Scarp monitoring, early warning and documentation of successive landslides occurred after the quick clay slide in Ask, Gjerdrum (Norway)

A temporary monitoring and early warning system was organized during the response and recovery phase following a quick clay slide that occurred December 30, 2020, at Ask, Gjerdrum (Norway). The landslide left a steep and active scarp, ca. 1230 m long and 20 m high around a depleted zone. A monitoring system was established to warn rescue personnel, working below the scarp, about the possible occurrence of successive landslides. The scarp was initially monitored with visual observations (from ground) combined with drones (using both optical and thermal cameras). After a week, ground-based radars (InSAR) were also installed. The landslide scarp was monitored for two years. Successive landslides occurred in 60 days of the monitored period. One landslide was warned with the use of visual observations and about 20 landslides were detected and predicted with the use of radar. The radar was able to detect areas of accelerating velocities, typically in a period of a few hours prior to failure, with the total amount of deformation up to 10 mm. Ca. 145 slides of different size were mapped with the use of photos, video from drone, field observations and radar data, documenting spatial and temporal distribution, magnitude, type of mechanisms and acceleration for some of them. The largest successive slides occurred where there still was quick clay at the foot of the scarp and they remobilised the remoulded quick clay already present in the runout area. The slides occurred along the entire scarp, but mainly in the eastern and northern part, which can be explained by the steep slope gradient. Ground-based radar (InSAR) data has not been used for monitoring quick clay slide scarps previously, and it was uncertain if it would give valuable data. Radar was a very valuable tool and probably the most effective and reliable method for continually monitoring scarp deformations during search, rescue and mitigation works.