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Segmental retaining walls come of age

TECHNICAL NOTE

Segmental (block) retaining walls (SRW) have been available in the UK for many years but most product development and associated growth in sales and popularity has been in the US. The market there is mature and there are many block manufacturers offering a wide range of systems.

A typical SRW comprises a mass of granular soil reinforced with a polymeric geogrid and faced with hydraulically pressed 'semi-dry' concrete facing blocks (Figures 1a and 1b).

Early in the development of SRW structures, the US National Concrete Masonry Association (NCMA) took a lead in setting standards for design, construction and testing.

Its Design manual for segmental retaining walls 1 first appeared in 1993 and rapidly became the de'nitive guide for engineers and architects considering using this method of retaining wall construction.

As well as providing a detailed and standardised engineering approach for the analysis and design of both reinforced and unreinforced segmental walls, the design manual provides practical advice on construction and several generic contract specifications. Importantly, it also proposes test methods for block interface shear strength and geogrid connection strength, both of which are widely used by manufacturers of SRW systems around the world.

Initially, SRWs were used predominantly in the commercial and house building sectors but latterly have been adopted by the civil engineering industry, prompting the American Association of State Highway and Transportation Officials (Aashto) to introduce a slightly modified approach into its own guidance.

In the UK most walls are designed following the NCMA guidance but there are plans to incorporate a design approach for SRWs into BS 8006: 1995, Code of practice for strengthened/reinforced soils and other fills 2, which is now undergoing its 10-year review.

The intention is that the revised British Standard will provide a unified design approach for SRWs in a truly UK context and provide a path towards universal and regular use as has happened in the US.

At present, however, the NCMA design manual remains the most comprehensive design guidance for engineers.

Segmental walls offer many advantages when compared to other modular retaining wall systems.

These include ease and speed of construction, an attractive range of colours and finishes, durability and flexibility in design.

Concrete SRW units have high compressive strength and low absorption, making them resistant to weathering. They can be designed with suitable geogrids to meet typical house building, commercial development and civil engineering design requirements.

Available in a wide range of face sizes, styles and colours, SRWs can be built in a variety of architectural effects and finishes to match or complement the surrounding built environment (Figure 2).

The modular nature of many SRW systems enables the construction of tight curves and corners in dificult access situations. They can be built quickly and safely, requiring less skilled labour and plant than many other systems.

Development of bespoke lifting tools and time and motion studies have demonstrated how walls may be built safely in accordance with manual handling health and safety legislation.

Typically constructed using compacted granular backfill, site won or imported, work has also been undertaken to prove the viability of using recycled/stabilised backfill to accommodate the increasing environmental pressures to safeguard natural resources.

An important issue in the design of segmental walls is the geogrid/ block connection strength capacity and there are a wide variety of connection systems used. The various connection types may be broadly divided into two categories: friction connections (with and without locating pins), and positive mechanical connections.

The pull-out resistance of friction connections is directly related to the dead weight of the blocks above the geogrid trapped directly against the block below.

Positive mechanical connections, on the other hand, gain their pull-out resistance primarily from the mechanical interlock of the geogrid and the block, often in combination with a polymeric locking device.

Understandably, positive mechanical connections provide the greatest pull-out resistance but friction connections provide more than adequate pull-out resistance for most circumstances and the majority of walls built in the US have used friction connection systems.

Anchor Walls Systems is one of the SRW pioneers in the US and over the years has developed a full range of both friction and positive mechanical connection systems, including its Anchor Vertical (BBA Cert No 03/4032) and Anchor Diamond blocks, reinforced with Paragrid, and latterly the Anchor Landmark (BBA Cert No 04/R138) positive mechanical connection system, reinforced with EnkaGrid PRO.

Although most walls in the US are built using friction connection systems, as they tend to be more cost effective and easier to build, where there is a need for a more robust engineering structure a positive mechanical connection system is usually adopted (Figure 3).

As the use of SRW systems becomes more widespread there is a need to establish standards for design and regulate their use, particularly if they are to gain universal acceptance on government-funded highway schemes.

Third party accreditation or approval of the system can be of great assistance in this process.

Anchor Wall Systems recently completed an independent evaluation of its SRW system in both the UK and the US, establishing its credentials for use on large civil engineering projects.

The principal benefits of accreditation are to ensure:

appropriate quality standards for components;

adoption and use of consistent and safe design methodologies; and l best practice in the construction, use and maintenance of the finished structure.

It has been recognised in the US and the UK that it would be expensive and time consuming to attempt to meet the testing standards of 50 autonomous states (US) or the assessment criteria of the various local authorities (UK).

In the US, third party evaluation is provided by Hitec (Highway Innovative Technology Evaluation Center), a nationally recognised clearing house for implementing highway innovation in the US.

In the UK, the organisation for evaluation is the BBA (British Board of AgrÚment), which provides authoritative and independent information on the performance of building products.

An important factor in the selection of these organisations is that they are both universally recognised throughout the local engineering industries. Moreover, they are both recognised by the respective highway regulatory bodies, the Federal Highway Administration (FHWA) in the US and Highways Agency in the UK.

Much of the initial work on developing agreed standards was done in the US. Hitec was asked to produce third party evaluation criteria and develop an agreed set of standards.

Established 10 years ago under an agreement between the Civil Engineering Research Foundation (Cerf) and FHWA, Hitec worked closely with Aashto in developing a standard, ensuring that the final results would be generally acceptable in all 50 states.

In the UK, the BBA had to approach the initial evaluation slightly differently because SRWs have less history in the UK and local methodologies have not yet fully developed. Consequently, there was recognition that documentation from sources such as the NCMA and Aashto would need to be considered.

Material characteristics would be evaluated according to local BS EN standards and for aspects such as connection strength, US methods would be employed.

The BBA decided that since it would be evaluating a system comprising a particular block and geogrid combination, for example, Anchor Landmark segmental blocks with Enkagrid PRO geogrid, the geogrid should also have been evaluated and achieved BBA (Roads & Bridges) certification.

The Anchor Landmark system is one of the few SRW systems to have completed both a Hitec 3 evaluation and to have received BBA (Roads & Bridges) certification, making it suitable for use on many highway schemes in the UK and the US.

Innovation among segmental block wall manufacturers continues, as does the technology transfer between the UK and the US, and a relatively recent innovation is the use of Anchor Landmark system in combination with soil nails or ground anchors (Figure 4).

Used where there is insufficient space to allow placement of conventional geogrid reinforcement, the system's unique hollow block structure can accommodate galvanised steel beams (Figures 5a and 5b) to connect with the soil nail or ground anchor and so provide a robust structural facing.

Segmental retaining walls have come of age and can no longer be considered radical new technology.

Over 25 years of experience in the US and 10 or more in the UK have proved SRWs to be robust and cost effective retaining wall solutions in a wide range of soil conditions.

Standardisation of test methods and development of UK and European design standards in the coming years should enable industry to take advantage of this increasingly common technology.

1 Collin JG (1997). Design manual for segmental retaining walls, 2nd Edition, National Cement Masonry Association, Publication No TR 127A.

2 BS8006 (1995). Code of practice for strengthened/reinforced soils and other fills.

3 HITEC Report No 40677 (2003). Evaluation of Anchor Wall Systems' Landmark reinforced soil wall system, ISBN 0-7844-0677-4.

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