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Gaining ground

Reinforced soil techniques are increasingly popular as alternatives to conventional retaining solutions . Kim Worrall charts the developments over the last 35 years.

In the 35 years since the first commercial use of reinforced soil, more than 50,000 structures have been constructed using the method worldwide. The technique is now not only commonplace, but in many countries is the first port of call for clients who need to procure retaining structures.

The range of applications has been enormous, from routine retaining walls, through to major port and marine works, railway structures and even dams.

Retaining walls so high that they simply could not be achieved using conventional methods are now routine for reinforced soil.

The UK's contribution to this global effort was slow to start and even today is fairly small in numerical terms when compared with other countries.

However, this should not be interpreted as the UK being a minor player in the world of reinforced soil.

The first use of reinforced soil in the UK was as a retaining wall at Granton, Edinburgh in 1972. At this time the technique was still in its infancy, with most structures being built in France - where it all began.

Today, there are over 500 structures in the UK. Along the way many significant milestones have been passed - many without general recognition in the civil engineering industry. These cover the full spectrum including materials, codes and standards, companies, patents and of course, structure applications.

Introduction into the UK was a milestone in itself. No codes or standards existed at that time, nor was there even an established company responsible for promoting the technique. However, several structures followed, mostly built by contractors Monk and Tarmac, which enjoyed a form of licence from the French company La Terre Armee. At this time, the technique was patented (by the French) and therefore rightly provided a certain exclusivity to its use in the UK.

In 1980, following an agreement between patent holder Henri Vidal and the Ministry of Transport the technique became available for use on Crown projects. The UK's first reinforced soil firm - The Reinforced Earth Company - was established in Telford as the UK arm of the Terre Armee Group and the technique was all set to take off. The firm enjoyed exclusive use of the ribbed strip as a result of its UK patent granted in 1976, the main material patent in place at the time. Not until its expiry in 1996 could others make use of the advantages provided by the material.

The first official code or standard for the technique was the Department of Transport's Technical Memorandum BE3/78 published in 1978. This document signalled official approval, and together with the 1980 Crown agreement and the formation of The Reinforced Earth Company, constituted a period of renewed vigour.

This first DoT publication was acknowledged to be conservative and initially only recognised the use of metallic reinforcement as standard materials. However, this was addressed in a 1987 revision, which recognised the growing development of synthetic materials.

The introduction of synthetic materials as reinforcing elements was originally made by the firm Soil Structures in the early 1980s. The material was part of the Websol system and constituted a webbing of polyester fibres bound by a PVC sheath or coating.

This was soon followed by other materials, notably Netlon's polymer geogrid (now branded Tensar), an extruded poly- ethylene. Since then there has been a steady increase in companies and manufacturers offering synthetic reinforcements. In fact, widening of the south and south west sections of the M25 in the mid-1990s saw large scale use of polymer reinforcements, making a considerable impact on engineers' awareness of the potential of the method.

The increasing acceptance and use of the technology led to the formation in 1985 of a British Standards Committee charged with preparing a code of practice on reinforced soil. The standard, BS 8006, took 10 years to deliver and is undoubtedly the most comprehensive document on reinforced soil anywhere in the world. It embraces the 'soup to nuts' approach with all aspects of the technology contained in one document. The standard is now used extensively outside the UK, and has also been the basis for the preparation of other countries' national standards. Significantly, it is also the base document for the draft European execution code covering construction of reinforced soil. The important adoption of BS 8006 by the Highways Agency was made in early 1997 with the publication of BD70.

BS 8006 and BD70 continue to act as catalysts to the use of reinforced soil.

However, it is important to note that they have not been specifically responsible for milestone applications of the art in this country. As with most similar technologies, the types of application start with simple or routine examples. It is only with increasing experience that the technique demonstrates its true potential.

Throughout the 1980s there were few technical advancements as far as project or structure applications were concerned. An obvious exception was the first use of reinforced soil to support a UK motorway. This was as early as 1981 at Waltham Cross, where the M25 is supported on nearly 5000m 2of retaining wall.

The 1980s also saw use of reinforced soil being substantially client driven, with structures being designed for them pre-tender. The tender documents would then specify the works, the system and also the materials - along with the traditional 'or similar approved' sticker. Such contract procedures were not that conducive to technical innovation.

However, as civil contracts gradually changed to include more contractor-designed structures, the psychological handcuffs were removed to contractor innovation. A major step was the subsequent move to build, operate and transfer projects, which gave contractors even more freedom to identify applications for reinforced soil and to use it to its full potential. It was for these reasons that the main technical milestones in the UK were achieved during the 1990s.

One of the most visually and technically impressive structures was the temporary loading ramp constructed between tides to carry the Second Severn Crossing bridge caissons, including the UK's largest moving load.

The structure was the winner of a British Construction Industry Award in 1993, still the only reinforced soil structure to receive such an accolade.

The UK's largest reinforced soil project remains the extension of the A465 trunk road along the Neath Valley in South Wales, where about 12,000m 2ofretaining wall was built in 1995. The following year saw the contract with the most individual structures anywhere in the world. The M1/A1 at Leeds involved around 80 structures including abutments for the UK's longest single span bridge supported on reinforced soil - 53m across the M1 motorway.

Reinforced soil had already been used to support mainline rail traffic, most notably at the Folkestone terminal of the Channel Tunnel. One of the world's largest rail supporting structures was constructed adjacent to the M20 motorway, right beside the high voltage cables joining the national grids of England and France. However, it was in 1997 that the UK's first (and to date only) rail over rail bridge was built using reinforced soil for the abutments for the DC tramlink line.

It should not be forgotten that reinforced soil technology can only develop as long as there are companies to take the technique forward. The first and probably best known was the Reinforced Earth Company. Now owned by one time competitor Freyssinet, the firm was responsible for carrying the technique through the 1980s.

The 1990s has seen a steady increase in the number of companies offering complete packages of design, materials and construction of reinforced soil structures. The first UK company to make an impact was EC Civil Engineering.

Although no longer in business, the firm designed, supplied and built dozens of reinforced soil structures.

Reinforced soil has passed through various phases during its lifetime.

Eventually, it could become similar to reinforced concrete in that all engineers, companies and organisations will become familiar with it, design with it and use it routinely.

But for this to happen, all materials and components will have to become freely and widely available from a large variety of competing suppliers.

Arguably, the days of package providers as the main source of components is ending. Soil Solutions International was formed last year with the concept of providing a new way for clients to procure reinforced soil structures. Working with contractors, the company assists in procuring all the components directly from local manufacturers, with potential cost savings.

Reinforced soil has had a significant impact in the field of civil engineering over the past 30 years in the UK. The Institution of Highways & Transportation is now preparing a formal, historical archive on the building of the UK's motorway and major highway network. It has identified reinforced soil as having a specific role to play, so much so that the technique will have a section in its own right.

Kim Worrall is managing director of Soil Solutions International.

He has spent 20 years in the field of reinforced soil, is a member of the BS 8006 committee and a member of the Eurocode monitoring committee on the construction and design of reinforced soil.

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