The causes of the high friction angle of Dutch organic soils

Dutch organic soils have been found in past experiments to possess extremely high effective strength parameters. Since this finding is not expected and the phenomenon has yet to be explained, the high yield strength value is not used in practice. Understanding the abnormal properties of Dutch organic soils would thus be beneficial from the practical point of view. A programme aimed at understanding the unusual properties of Dutch organic soils, non-peat soils in particular, was performed on the representative organic soils in Dutch nature reserve park, Oostvaardersplasen (OVP) near Almere. Highly variable fabric of these organic soils was characterized by Computed Tomography X-ray scanner and environmental electronic microscope. Recognized fabric is in line with the geology of the OVP site. The multi-scale investigation as presented eventually identified the major role played by subhorizontal laminae and other non-organic microstructural elements (microfossil skeleton) in the high φ′ values of OVP organic soils. Deformation mechanisms of the microstructural elements are proposed and these make the unusual geotechnical properties explainable. Organics as involved were believed to have a primary contribution in increasing Atterberg limits and compressibility, and to allow the generation of high pore water pressures and low effective confining pressures during shearing. It has been also observed that high φ′ value is always correlated to the low effective confining pressure.