1. 1 N.K. Tovey Н.К.Тови Landslide just west of Maracas Beach, Trinidad December 2002 Types of Landslide Why Landslides occur Landslide Hazards

2. 2 Landslide Consequence Remedial Measures Remove Consequence Safe at the moment Cost Build Landslide Warning No Danger Temporarily Safe Design Landslide Preventive Measures Stability Assessment Slope Profile Geology Erosion/Deposition Glaciation Weathering Geochemistry Cut / Fill Slopes Construction Drainage Pumping Man’s Influence (Agriculture /Development) Earthquakes Material Properties (Shear Strength) Ground Loading (Consolidation) Slope Management Hydrology (rainfall) Ground Water Surface Water GIS

3. 3 Landslides: Types of Landslide Cut Slopes Fill Slopes Retaining Walls Hybrids: Cut/Retaining Wall / Fill/Retaining Wall “Natural” Slopes - is there a better word?

4. 4 Fill Slope Retaining Wall “Natural” Slope Cut Slope Cut Slopes Fill Slopes Retaining Walls Landslides: Types of Slope Hybrids: Cut/Retaining Wall / Fill/Retaining Wall “Natural” Slopes - is there a better word?

5. 5 Cut Slopes and Fill Slopes Fill Slope Retaining Wall“Natural” Slope Cut Slope Failure of “Natural Slope” – cut slope and retaining wall unaffected

6. 6 Is there such a thing as a “Natural Slope?” ? slopes where there has been no anthropogenic activity, or where there is such activity it causes small changes to the geometry of the slope so that the Factor of Safety is largely unaffected. Landslides triggered by anthropogenic activity Deep seated landslide unaffected by anthropogenic activity

7. 7 N S =W N S  W Relationship between mobilizing & resisting forces Force (S) required to move block is proportional to Normal Force (N) On a slope N depends on weight and  N = W cos  S also depends on weight and  N = W sin  S1S1 N1N1 S2S2 N2N2 S3S3 N3N3 N S  W  W Normal Force Shear Force

8. 8 Properties of Soils Coulomb: a French Military Engineer Problem: Why do Military Fortifications Fail? N F F = N tan ......4.3  is the angle of internal friction F N  Is there a relationship between F and N?

9. 9 Suppose there is some “glue” between block and surface Initially - block will not fail until bond is broken N F F = C + N tan ......4.4 C is the cohesion F N  C Block will fail Block is stable Properties of Soils

10. 10 Three types of material –granular (frictional) materials - i.e. c = 0 (sands)  =  tan  –cohesive materials - i.e.  = 0 (wet clays)  = c –materials with both cohesion and friction  = c +  tan  F = C + N tan  above equation is specified in forces In terms of stress:  = c +  tan  Properties of Soils

11. 11 Stress Point at B - stable Stress Point at A - stable only if cohesion is present if failure line changes, then failure may occur. Properties of Soils F N F - F G - G B A Implication: Vertical slopes stable only to a height of 2c/  Where  is unit weight = ρg stable just stable stable unstable

12. 12 Properties of Soils – Effects of Packing and Water Loose: grains can slip over each other easily Dense: grains have to rise up to slip over each other. Sample must EXPAND Water filling voids forces grains apart +pwp Water partly filling voids causes suction - pwp

13. 13 F N F - F N NNNNNN N Displacement  dense loose Peak in dense test is reached at around 1 - 3% strain Properties of Soils What happens if residual strength is used compared to peak strength?

14. 14 Properties of Soils Displacement  dense loose Displacement Change in volume Displacement volume dense loose Dense / overconsolidated soils expand on shearing Loose / normally consolidated soils contract on shearing Eventually a common void ratio and shear strength What Shear Strength should be used? At critical voids ratio

15. 15 Distance stress point is from failure line is a measure of stability. Greater distance > greater stability F s = CA / BA Properties of Soils – Effects of water   Mohr - Coulomb B -ve pwp moves stress point to right Moves point further from failure line  greater stability Moves point closer to failure line  less stability +ve pwp Slopes near Hadleigh, Essex are only stable because of -ve pwp C A  = c + (  - u) tan 

16. 16 -ve pwp +ve pwp Water squeezed out Water sucked in e log  Properties of Soils – Effects of previous history e log  Critical State Line Sedimentation / glaciation/ anthropogenic loading Erosion / unloading Consolidation and Rebound Void ratio (e) = volume of voids volume of solids Dense / heavily overconsolidated loose / lightly overconsolidated What happens if slope movement does not allow volume change? If dense, sample tries to expand > - ve pwp >>>> more stable If loose, sample tries to contract > +ve pwp >>>> less stable

17. 17 Strata are parallel to surface Failure Surface is parallel to surface Water Table is parallel to surface Analysis is relatively straight forward assuming a block sliding and relevant properties. Extensive Slope of nearly constant angle Many slopes approximate to this Water table Types of Slope Failure Infinite Slope W Failure Surface

18. 18 Straight Line failure Only applicable for slopes ABOVE water table Postulate failure mechanism Need to test for minimum factor of safety. Types of Slope Failure: Finite Slopes W  FsFs 

19. 19 Cracking at surface at crest Reduces length of shear resistance Allows water to fill crack and cause destabilising pressure In dry summers keep crest damp to prevent crack formation!!! Types of Slope Failure – problem of cracks W 

20. 20 Slope failure Toe failure Deep seated Base failure Types of Slope Failure Summer water table Winter Water Table Backward tilting surface at crest of failure ~ 10 o.

21. 21 Types of Slope Failure If water is under pressure then debris from landslide becomes fluid. Controlled disaster becomes a major disaster - Aberfan

22. 22 Types of Slope Failure: Progressive Failure Bulging must occur before failure takes places Displacement  dense

23. 23 Types of Slope Failure: Method of Analysis Divide up slope into slices and sum up the stability of all slices. Detailed analysis allows for estimating inter-slice forces Can be ignored as first approximation as these are conservation assumptions Leads to lower F s than actual S N W

24. 24 Slope Failure: Remedial Action Create berms with longitudinal drains to remove surface water Reprofile Slope – remove material at middle – top Add weight to toe Lower water table

25. 25 Analysis of safety involves –Soil Sampling –Field Surveying –Location of Water Table –Laboratory Testing of samples –Analysis of stability –Some parts of analysis are conservative - –Other parts over estimate factor of safety and may give false sense of security Types of Slope Failure

26. Appraising Test Data Several tests on soil samples from a slope What value should be used in analysis strength 1 2 3 4 5 Test No.

27. 27 Errors in Interpretation of Field Data Two Boreholes Both hit solid rock F s ~1.4 Slope Failed Inappropriate Failure mechanism