The Rissa Landslide

By Peter Salvin & Oliver Daffarn

In this presentation….

• Introduction• What happened at

Rissa• Why it happened• What lessons have

been learnt

Introduction (1)

• Rissa on West coast of Norway

• Situated near Lake Botnen

• 4500 inhabitants

Introduction (2)

• What Happened?– In 1978, a farmer was building a new barn

near lakeside

– Excavated foundations and stockpiled material

– Triggered failure of land

Introduction (3)

• What happened (2/2)• 70-90m of shore slipped into the lake

• 8-9 million cu. yards of debris

• 7 farms swept away. A total of 0.127 sq.miles of land affected

videos

Failure Mechanism (1)

• What is a landslide?• Sudden downward movement of rock, soil or

debris on slopes due to gravity

• Landslide types• Rock falls• Sags• Slides• Flows

Failure Mechanism (2)

• Landslip causes• Geometrical change• Loading• Unloading• Ground vibrations• Water change• Seepage erosion• Weathering

• For Rissa;• A combination of load addition and subtraction in

partnership with local geology

Local Geology (1)• Quick clay

– What is it?– Where is it found?

Quick Clay: Formation (1)

• Clay laid down in glaciomarine conditions• Glacier erodes crystalline bedrock• Collects and deposits minerals

Quick Clay: Formation (2)

• Clays rise above sea level

• Has the effect of reducing contact between grains

Glacier retreat = Isostatic rebound

Quick Clay: Formation (3)

• Leaching of salts occur creating a chain of events…

• Lower salt content in the soil» therefore….

• Repulsive forces between grains» therefore

• Reduction in grain to grain contact» therefore

• Pore water pressure rises

Voila…….Quick Clay!!!!

• Any slight disturbance = Sudden failure!!!

Since Rissa….

• Program of surveys introduced

• Mapping of Quick clay sites

• Geotechnical Investigations

Treatment Options

• Add salt

• Explosions

• Electrokinetic