GreenTips

On any site the aim should be to minimise the amount of excavation as this can lead to habitat destruction, erosion and silting up of waterways. Taking away topsoil removes most or all of the biological systems that help maintain the soil in place.

If excavation is still necessary, methods can be employed to minimise the amount of silt runoff, such as the placement of hay bales at the lower end of the site.

The choice of retaining wall needs to balance durability (offered by materials such as masonry), with low embodied energy (offered by products such as timber.) Unfortunately if timber is used, there is little option but to use heavily treated timber. The New Zealand Timber Preservation Council provides a summary of the different classifications of timber treatment. Treated timber should always be used with care . The use of a treatment system which will not leach into the soil is preferable in retaining walls which are in constant contact with moisture.

The use of locally sourced natural stone will usually be the optimum choice environmentally but concrete and reinforcement required for structural stability will significantly increase the environmental impact of the wall. The Dry Stone Walling Association of Great Britain provides information on the construction of mortar free stone walling. However these techniques must be used with care in countries like New Zealand which are prone to earthquakes. Engineering advice should be sought if there is any doubt.

Concrete manufactured from cement with a recycled component can reduce the embodied energy of concrete walls. The use of crushed demolition materials as hardcore can reduce the volume of virgin materials while the use of locally sourced sand and backfill materials will reduce the transport energy required to move these bulky materials. A good source of background information on the use of concrete can be found at the Cement and Concrete Association website.

Crib and stepped walling constructions can offer the opportunity to increase the amount of natural planting in order to increase the site biodiversity.

In some cases the clever use of recycled materials such as car tyres can be appropriate.

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