabion wall face to assist drainage.

 

The single carriageway road construction was then undertaken by I & H Brown. Some 2000cu.m of site won material and 2000 cu.m of recycled concrete were placed and compacted followed by a conventional, hot rolled, multi layer, bound pavement with verge drainage.

 

Construction of the £5.5m, A93 Craighall Gorge realignment project began in October 2007 and is due for completion in August 2008.