Retaining Wall Solutions
SEGMENTAL RETAINING WALLS – THE RIGHT SOLUTION
The shortage of new flat land available for housing development has put the spotlight on ground works. As ideal sites for new homes become increasingly scarce, house builders are now redeveloping existing sites and building on hilly and uneven terrain that would have been previously considered unsuitable for housing. This in turn has created a whole raft of new challenges associated with the selection of cost effective aesthetically pleasing and sustainable retaining wall and soil reinforcement solutions.
Technical performance is not the only criterion in specification. Increased importance is now being placed on environmental considerations and the aesthetics of the finished development. Soil reinforcement and/or retaining walls must be visually appealing and in harmony with the surrounding environment. Solutions which may be applicable for commercial and industrial applications may be very much less suitable for domestic housing in rural settings.
Many specialist suppliers including Acheson & Glover and its engineering partner, Maccaferri, now offer a whole range of retaining wall and soil reinforcement solutions, often with the option of technical advice and engineering design. Since the introduction of reinforced engineered retaining walls (SRW’s) by Acheson & Glover and Maccaferri into the Irish Market, House Building Companies and Consultant Engineers quickly recognised the inherent advantages of the system, to be able to quickly and cost effectively build reliable, aesthetically appealing retaining structures with design life and performance to match ‘conventional systems.’ Working closely with house builders and consultant engineers they can offer valuable assistance in selecting and developing the most appropriate solution.
Why are engineered retaining walls specified? The ground may slope away steeply, requiring reinforcement to support the perimeter slope. There may be limited ground for development and a retaining wall may be specified to extend the area of level ground available for building. Alternatively, walls may simply be required to create terraces and attractive landscaping, particularly in the case of more prestigious developments.
The choice of retaining wall solution will depend on a number of factors including the steepness of the slope, the underlying soil conditions and the precise purpose of the wall. These considerations will all need to be factored into the final geotechnical solution. In most instances, the architects will indicate on the initial plans the overall finish required and this will also be a major factor influencing the final specification.
A wide range of modular retaining systems now competes with reinforced concrete walls. Most of these are quicker and simpler to build and more interesting and pleasing to the eye. Over the last five years or so, split faced concrete segmental block walls – originally developed in the US – have grown in popularity. Systems such as the BBA certified Anchor Vertica® system allow the construction of walls with a ‘natural stone’ appearance up to five or six metres in height with Paragrid geosynthetic soil reinforcement. Structural stability is achieved through the frictional interaction and interlock of the soil particles with the geogrid and by the frictional connection of the geogrid to the facing units.
For walls of greater height, a system such as the BBA Roads and Bridges certified Anchor Landmark system is likely to be more appropriate with its unusual split faced portrait block and positive geogrid connection for additional face stability.
A typical reinforced SRW comprises a mass of granular soil reinforced with a polymeric geogrid and faced with hydraulically pressed ‘semi-dry’ concrete facing blocks. Concrete SRW units have high compressive strength and low absorption, making them resistant to weathering and can be designed to meet typical House Building, Commercial and Civil Engineering design requirements.
The modular nature of SRW systems enable the design and construction of tight curves and corners in difficult access situations and they can be quickly and safely, requiring less skilled labour and plant than many other systems.
Development of bespoke lifting tools and time & motion studies have demonstrated how walls may be constructed safely in accordance with current manual handing Health & Safety legislation. Typically constructed using compacted granular backfill, either 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. As with most retaining walls, a column of free-draining aggregate is required immediately behind the face of the wall to prevent the build up of water.
Here in the UK most walls are designed following NCMA guidance, but there are plans to incorporate a design approach for SRW’s into BS 8006: 1995 Code of Practice for strengthened / reinforced soils and other fills which is currently undergoing its ten year review. It is intended that the revised British Standard will provide a unified design approach for SRW’s in a truly local context. The adoption of the technology is continuing in parallel through many other parts of Europe and European design standards should be expected to follow.
In conclusion, Segmental Retaining Walls offer many advantages when compared to traditional methods and other modular retaining wall systems. These include: ease and speed of construction; attractive range of colours; variety of face finishes; durability; flexibility in design.
Modular retaining walls and reinforced soil retaining systems continue to grow in popularity and this is expected to continue as the reinforced segmental block wall technology developed over the past 25 years in the US continues to be transferred to Europe.
