I am curious to know if there are any published values for the undrained shear strength of peat? Also, if available, what is the common value(s) used in practise?
Based on my research and field investigations I have used a shear strength value of 15 kPa in the past. "Caution" would have to be the word I would use when globally applying a shear strength value to all peats as the strength is dependent on the amount of fibrous content, water table depth and most importantly local experience. I have also found that although many types of peat may be low in shear strength, it is the underlying organic silt that is the real design problem. While the peat may be near 15 kPa shear strength the organic silts can have strengths less than 10 kPa.
It would also be wise to examine settlement related issues of peat in relation to your proposed development.
BEWARE of relying on any given value of shear strength for peat. Remember that it is an ORGANIC material. Consider whether the properties assigned to peat in any design process (even if there is plenty of justification) will remain constant in the particular circumstances of your project.
Do not rely on any "undrained" measurements made in fibrous peat - they are completely meaningless in terms of its real behaviour.
The comment about looking at the decomposed layer immediately below the fibrous zone is very important - you will usually find that this is the weakest zone, as the fibrous layer can actually exhibit some "tensile" or extensional strength.
I recently consulted on several adjacent landfill problems where the owners had removed the peat prior to constructing high fills - much to their chagrin they later found that the toe of their fills were unstable and were affecting a large zone outside their properties. They would have done better to leave the fibrous layer intact, or better still, retain an experienced geotechnical engineering firm before commencing landfill operations.
