Correlations between dynamic probe blow count and undrained shear strength for peat at a well-characterised raised bog in Ireland

Peat deposits are generally heterogeneous, with large variations over small areas. Peat has an extremely high water content, high compressibility and low shear strength. This presents a major issue in studying the geomechanical behavior of peat. For this study, field and laboratory tests were conducted to establish the undrained shear strength (su) of peat at a cutover industrial peatland in Ireland which had been extensively characterized in the past. The new field work included TRL-type Dynamic Cone Penetrometer, Mackintosh probe and field vane shear tests. A correlation was developed between the Mackintosh probe “M-value” blow count and the undrained shear strength of peat. The correlation can be expressed as su=16.54M0.373. Unconsolidated undrained triaxial tests and laboratory vane tests were carried out on samples retrieved from the field. The shear strength results thus derived were lower than the field test results, but consistent with each other. The suggested reason for the reduced undrained shear strength measured in the lab is sample disturbance during extraction, transport and storage prior to testing.