by Mark Shaw, Technical Director, Maccaferri Ltd

08/06/2005

Segmental (block) Retaining Walls (SRW) have been available in the UK for many years but it has been in the United States where most of the product development and associated growth in sales and popularity has occurred.  In the United States the market 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 (Ref Figure 1 a) and 1b).

Figure 1a) – Typical SRW during Construction

Figure 1b) – Typical SRW cross-section

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 (1) first appeared in 1993 and rapidly became the definitive 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 un-reinforced 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 utilised 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.

Here in the UK most walls are currently 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 currently undergoing its 10 year review.  It is intended 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 United States.  Currently, 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
• Attractive range of colours and finishes
• Durability
• Flexibility in Design

Concrete SRW units have high compressive strength and low absorption making them resistant to weathering and 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 (Ref Figure 2). 

Figure 2 – SRW utilised within a new housing development.

The modular nature of many SRW systems enables the design and construction of tight curves and corners in difficult access situations and they can be constructed 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 constructed safely in accordance with current 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 backfills 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 currently being used.  The various connection types may be broadly divided into two categories:-

• Friction Connections (with and without locating pins)
• 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 constructed to date within the US use friction connection systems.  

Anchor Walls Systems Inc is one of the SRW pioneers in the United States and over the years has developed a full range of both friction and positive mechanical connection systems, including its Anchor Vertica (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.  As indicated earlier, most walls in the United States are constructed using friction connection systems, as they tend to be more cost effective and easier to build, but where the is a need for a more robust engineering structure a positive mechanical connection systems is usually adopted (Ref Figure 3).

Figure 3 – Anchor Landmark positive mechanical connection system

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 Inc has 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:-

1. appropriate quality standards for components, 
2. adoption and use of consistent and safe design methodologies and
3. best practice in the construction, use and maintenance of the finished structure.

Both in the United States and in the United Kingdom, it has been recognised 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 was provided by HITEC (Highway Innovative Technology Evaluation Center) which is a nationally recognised clearinghouse for implementing highway innovation in the US.  In the UK, the organisation for evaluation was the BBA (British Board of Agrément), an organisation that provides authoritative and independent information on the performance of building products.

An important factor in the selection of these organisations was that they are both universally recognised throughout the local engineering industries and moreover, they are both recognised by the respective highway regulatory bodies, 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 United States.  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 autonomous 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 a 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. It was also determined by BBA that since they would be evaluating a system comprising a particular block and geogrid combination, for example, Anchor Landmark segmental blocks with Enkagrid PRO geogrid, that, the geogrid should also have been evaluated and achieved BBA (Roads and 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 the system suitable for use on many highway Schemes in both the UK and the US. 

Innovation amongst the 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 (Ref Figure 4).

Figure 4 – Anchor Landmark soil nail project nearing completion

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

Figure 5a) – Anchor Landmark soil nail connection detail

Figure 5b) – Anchor Landmark soil nail connection detail in-situ

In summary, Segmental Retaining Walls have come of age and can no longer be considered radical and new technology.  Over twenty five years of experience in the US and ten or more years experience in the UK has 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 everyone to take advantage of this increasingly common technology.

References

1)   Collin, J.G. (1997), Design Manual for Segmental Retaining Walls, 2^nd 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.