Geotechnical characteristics of post-glacial organic sediments in Lake Bergsee, southern Black Forest, Germany

In Lake Bergsee, southern Germany, the upper 13 m of gyttja and detritus mud were deposited during the latest Pleistocene and Holocene. These have been investigated for physical properties using mainly geotechnical methods and X-ray computed tomography. These deposits are extremely weak even compared with similar deposits in Scandinavia and elsewhere. It has been possible to recognise a significant change in the density and in the undisturbed undrained shear strength at a depth of 6.5 m, which can be related to a change in the sediment type. Other variations in properties can be related to organic fibre length and mineral growth. Although compressibility has not been measured directly, the deposits have the properties of a highly compressible sediment and estimates of effective stress and compression have been made. The sediment column seems to have experienced very little shortening, apparently because the microstructure of the organic sediments would hinder the movement of water under the expected sediment loads.