1. The Rissa Landslide By Peter Salvin & Oliver Daffarn

2. In this presentation…. Introduction What happened at Rissa Why it happened What lessons have been learnt

3. Introduction (1) Rissa on West coast of Norway Situated near Lake Botnen 4500 inhabitants

4. Introduction (2) What Happened? –In 1978, a farmer was building a new barn near lakeside –Excavated foundations and stockpiled material –Triggered failure of land

5. 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

6. 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

7. 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

8. Local Geology (1) Quick clay –What is it? –Where is it found?

9. Quick Clay: Formation (1) Clay laid down in glaciomarine conditions Glacier erodes crystalline bedrock Collects and deposits minerals

10. Quick Clay: Formation (2) Clays rise above sea level Has the effect of reducing contact between grains Glacier retreat = Isostatic rebound

11. 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!!!

12. Since Rissa…. Program of surveys introduced Mapping of Quick clay sites Geotechnical Investigations

13. Treatment Options Add salt Explosions Electrokinetic