24/01/2005
The shortage of new land available for development has put the spotlight on groundworks. As ideal sites for new homes become increasingly scarce, housebuilders 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 slope reinforcement solutions.
Technical performance is not the only criteria 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 Maccaferri, now offer a whole range of retaining wall and soil reinforcement solutions, often also providing a technical advice and engineering design service. Working with housebuilders and their 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 be simply required to create terraces and attractive landscaping, particularly on the 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 precise purpose of the wall. These considerations will all need to be factored into the final geotechnical solution, working with the consulting engineers for the development. In most instances, the architects will indicate on the initial plans the overall finish required and this also will be a major factor influencing the final specification.
A wide range of modular retaining systems now exist to compete with conventional reinforced concrete walls, most of which are quicker and simpler to build and more interesting and pleasing on the eye. Gabions still remain the most cost effective solution in many instances and can look stunning when properly faced with natural quarried stone. As an alternative, reinforced soil "green" solutions incorporating geosynthetic or double twist wire reinforcement can be used to create engineered slopes, appearing to the untrained eye to be a simple grassy slope. In many circumstances, with appropriate selection and testing, site won material can be re-used as engineered fill, preventing the expensive import or export of material from site.
Over the last five years or so, split faced concrete segmental block walls introduced from the U.S. have grown in popularity. Systems such as the BBA certified Anchor Vertica system allow the construction of 'natural stone’ looking walls up to 5m or 6m high, reinforced 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 & 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. 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. Offering housebuilders the visual appeal of conventional brick and natural stone walls, most segmental retaining wall systems are available in a range of standard architectural masonry colours, ensuring the wall blends in with the surrounding homes.
Modular retaining walls and reinforced soil retaining systems continue to grow in popularity and this is anticipated to continue as the reinforced segmental block wall technology developed and established for over 25 years in the U.S. continues to be transferred into Europe.
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