Sylvia Stegmann
University of Bremen
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In: Landslide Science for a Safer Geoenvironment. Springer, Heidelberg, Germany, pp. 549-555. ISBN 978-3-319-04995-3 | 2014
Fei Ai; Annika Förster; Sylvia Stegmann; Achim J Kopf
Submarine slope failures of various types and sizes are common along the tectonic and seismically active Ligurian margin, northwestern Mediterranean Sea, primarily because of seismicity up to ~M6, rapid sediment deposition in the Var fluvial system, and steepness of the continental slope (average 11 � ). We present geophysical, sedimentological and geotechnical results of two distinct slides in water depth >1,500 m: one located on the flank of the Upper Var Valley called Western Slide (WS), another located at the base of continental slope called Eastern Slide (ES). WS is a superficial slide characterized by a slope angle of ~4.6 � and shallow scar (~30 m) whereas ES is a deep-seated slide with a lower slope angle (~3 � ) and deep scar (~100 m). Both areas mainly comprise clayey silt with intermediate plasticity, low water content (30‐75 %) and underconsolidation to strong overconsolidation. Upslope undeformed sediments have low undrained shear strength (0‐20 kPa) increasing gradually with depth, whereas an abrupt increase in strength up to 200 kPa occurs at a depth of ~3.6 m in the headwall of WS and ~1.0 m in the headwall of ES. These boundaries are interpreted as earlier failure planes that have been covered by hemipelagite or talus from upslope after landslide emplacement. Infinite slope stability analyses indicate both sites are stable under static conditions; however, slope failure may occur in undrained earthquake condition. Peak earthquake acceleration from 0.09 g on WS and 0.12 g on ES, i.e. M5‐5.3 earthquakes on the spot, would be required to induce slope instability. Different failure styles include rapid sedimentation on steep canyon flanks with undercutting causing superficial slides in the west and an earthquake on the adjacent Marcel fault to trigger a deep-seated slide in the east.
Archive | 2014
Sylvia Stegmann; Achim J Kopf
The upper shelf of the landslide-prone Ligurian Margin (Western Mediterranean Sea) off Nice well-known for the 1979 Airport Landslide is a natural laboratory to study preconditioning factors and trigger mechanisms for submarine landslides. For this study low-stress ring shear experiments have been carried out on a variety of sediments from >50 gravity cores to characterise the velocity-dependent frictional behaviour. Mean values of the peak coefficient of friction vary from 0.46 for clay-dominated samples (53 % clay, 46 % silt, 1 %) sand up to 0.76 for coarse-grained sediments (26 % clay, 57 % silt, 17 % sand). The majority of the sediments tested show velocity strengthening regardless of the grain size distribution. For clayey sediments the peak and residual cohesive strength increases with increasing normal stress, with values from 1.3 to 10.6 kPa and up to 25 % of all strength supported by cohesive forces in the shallowmost samples. A pseudo-static slope stability analysis reveals that the different lithologies (even clay-rich material with clay content ≥50 %) tested are stable up to slope angles <26° under quasi-drained conditions.
Archive | 2016
Sylvia Stegmann; Stefan Kreiter; Jean-Sebastien L’Heureux; Maarten Vanneste; David Völker; Nicole J. Baeten; Siren Knudsen; Leif Rise; Oddvar Longva; Jo Brendryen; Haflidi Haflidason; Shyam Chand; Tobias Mörz; Achim J Kopf
High-resolution geophysical data reveal the presence of several spatially-isolated, small-scale landslides along the gently dipping (~3–4°) upper slope off Vesteralen, Northern Norway. Dynamic slope stability analysis suggests that seismicity may be largely responsible for the occurrence of these slope failures. The landslides are clustered in two groups, with one group of parallel features with their headwalls in ~500 m water depths. The second group is found in ~800 m water depths.
Archive | 2016
Max Oke Kluger; Stefan Kreiter; Jean-Sebastien L’Heureux; Sylvia Stegmann; Vicki G. Moon; Tobias Mörz
Earthquake induced cyclic loading has the potential to destabilize submarine slopes either by liquefaction in coarse-grained deposits or by cyclic softening in cohesive sediments. Vibratory cone penetration tests (VCPTU) represent a new approach for the evaluation of cyclic softening in fine grained sediments. In the past, VPCTU were utilized to evaluate liquefaction potential of sands, but cyclic softening of fine-grained marine sediments has not yet been tested with VCPTU in situ. At the study site in Orkdalsfjord, mid Norway marine clayey silt deposits are interbedded with coarse silt and clay layers. Static and vibratory CPTU were performed down to 19 m penetration depth using the Geotechnical Offshore Seabed Tool (GOST) and in addition, two gravity cores were taken for cyclic triaxial testing and geotechnical index tests. From static and vibratory CPTU a number of coarse silt layers with a distinct drop in cyclic cone resistance were identified. Compared to surrounding finer sediments the coarse silt layers exhibited a higher potential for cyclic softening. This assumption is supported by cyclic triaxial tests on very coarse and surrounding medium-coarse silts, respectively, revealing a strong loss of cyclic shear strength in a controlled and documented stress-strain regime. This study highlights the potential for VCPTU as a promising tool to qualitatively evaluate the vulnerability of marine silts to cyclic softening. In combination with advanced laboratory tests these results are envisioned to help better identifying submarine slopes subjected to failure during earthquakes.
Marine Geology | 2007
Michael Strasser; Sylvia Stegmann; Felix Bussmann; Flavio S. Anselmetti; Beat Rick; Achim J Kopf
Geophysical Research Letters | 2007
Sylvia Stegmann; Michael Strasser; Flavio S. Anselmetti; Achim J Kopf
Canadian Geotechnical Journal | 2010
Nabil Sultan; Bruno Savoye; Gwenael Jouet; D. Leynaud; Pierre Cochonat; Pierre Henry; Sylvia Stegmann; Achim J Kopf
Geo-marine Letters | 2008
Annedore Seifert; Sylvia Stegmann; Tobias Mörz; Matthias Lange; Thomas F. Wever; Achim J Kopf
Archive | 2006
Sylvia Stegmann; Tobias Mörz; Achim J Kopf
Marine and Petroleum Geology | 2009
Achim J Kopf; Sylvia Stegmann; Georg Delisle; Behrouz Panahi; Chingiz S. Aliyev; Ibrahim S. Guliyev