Presentation on theme: "ENV-2E1Y: Fluvial Geomorphology:"— Presentation transcript:
1 ENV-2E1Y: Fluvial Geomorphology: 2004 - 5 Slope Stability and GeotechnicsLandslide HazardsRiver Bank StabilitySection 4 - Shear Strength of SoilsN.K. ToveyН.К.Тови М.А., д-р технических наукLandslide on Main Highway at km 365 west of Sao Paulo: August 2002
2 ENV-2E1Y: Fluvial Geomorphology: 2004 - 5 IntroductionSeepage and Water Flow through SoilsConsolidation of SoilsShear Strength ~ 1 lectureSlope Stability ~ 4 lecturesRiver Bank Stability ~ 2 lecturesSpecial TopicsDecompaction of consolidated Quaternary depositsLandslide Warning SystemsSlope ClassificationMicrofabric of Sediments
3 Section 4 - Shear Strength of Soils Definitions:a normal load or force is one which acts parallel to the normal (i.e. at right angles) to the surface of an objecta shear load or force is one which acts along the plane of the surface of an objectthe stress acting on a body (either normal or shear) is the appropriate load or force divided by the area over which it acts.Stress and Force must NOT be confused
4 Section 4 - Shear Strength of Soils EQUILIBRIUMThere are three conditions:the net effect of all forces parallel to one direction must be zerothe net effect of all forces orthogonal (at right angles) to the above direction must be zerothe sum of the moments of the forces must be zerothe first two conditions can be checked by resolving forces (e.g. see Fig. 4.1)
5 Section 4 - Shear Strength of Soils Resolution of ForcesAt Equilibrium:Resolve forces parallel to P1 :-P1 = P2 cos 2 + P3 cos 3Similarly at right angles to P1P2 sin 2 = P3 sin P1P3P232
6 Section 4 - Shear Strength of Soils Coulomb: a French Military EngineerProblem: Why do Military Fortifications Fail?
7 Section 4 - Shear Strength of Soils Coulomb: a French Military EngineerProblem: Why do Military Fortifications Fail?Is there a relationship between F and N?NFNFF = N tan is the angle of internal friction
8 Section 4 - Shear Strength of Soils Suppose there is some “glue” between block and surfaceInitially - block will not fail until bond is brokenNFNBlock will failFBlock is stableCF = C + N tan C is the cohesion
9 Section 4 - Shear Strength of Soils F = C + N tan above equation is specified in forcesIn terms of stress: = c + tan Three types of materialgranular (frictional) materials - i.e. c = (sands) = tan cohesive materials - i.e. = 0 (wet clays) = cmaterials with both cohesion and friction = c + tan
10 Section 4 - Shear Strength of Soils Stress Point at B- stableStress Point at A- stable only if cohesion is presentif failure line changes, then failure may occur.FNF - FG - GAB
11 Section 4 - Shear Strength of Soils F - FDisplacementdenseloosePeak in dense test is reached at around 1 - 3% strain
12 Section 4 - Shear Strength of Soils displacementIncreasing normal stress/denselooseDisplacementNormalising curves to normal stress leads to a unique set of curves for each soil.
13 Section 4 - Shear Strength of Soils Types of Shear TestStress controlled testStrain controlled test (as done in practical)Failure in stress controlled testBANG!DisplacementFNNNNNNReadings cannot be taken after peak in a stress controlled test
15 Section 4 - Shear Strength of Soils Plot volume changes as Void RatioVoid RatiodisplacementlooseCritical void ratiomediumdenseAll tests eventually come to same Void Ratio
16 Section 4 - Shear Strength of Soils Effects of Water Pressure = c + tan Does not allow for water pressure.Principal of Effective StressFrom ConsolidationTotal Stress = effective stress + pore water pressureor ’ = uIn terms of stresses involved water cannot take shearso = c + ( - u ) tan or = c + ’ tan Mohr - Coulomb failure criterionif pore water pressure = 0 then original equation applies
17 Section 4 - Shear Strength of Soils Distance stress point is from failure line is a measure of stability.Greater distance> greater stabilityMohr - Coulomb-ve pwp moves stress point to rightA+ve pwpMoves point closer to failure line less stabilityMoves point further from failure line greater stabilitySlopes near Hadleigh Essex are only stable because of -ve pwp
18 Section 4 - Shear Strength of Soils The Triaxial TestProblems with Standard Shear BoxShear zone is complexDifficult to get undisturbed samples which are squareDifficult to do undrained or partially drained testssands - always will be drainedclays - may be partially drained - depends of strain rate.
19 Section 4 - Shear Strength of Soils The Triaxial TestLoadCell PressureSample in rubber membranePorous stone
20 Section 4 - Shear Strength of Soils The Triaxial TestCell pressure can be varied to match that in groundcylindrical samples can be obtainedsample can be sealed to prevent drainage or to allow partial drainagecan perform both undrained and drained tests
21 Section 4 - Shear Strength of Soils Drained Testallow complete dissipation of the pore water pressure.speed of the test must allow for the permeability of the material.for clays time is usually at least a week.measure the volume of water extruded from or sucked into the sample in such tests.Undrained Testno drainage is allowed.measure the pore water pressures during the test.
22 Section 4 - Shear Strength of Soils Drained Testresponse to load and volume change is similar to standard shear box.Undrained Testburette is replace by a pore water pressure measuring device.Since drainage is not required, test can be rapid.Shear stress will be lower than in drained test if positive pore water pressures develop
24 Section 4 - Shear Strength of Soils 4.8 Failure modes in the Triaxial Test.Loadingits length will shorten as the strain increasessome bulging towards the end.Over consolidated samples (and dense sands),usually a very definite failure plane as peak strength is reached.Normally consolidated clays and loose sands,failure zone is not visibleusually numerous micro failure zones criss-crossing the bulging region.Undrained testorientation of the failure zone is at 45o to the horizontal,Drained testorientation will be at (45 + /2), - often not as well defined.
25 Section 4 - Shear Strength of Soils -ve pwp+ve pwpelog Water squeezed outWater sucked inCritical State LineDiagram gives an insight into why some slopes appear to fail soon after they have formed, while in other cases they are initially stable, but fail much later.
26 Section 4 - Shear Strength of Soils 4.9 Unifying remarks on the behaviour of soils under shear.DrainedSome soils expandSome soils contractDepends on initial compaction.UndrainedSome samples +ve pwp developSome samples -ve pwp developAll samples move towards Critical State Line (CSL)What happens if sample has OCR consistent with CSL?sample shears with no volume change in dense testor no pore water change in undrained test.