Should we continue assuming that hazard classes are static in zones prone to quick clay landslides? (learning from the 2020 Gjerdrum landslide)

As a consequence of the quick clay landslide in Gjerdrum on 30 December 2020, the Norwegian government appointed an independent commission to investigate the causes of the landslide and to give recommendations for risk management. In the commission first report published on 29 September 2021 (Gjerdrumutvalget, 2021), three causes were identified: low slope stability conditions, erosion in the brook Tistil (Tistilbekken), and the effect of the wet autumn season in 2020.

The second cause (erosion in Tistilbekken) was identified in a report by the Geological Survey of Norway (NGU) (Penna and Solberg, 2021). Their methodology included, among other things, the calculation of changes in terrain elevation over several years. The results showed reductions in ground-level at Tistilbekken up to 2,8 m, which were caused by erosion. See Figure 1.

Regarding the third cause (effect of the wet autumn season in 2020), meteorological and hydrological data show that autumn 2020 was the wettest season (3-month period) in the area since autumn 2000.

Seasonal variations in groundwater conditions produce stability conditions that vary with time (see the black periodic curve in Fig. 2). The combination of the aforementioned second and third causes produced large reductions in instability over some years (due to erosion from 2007 to 2015) and over a few weeks/months (due to the wet autumn in 2020).

In the current Norwegian guidelines, the classification of hazard in areas prone to quick clay landslides is static, i.e., the hazard class is not automatically updated based on some sort of monitoring. This updating has not been deemed relevant or necessary because, historically, the main triggering factor for quick clay landslides has been anthropic factors (fills, cuts, etc.). The risk class, which results from combining hazard and consequences, is used by the Norwegian authorities in decision-making for prioritizing mitigation measures.

Based on the experience from the Gjerdrum landslide, would it be relevant or useful to modify the current methodology for hazard evaluation such that a periodic updating of hazard (and eventually, risk) is included? This thesis is aimed to shed some light on the answer to this question. This will be done by carrying out assessments in selected areas prone to quick clay landslides. The evaluations will include changes in topography over time, seasonal variations in groundwater, slope stability calculations, etc.

Image may contain: Ecoregion, White, Rectangle, Slope, Map.
Fig. 1 Example of two cross sections in the Tistil brook (Tistilbekken) that show the ground surface in 2007, 2015 and 2020. Here, the erosion produced a lowering of the ground level in the brook up to 2,5 meters. Source: Gjerdrumutvalget (2021). Click here for a bigger version.
Image may contain: Rectangle, Product, Slope, Font, Parallel.
Fig. 2 Conceptual illustration that shows that the stability of the slope west of Holmen was initially very low and was reduced by erosion and the wet autumn in 2020. Stability conditions vary seasonally due to changes in groundwater (which in turn is affected by precipitation).
Source: Gjerdrumutvalget (2021). Click here for a bigger version.

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Published Oct. 5, 2021 4:10 PM - Last modified Oct. 7, 2021 11:49 AM

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