Slope stability assessment of the Helland Hansen area offshore the mid-Norwegian margin

Numerous large-scale submarine slides affected the continental slope offshore mid-Norway during the Holocene. One of them, the Storegga slide, is located south of the Vّring Plateau, stretching from the North Sea Fan to the Helland Hansen arch. With a volume of 3200 km3 and an influenced area of 95,000 km2, the Storegga slide, which occurred about 8250 years before present (BP), represents one of the largest submarine slides worldwide. The slope stability assessment concentrates on the northern sidewall (Helland Hansen area) where gas hydrates are suspected from the presence of a bottom-simulating reflector (BSR) on reflection seismic data and where few slope stability investigations have been performed compared to the Ormen Lange area. The limit equilibrium and finite element methods (FEM) were used for the evaluation of static and dynamic (seismic) stabilities of the dipping seabed. To account for the uncertainties in the soil parameters, a probabilistic approach was applied by coupling the limit equilibrium model with the first- and second-order reliability methods (FORM and SORM) and the Monte Carlo simulation. The finite element simulation for the seismic loading indicates that a strong earthquake (>6.5 Ms) could be a potential trigger for slope failure but only down to 30 m subsurface. Obviously, one needs a significant pore pressure build-up and loosening of the sediments to explain the slide on such gentle slopes. More critical preconditions to failure in the deeper sediments should exist to explain the thickness of the sliding slab, knowing that the northern sidewall is approximately 150 m high. Cyclic loading due to a series of earthquakes could explain the slide, affecting the shearing resistance in a marine clay unit (weak layer) by excess pore pressure generation (drained conditions). The degradation of undrained shear strength with cyclic loading plays a major role in the instability process.