Cob: foundation idea

[Matthew HALL(SED)]

Dear Robyn
This sounds like a very interesting theory, what you are basically describing is using loose
hardcore for foundations possibly extended above ground level with the use of retaining formwork.
There is a technique of using loose limestone chippings for foundations called vibroflotation
whereby the loose rubble is compacted by means of a large vibrating poker. This is however
expensive and i do not believe that loose rubble on its own would be very good for foundations to
any wall. It would be far better to dig the foundations in the perimeter of the house design, as
you suggested, and part fill with hardcore followed by concrete strip foundations. I presume the
idea of concrete is unattractive to you, in this case you could still use this design but use a
lime & pozzolan concrete instead of a portland cement one. Lime is much more of an eco-material etc
etc. 1 foot sounds far too shallow to me, even for a single storey house and especially for heavy
earth walls. I would suggest a minimum depth of 600mm (2ft) and the width of the strip should
exceed the width of the wall by a distance of at least a few inches. Somebody suggested using
gabian walls for the plinth (rocks tied inside metal cages). The compressive strength of the rocks
is more than adequate but you are relying on the integrity of the metal retaining cage which is not
designed for this level of stress. Corrosion can also be a problem...You would be much better off
using the natural stone to make a plinth wall to eliminate the rising damp problem. If you have
access to a large amount of lime chippings you could even mix them with lime and build the plinth
like rammed earth inside shuttering - a compressed rammed limestone if you will. I know this has
been done and is very strong, durable etc. I would strongly advise against any sort of plinth
comprsing of loose aggregate that has been retained either by shuttering or cages. Hope this is of
use.
Regards
Matthew
____________________________________________________________________
Matthew Hall BSc (Hons) GradBEng
Centre for the Built Environment
Unit 9 Science Park
Sheffield Hallam University
Pond Street
Sheffield S1 1WB
England

Tel: +44 (0) 114 225 3200
Fax: +44 (0) 114 225 3206
E-mail: M.Hall at shu.ac.uk
Webpage: http://www.shu.ac.uk/schools/sed/earth