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Submarine slope failures occur when the downslope component of the of the submerged weight of the mass exceeds the shear resistance along the most critcal slide base. The consequence of the failure event depends on the affected mass and area as well as the dynamics of the event and may range from nil to severe damage to subsea installations and infrastructure and to tsunami generation and widespread damage along surrounding coastlines.
Large slide events are seen as slide scars and extensive marine transport deposits on the contiental slopes in the vicinity of river deltas and glacial fans. They take place at very low slope angles and may extend from large water depths up to and into the shelf edge. In these areas large and relative rapid accumulations of sediments have been generated at during the Pleistocene time, piling up on top of massive sediments from Pliocene and earlier periods. The major Pleistocene glaciations and the associated sea level variations shifted the position of beaches and river deltas in lower latitudes. Current erosion of the shelf generated density flows of resuspended material over the shelf edge in addition to the river transported sediments. At higher latitudes, glaciers transported eroded mass over the shelf edge causing a rapid progradation of the shelf and thick accumulations of glacial debris flows on the slope interbedded with marine hemipelagic clays from interglacial periods.
Rapid deposition of fine grained material leads to excess pore pressure (overpressure) in the whole sediment column. Increased deposition rates during sea level rise and fall periods may lead to rapid weight and pore pressure increase in the top sediments and a gradual and more slow increase in pore pressure level in the deeper sediments. Simulation of the deposition process applying compaction and permeability models derived from geotechnical site investigations, show that excess pore pressure may reach very high values resulting in low shear strengths relative to the weight of the overburden.
The strain softening behaviour of marine clays is another important contributor to slope instability and especially to the development of large scale slide events. When subjected to a changes in shear stress exceeding the strength of the material, these clays may typically loose 60 to 80% of the peak strength. The mobility of the slide material will thus increase considerably after failure initiation, and in areas with a low factor of safety against slope failure a local instability may spread progressively and retrogressively over large distances.
Geomechanical modelling of sedimentation processes, strain softening behaviour and retrogressive slide dynamics allows explanation of major slide events like the Storegga slide and extensive local instabilities in the upper strata observed in many delta areas. The methods are presently being applied in evaluation of slide risk for offshore petroleum development projects.
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