Soil investigations, offshore mid Norway: A case study of glacial influence on geotechnical properties

Fine-grained glaciomarine and glacial deposits on the outer Mid Norwegian continental shelf show complex variations in shear strength and degree of consolidation. At the Smorbukk Sor field (approx. 65 degrees N, 7 degrees E) about 80 m of variably overconsolidated clayey till is found on top of normally- or possibly underconsolidated glaciomarine and marine sediments. A high gas content is found below 60 m, and the porewater in the rather soft sediments in the lower part of the borehole may have been partly trapped by gas hydrates when the overlying hard till was deposited. The variations in geotechnical properties of the 80 m thick till section are suggested to result mainly from compaction below grounded ice during the last glaciation, and we interpret the strength variations in the till to reflect shifts between freeze of sediment porewater onto the base of the ice causing highly overconsolidated intervals, and thermal equilibrium or melting at the ice sole resulting in intervals of softer till. A large ridge northwest of Smorrbukk (Skjoldryggen) is probably partly formed by ice push during ice flow oscillations at the same time as the described till at Smorbukk was deposited, This ridge retarded the ice flow and hence the frictional heating at the end of surges, creating pulses of net freezing base ice and consolidation of the substratum. The described process has possibly affected a wide area up-ice of Skjoldryggen, and may be common in areas with a complex glacial history and a fine-grained substratum.