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Download Lecture 2 Why Landslides occur
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Landslide Hazards Types of Landslide Why Landslides occur N.K. Tovey Н.К.Тови Landslide just west of Maracas Beach, Trinidad December 2002 1 Man’s Influence (Agriculture /Development) Pumping Drainage Hydrology (rainfall) Earthquakes Ground Water Ground Loading Surface Water Material Properties Landslide Preventive Measures Design Construction GIS Geology (Shear Strength) Erosion/Deposition Glaciation Weathering Geochemistry Stability Assessment Slope Profile (Consolidation) Slope Management Landslide Warning Landslide Cost Build Cut / Fill Slopes No Danger Safe at the moment Temporarily Safe Consequence Remedial Measures Remove Consequence 2 Landslides: Types of Landslide • • • • • Cut Slopes Fill Slopes Retaining Walls Hybrids: Cut/Retaining Wall / Fill/Retaining Wall “Natural” Slopes - is there a better word? 3 Landslides: Types of Slope • Cut Slopes • Fill Slopes • Retaining Walls Cut Slope “Natural” Slope Fill Slope Retaining Wall • • Hybrids: Cut/Retaining Wall / Fill/Retaining Wall “Natural” Slopes - is there a better word? 4 Cut Slopes and Fill Slopes Cut Slope “Natural” Slope Fill Slope Retaining Wall Failure of “Natural Slope” – cut slope and retaining wall unaffected 5 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. 6 Relationship between mobilizing & resisting forces Shear Force SS321 N1 N2 N3 Normal Force S Force (S) required to move block is proportional to Normal Force (N) On a slope W W W N =W N depends on weight and N = W cos S also depends on weight and N = W sin 7 Properties of Soils Coulomb: a French Military Engineer Problem: Why do Military Fortifications Fail? Is there a relationship between F and N? N F F F = N tan ......4.3 is the angle of internal friction N 8 Properties of Soils Suppose there is some “glue” between block and surface Initially - block will not fail until bond is broken N F Block will fail F F = C + N tan C is the cohesion ......4.4 Block is stable C N 9 Properties of Soils F = C + N tan above equation is specified in forces In terms of stress: = c + tan • 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 10 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 A B N Implication: Vertical slopes stable only to a height of Where is unit weight = ρg 2c/ 11 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 12 Properties of Soils N N N N N N N N F dense loose N Displacement Peak in dense test is reached at around 1 - 3% strain What happens if residual strength is used compared to peak strength? 13 Properties of Soils dense loose At critical voids ratio volume loose dense Displacement Change in volume Displacement Displacement 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? 14 Properties of Soils – Effects of water • Distance stress point is from failure line is a measure of stability. • C +ve pwp • Greater distance > greater stability Fs = CA / BA -ve pwp moves stress point to right B Moves point closer to failure line Moves point further from failure line less stability greater stability A = c + ( - u) tan Slopes near Hadleigh, Essex are only stable because of -ve pwp 15 Properties of Soils – Effects of previous history Sedimentation / glaciation/ anthropogenic loading e Consolidation and Rebound volume of voids Void ratio (e) = Erosion / unloading Dense / heavily overconsolidated loose / lightly overconsolidated log 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 Water sucked in e volume of solids Water squeezed out Critical State Line log +ve pwp -ve pwp 16 Types of Slope Failure Water table Infinite Slope Failure Surface Extensive Slope of nearly constant angle W • • • • Many slopes approximate to this 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. 17 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. 18 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!!! 19 Types of Slope Failure • Backward tilting surface at crest of failure ~ 10o. • Summer water table • Winter Water Table • Slope failure • Toe failure • Deep seated Base failure 20 Types of Slope Failure • If water is under pressure then debris from landslide becomes fluid. Controlled disaster becomes a major disaster - Aberfan 21 Types of Slope Failure: Progressive Failure dense Displacement • Bulging must occur before failure takes places 22 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 23 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 24 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 25 strength Appraising Test Data 1 2 3 4 5 Test No. • Several tests on soil samples from a slope • What value should be used in analysis Errors in Interpretation of Field Data • Two Boreholes • Both hit solid rock • Fs ~1.4 • Slope Failed • Inappropriate Failure mechanism 27
