One of our favourite methods of building low cost foorings for straw bale walls. You will need to adventurous to accomplsish this howver the savings are substantial if you go about it the right way. On one of our rubble trench footings in our own straw bale home we have saved costs by a factor of 4 and environmentally by at least a factor of 10. You will need a structural engineer prepared to certify these types of footings based on your soil type. Anyway I will let the photos below tell the story enjoy and be adventurous and keep thinking there is a better way to build and it may even be cheaper.