It was a piece of information that I retrieved from the recesses of my brain - but I'm reasonably confident I found it in the British Standards Institute, 1986, BS8004, Code of Practice for Foundations. More than being a technical document it tackles issues in a more conversational style and is therefore a fairly bulky document. Unfortunately I cannot tell you where in that document the reference is located because I can't remember! I have a feeling that there is a section on compressible soils in the document if that's any help...
Clause 2.2.2.3.4 refers to Peat and Organic Soils and recommends piling through the organic layer to sub-soil. It does not unfortunately give guidance on an acceptable limit for inclusion of organic materials within structural fill, I have therefore always adhered to the view that all structural fill should be free from organic material.
Clause 2.1.2.3.3 refers to Settlement and states that in organic soils settlement can occur indefinitely (a "secondary settlement" occurs) and this needs special consideration. I have not found any indication as how to predict the rate of consolidation in organic soils - I would venture they are too variable to predict. I would therefore again offer the view that all structural fill should have no organic content to guarantee that the bulk of the settlement will occur with in the first 6 months to a year at a magnitude we would normally expect.
Clause 2.2.2.3.4 also refers to Settlement and states that in organic soils the secondary settlement can be large and prolonged - the BS recommends specialist advice be sought if bearing on organic soils is not avoidable.
Therefore, depending upon the depth of organic material concerned, I would recommend removing and replacing with non-organic fill, or piling through the organic layer to the sub-soil, or seeking further specialist advice, depending upon which represents best value to the client.
