Back analysis of a coastal cliff failure along the Forkastningsfjellet coastline, Svalbard: Implications for controlling and triggering factors

ABSTR A C T Based on a morphostructural analysis of a cliff coast segment of Forkastningsfjellet, back analysis of the August 12th, 2016 rock slide situated at 78 degrees 19'10 '' N/15 degrees 39'52 '' E was carried out. This rock slide comprises a volume of 175,000 m(3), and indicates a partial reactivation at the front of the similar to 100 million m(3) large postglacial Forkastningsfjellet rock slide. We studied the controlling and triggering factors of the reactivation using a 2-D limit-equilibrium calculations and a 3-D simulation with Scoops3D. Slope instability initiated along a pre-existing listric block fault that was inherited from the postglacial Forkastningsfjellet rock slide. The cause of the failure is attributed to a strength decrease and additional water pressures along the pre-existing sliding plane, possibly in combination with a degradation of the affected weak shales of the Rurikfjellet Formation, which build up a major part of the steep slope. Although the analysis suggests a structural control on the type and mechanism of slope failure, a significant impact of climate-related factors is inferred. Increasing temperatures and changing precipitation trends are reported from Svalbard. These are interpreted to foster permafrost degradation and reduce bonding forces in the thawing ice-filled fractures at the site. In addition, progressive weakening by more frequent frost and thaw cycles of the slaking shales and the introduction of additional water pressures to the rock mass are considered to contribute to the insta-bility. The final trigger of the 2016 failure is attributed to a two-day rainfall that had preceded the event. The application of the Scoops3D software tool showed that it is capable of predicting the locations and affected volumes of landslides with reasonable accuracy, when the geological and structural setting is well established. Under such premises the tool can be used to support preliminary susceptibility assessments in study areas with comparable geological and morphostructural settings. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study analyzed the rock slide event that occurred in Forkastningsfjellet on August 12th, 2016, and identified potential triggering factors through back analysis. It was found that the previous large rock slide played a significant role in the occurrence of this event, and the two days of continuous rainfall in 2016 was considered as the final trigger.