|A retaining wall on the A30 in Cornwall|
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|Parapet • Cuttings and Embankments|
Retaining Walls are built to serve two main purposes on the road network, both generally associated with roads in hilly locations. Firstly, they are used to support the lower side of a road built up on a slope, and often topped with a Parapet. This could be used to allow a road to cross a gulley without affecting the camber, or to support a road built on a steep slope.
The other used of a retaining wall is where a road is cut into a slope, rather than built up on one. The retaining wall is then constructed on the uphill side of the road to hold back the hillside above. This type of construction is less favoured by many, as a landslip above the road could have a more serious impact.
Some roads, of course, feature retaining walls on both sides to even out a slope.
Retaining walls have been used for centuries, and were originally constructed with stone, the wall starting with a wide foundation and then tapering upwards. The outer, visible, slope is generally much less than the inner, hidden slope, although on the uphill side of the road this is not always the case. In the 20th Century, reinforced concrete was introduced for construction and whilst this is generally much more resistant to ground movement, if a failure did occur it could be much more severe than the collapse of a few stones on an older wall.
Methods of construction
These are usually built on a concrete footing and they rely on the weight of the wall itself to prevent overturning. For smaller walls the footing may not be much wider than the wall itself. As the wall gets bigger the footing may extend under the retained soil. Gabion baskets also rely on gravity for their strength. They are wire baskets filled with rocks, and are then built up in layers to retain the soil behind. As they are porous, there is no build up of water pressure behind, however they should be lined with a geotextile fabric to prevent fine soil particles from being washed through the wall.
These may be formed from steel sheet piles, bored 'secant' piles, or large concrete diaphragm walls. They all rely on the depth the piles are inserted into the ground and the inherent strength of the piles to retain soil. As such they are ideally suited to retaining the side of a cutting as they do not require the material behind the wall to be removed to enable construction. Sheet piles can also be used to retain embankments as the tops of the piles can be left proud of the original ground level, and filled behind to construct the embankment, for instance at Henlys Corner on the A406.
Sheet Pile Wall
Sheet piles may be left untreated (i.e. bare metal) or may be painted with a weatherproof coating. The latter must be repainted from time to time to maintain the integrity of the pile. The former case is known as weathered steel, and the pile is designed to be thicker than initially required to support the load. As the pile rusts, the effective thickness reduces and the rust itself forms the surface coating, darkening with age. It is evident that the design life of the wall is important – the longer the design life, the thicker the steel needs to be. When life expired, the steel could be cleaned and painted to extend its lifespan.
Secant Piled walls
Bored secant pile walls comprise a line of bored concrete piles interspersed with bored soft piles, providing a continuous waterproof wall. The soft piles are bored first and are filled with a bentonite clay/cement mix which sets moderately hard, but has little structural strength. The concrete piles are then bored in the gaps between the soft piles, sealing the wall and providing its strength.
Diaphragm walls formed by excavating a deep trench, into which a steel reinforcement cage is placed, and then backfilled with concrete. To prevent the sides of the trench collapsing in during construction, the trench will be filled with bentonite clay which is soft enough to allow the excavator to work and the cage to be inserted, but provides enough strength to support the sides. The concrete is then piped into the bottom of the trench, and as the concrete level rises, it displaces the bentonite, which is pumped back to a storage tank.
Cladding and Capping Beams
Bored pile and diaphragm walls could be left exposed when the adjoining soil is removed, however they are usually hidden by cladding, often precast concrete panels, but they could be brick or another material.
Piled walls are often topped with a capping beam, usually made from reinforced concrete, though sheet steel has also been used, for example on the north west quadrant of the M25. The capping beam ties the piles together, which means that the piles themselves need not be quite so substantial as if they were each independent of each other. The capping beam also acts as a base for a parapet if required.
Reinforced Earth Walls
There are two main types of reinforced earth walls. The first uses geotextiles wrapped around layers of fill to hold the fill material in place, and the second uses precast concrete panels anchored into place with long steel straps embedded in the fill material. Both types lend themselves to retaining new embankments as they rely on their self weight to hold them in place.
As the first type has no facing panels, strictly speaking it is not a wall, but is a reinforced earth embankment. A geotextile can be a fabric but for reinforced earth structures is usually a high strength plastic mesh that is wrapped under and then over each layer of fill to stop the edge material moving out of place. In this way, embankments can be built with much steeper sides than a conventional earth embankment.
The second type lends itself to bridge abutments. Precast concrete panels are held in place by long steel straps embedded under the weight of the fill material. This technique is also used to support the sides of an embankment. Soil Nailing is a similar technique used to support the side slopes of a cutting. Here, holes are bored into the ground with long rods grouted into place. large plates are fixed to the rods at the exposed soil surface, and these plates in turn hold a geotextile in place to retain the soil.
Work in progress....
Retaining walls do fail from time to time, causing disruption to traffic. The A82 above the River Gloy was eventually re-routed further from the river after the retaining wall partially collapsed in December 2012, and of course the problems at Pulpit Rock further south on the A82 were due to similar issues.