N.K. Tovey Н.К.Тови

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

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

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

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

6. Is there such a thing as a “Natural Slope?” Landslides triggered by anthropogenic activity Deep seated landslide unaffected by anthropogenic activity • ? • 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.

7. Relationship between mobilizing & resisting forces S1 S2 S3 N1 N2 N3  S W S S   W W N N N =W Shear Force Normal Force 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 

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

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

10. Properties of Soils • 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 

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

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

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

14. At critical voids ratio loose volume dense  Displacement Change in volume Displacement Displacement Properties of Soils 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?

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

16. Properties of Soils – Effects of previous history Sedimentation / glaciation/ anthropogenic loading e volume of voids Void ratio (e) = volume of solids Erosion / unloading loose / lightly overconsolidated Dense / heavily overconsolidated +ve pwp log  Watersucked in e -ve pwp Water squeezed out Critical State Line log  Consolidation and Rebound 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. Types of Slope Failure W • 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. Water table Infinite Slope Failure Surface Extensive Slope of nearly constant angle Many slopes approximate to this

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

19. Types of Slope Failure – problem of cracks W • 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!!! 

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

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

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

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 Fs than actual S W N

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. Types of Slope Failure • 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

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

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