The landslide was over 100 m wide and occurred in a quick clay zone in Nittedal just outside Oslo. Slide masses slid down the slope towards the Nitelva river taking with it sections of road. As a consequence of the insecure situation, about 80 people and several companies and businesses in the area were evacuated. On Friday, March 20, 2020, about 6 months after the landslide, Police withdrew the evacuation order after consultation with NGI.

Assistance in the acute phase

In the acute phase immediately after the slide, NGI assisted in assessing the potential for further slide activity. NGI's assessments and recommendations for police and local authorities formed the decision basis for which homes and businesses had to be evacuated.

Within hours, a geotechnical drilling rig was in place to closely determine ground conditions and initiate site investigations. NGI's laboratory was then able to conduct running analyses of clay samples retrieved from the landslide site.

The landslide destroyed major parts of the infrastructure in the area. The consequences were considerable and put the municipality's resources to the test. There were practical problems with electricity and water supply, sewage pollution of the river, securing the landslide, access to and closure of school and sports ground, closure of the through road and serious communication issues.

The pressure was considerable for residents evacuated from their homes without warning, for businesses abruptly denied access to their premises, as well by others affected and media pressure.

Assessment of landslide process

NGI has assisted the local authorities in Nittedal with geotechnical expertise from the landslide event and until the evacuation order was withdrawn. In the first phase, potential landslide spread was evaluated, to form the basis for decision making on evacuation and restrictions on the use of the area in close proximity to the landslide, (companies, school, sports facilities, access roads etc.).

NGI also participated in meetings with the municipality's crisis team and in public meetings with those affected, (residents and businesses). NGI planned and conducted field and laboratory investigations, geotechnical consulting and design of permanent stability for the landslide and affected area.

Prosjektert nett av kalk sement peler 800

Sketch of designed net of lime-cement injected piles in the slide area down towards the river (in the upper part of the photo)

Design of mitigation measures

NGI presented a proposal for mitigation measures to secure and rehabilitate the area with recommendations for permanent stability. This was based on ground improvement by lime-cement piling combined with a supporting embankment.

Initially, up to 2 m of the landslide crest was removed to provide increased safety for subsequent remedial work. Thereafter, the landslide pit was cleared of trees, roots and vegetation. For this preparatory work, the municipality received assistance from NVE, which has access to an excavator with 18 m range.

Ground reinforcement was carried out with about 2,500 lime-cement piles, (totalling 21,500 running meters), in the landslide pit to increase the shear strength of the clay. Finally, a supporting embankment fill of rocks was laid down at the slope base before restructuring and levelling.

NGI also provided an automatic measurement system that monitored pore water pressure in the subsurface during the mitigation period.

Etter sikring og planering av omraadet 800

The area on completion of stabilization work

NGI was responsible for supervision of the contractor assigned to perform the mitigation work. After the backfill was in place, the police withdrew the evacuation order and people were allowed to move back on Friday 20th March 2020.