Cob: cob and rocks

[Matthew HALL(SED)]

Hi Angela
Angela.Broach at sas.com writes:
>I was wondering...  If you built a cob wall about 3 feet thick and had the outside of it with
>embedded rocks (about grapefruit sized) so that the outside looked like a stone wall with cob
>mortar

My project supervisor is Algerian and in his country the traditional method of earth construction
is rather like cob construction in that they are monolithic mud walls but have incorporated light
timber framing. When the walls are being erected and the mud is still wet they push use stones,
rocks and boulders into the wall facade and leave it to cure there in the hot sun. This technique
has been practices for centuries in northern africa and offers in creased resistance to erosion
from wind-blown sand etc. I do not think differential rates of contraction between the stones and
the mud would be a problem, the proof of this being the extensive use of the technique in the
sun-baked saharan regions i am describing. However, the weak point of this kind of wall in terms of
durability is the exposed mud which is basically acting as a mortar for the embedded stones. I
agree that preferential erosion of these joints will probably occur as a result of the
imperviousness of the stones. According to my supervisor (who h
as built houses with this technique, including his own home) they use large stones (grapefruit size
as you say) and progressively smaller stones to fill the gaps in between, until they reach the
stage where they are using handfuls of gravel to fill the small gaps. The effect is rather like a
random pebbledash that is unevenly embedded in the mud wall beneath. Obviously durability is not
much of a problem in Algeria, Morocco etc in relation to rainfall. As Charmaine points out the
technique is also used in trad. Scotland earth building where the weather is very wet & windy. Many
of these buildings are preserved by  the National Trust (UK), they have a good publication on this
technique as well as others. If you like i will forward you details of this book. 
Regards
Matthew
___________________________________________________________________
Matthew Hall - PhD Research Student
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