We know that organic soil is a problematic soil. We have already learned about the relation between foundation problem and organic soils. In organic soils there have large organic content which requires special attention to the soil. Especially when organic content increased greater than 75%, the soil has to be treated as special soil which shows behavior that hardly comply with our traditional soil specially at low stresses level.
Organic contents usually changes to new form by biological break down. The organic changes occur in contents below water table at a relative very slow rate. This changing in form is called decomposition. But in some cases, this decomposition of organic content may be associated with an accelerated rate and leads to problems to foundation.
The biological activity, taken place in this soil is measured by the analyzing production of gas during their decomposition process. The rate of production of gas indicates the decomposition rate of them. This decomposition significantly influences engineering properties of soil. The shear strength, settlement criteria and stability of an organic deposit are highly affected by this biological action.
Considering above degradation reaction, engineers usually do not recommended this soil for founding medium of structures or considered suitable as subgrade of embankment.