21/08/2006

David Wright, Regional Manager, Europe for retaining wall specialists, Anchor Wall Systems Inc, explains why segmental retaining walls are set to take off in the UK construction market

If all construction sites were billiard-table flat, there would be no need for slope engineering and retaining walls would never have been thought of. As we know, things are not quite that simple in the real world of heavily contoured ground of indeterminate composition and questionable stability. So, where significant changes in level are required and a retaining structure is needed, Engineers fortunately have a range of solutions from which to choose.

One method that has seen rapid growth in usage in the UK and Europe over the last few years is the segmental retaining wall (SRW) system. Originating in the USA in the 1980’s, SRWs combine the speed of building of and relatively low cost benefits of a simple reinforced soil structure with sophisticated aesthetic detailing created by a facing of specially designed concrete masonry units.

SRWs allow the scheme designer to incorporate features such as curves, steps and terraces; details that are difficult and costly to construct when using more conventional, in-situ concrete retaining systems. Design lives of 120 years are routinely achieved, so the end user can be confident that an SRW structure is a durable and practical engineering solution.

Just like any other retaining wall solution, it is critical that an SRW is carefully designed and constructed. With proper attention to detail before wall construction begins, the building process will be quick, economical and straight-forward to manage.

Design methodologies

The engineering design of an SRW is a logical process. Using basic principles of soil mechanics and engineering, early pioneers in the use of segmental retaining walls developed design methodologies to standardise their approach to such schemes.

Early in the development of SRW structures, the United States based National Concrete Masonry Association (NCMA) took a lead in developing and setting standards for design, construction and testing. Its Design Manual for Segmental Retaining Walls first appeared in 1993 and rapidly became the definitive guide for Engineers considering using this method of retaining wall construction.

Despite the evolution of alternative methodologies around the world and the development of codes of practice such as BS8006 : 1995, (Code of practice for strengthened / reinforced soils and other fills) the basic principles set out by NCMA are still used routinely worldwide.

Information gathering

In practical terms, how do these methodologies work?

The wall designer must first gather basic site information. It is vitally important to fully appreciate the way the wall will be expected to integrate with its surroundings so there must be a detailed assessment of slopes above and below the wall and to understand how surface and ground water will be directed around the wall. The designer will need to assess and quantify how the wall might be subjected to external loads. Above all, since over 90% of a segmental retaining wall consists of soil or selected engineered fill, soils properties must be fully understood and documented.

Design process

Beneath the surface, an SRW is essentially a reinforced soil structure. By facing with a skin of concrete masonry units, the integrity of the structure is improved and it is protected from environmental factors. Moreover, it becomes an aesthetically attractive structure that can easily accommodate a variety of face finishes and styles which, depending upon the chosen block system, can curve and flow to complement or contrast with the surrounding environment. At its heart however, it remains a reinforced soil structure which most engineers can readily relate to.

Designing reinforced soil is essentially an iterative process, using the input design parameters (as discussed above) to model internal and external failure mechanisms. The engineer must then model scenarios whereby, through the inclusion of layers of geosynthetic reinforcement, the failure mechanism is counteracted by a pre-determined safety margin. Following this, various comnbinations of lengths, strengths and placement locations of the geosynthetic reinforcement can be considered. Finally, the design will be refined so as to make it practical to build and optimised on the basis of total cost to build.

In practise, because the engineer must consider so many potential failure mechanisms (there are at least 10 conditions that must be considered) and design against each, this becomes laborious to say the least! In light of this, Anchor Wall Systems has developed software solutions to simplify these design calculations and make designing an SRW a straight-forward, efficient process.

Construction process

Having derived a detailed design, construction can begin. Building segmental retaining walls is generally the responsibility of specialist contractors but in reality that doesn't have to be the case. The skills that are needed to build anb SRW are easily acquired by most competent groundworkers, civil engineering contractors or similar trades. Leading suppliers of such systems are generally happy to provide training and supervision at the start of the works.

Most SRWs are built starting the actual construction from a granular aggregate levelling pad. Having set out the line and levels for the wall, typically a trench will be dug that is not less that 150mm below the level of the first block and which will extend at least 150mm in front and behind the wall.

Compactable aggregate, usually a 20mm material with fines, is laid into the trench and graded to level. The first course of block work is then simply laid onto the compacted levelling pad and adjusted for level. At this point, it is critical that the first course of blocks are butted together and levelled both side to side  and front to back. Because an SRW uses no mortar, there is no room for error, so it is important to ensure that every block in the 'foundation course' is accurately placed.

It could be argued that by now, much of the hard work is actually done. Laying succeeding courses is largely a case of placing the masonry units jon top of the preceding course and checking to main tain level and alignment. Most SRW systems have an inbuilt mechanism to ensure a constant face alignment and accuracty such as a rear lip or locator lugs on the top face of the block.

Because most SRWs 'sit back' a few degrees from vertical and, because of their inherent mass, they are well suited to be used as gravity walls up to a certain limit (this will vary according to the system and can be verified by the supplier and/or an engineer).

When taller walls are needed, SRWs really come into their own. Using geosynthetic reinforcement is a highly effective way to use the retained material to contricute to the overall strength and integrity of the structure. Of course, not every soil that will be found on site will be suitable for re-use but it is surprising how often on-site material can be re-used as a backfill.

In such cases where the SRW will be reinforced, the building process is very little modified. The engineering design will determine the optimum dimensions and grade of reinforcement and the elevations at which it is needed. The reinforcement is then laid between succesive courses of block-work and laid out flat onto the backfill material.

Special care must be taken to avoid driving heavy plant over the geosynthetic reinforcement prior to it being backfilled - it is easily damaged unless installed exactly in naccordance with the manufacturers instructions. Provided that the proper care is taken, it can be expected to perform for at least 120 years.

To improve the connection between the geosynthetic reinforcement and the masonry block facing, SRW systems such as Anchor Landmark employ locking devices that create a positive connection between the two components - some of these are capable of creating a connection that will operate regardless of variations in normal load.

The wall construction is then essentially a repetitive process until the proposed finished elevation is reached. Along the way, it is easy to incorporate curves, corners or steps and most SRWs can be finished with cap details and even fence, guard rail or barrier details.

Conclusion

Anchor Retaining Wall products are manufactured under license by the Acheson & Glover Group based in Dungannon, Co Tyrone and are marketed in the UK and Ireland in partnership with Engineering Consultants, Maccaferri Ltd.

In 2004, Acheson & Glover became the first retaining wall block manufacturer in Ireland to achieve British Board of Agrément [BBA] accreditation for their Anchor Landmark® and Anchor Vertica® Systems. Anchor Landmark® has additional Roads & Bridges accreditation.