When laying foundations, how can I minimise environmental impact?

Foundations are designed to spread the load of the building over a wide area to prevent sinkage. The base needs to be below the frost line (usually 750mm), free of organic matter (that could rot) and away from trees.

The environmental impact of concrete use can be minimized by installing pad or strip foundations and using recycled rubble as general fill. Concrete can be completely avoided by using hydraulic lime as the binder.

Adding large aggregate to bulk out the concrete saves cement. Recycled aggregate, such as crushed waste concrete, can be reused with hardly any impact on strength.

You can substitute up to 50% of the cement with pulverised fuel ash (PFA), a waste from power stations, e.g. from Scotash (www.scotash.com) or with GGBFS (ground granulated blast furnace slag), a byproduct of the steel and iron industry.

You could also seek advice from a local eco-builder/architect by looking through the AECB listings.

CAT ecobuilding experts gave the following preference list for foundations (from most to least preferable):

Masonry construction:

  1. Local stone foundation wall; DPCs/membranes of recycled polythene or bitumen; land drains around the house perimeter; minimise thermal bridging between foundations and the ground floor by using edge insulation.
  2. Second-hand bricks, sand/lime blocks or dense concrete blocks made with secondary aggregate, laid in a lime or weak cement mortar onto the smallest concrete strip necessary. Limecrete made from hydraulic lime and ballast, possible with additional pozzlan (setting) material.
  3. Concrete blocks made from new aggregate, or new bricks, laid onto the smallest concrete strip necessary.
  4. Trench-fill or reinforced concrete slab made from new aggregate. Short piles with reinforced concrete ground beams.

Timber construction:

  1. Timber (ground) beams spanning between timber piles or recycled brick piers on compacted re-used rubble. DPCs and membranes of recycled polythene or bitumen or slate.
  2. Piers as above on minimal concrete foundation. Posts set directly on a minimal concrete pad or short piles.
  3. Strip foundations and plinth wall as for masonry construction.
  4. Trench-fill or reinforced concrete slab made from new aggregate, unless ground conditions dictate. Short piles with reinforced concrete ground beam.

These recommendations are from 'The Whole House Book' (Borer & Harris), which is now out of print but may be available through a library or secondhand.

Contacts

AECB: the Sustainable Building Association - http://www.aecb.net - 0845 456 9773
  Network of builders, architects, manufacturers, and organisations. Aims to develop, share and promote best practice in environmentally sustainable building. There is a list of members on the website.

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