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):
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.