In situ cyclic softening of marine silts by vibratory CPTU at Orkdalsfjord test site, mid Norway

Abstract

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.