ENV-2E1Y: Fluvial Geomorphology: 2004 - 5

1. ENV-2E1Y: Fluvial Geomorphology: 2004 - 5 Slope Stability and Geotechnics Landslide Hazards River Bank Stability Section 4 - Shear Strength of Soils N.K. Tovey Н.К.Тови М.А., д-р технических наук Landslide on Main Highway at km 365 west of Sao Paulo: August 2002

2. ENV-2E1Y: Fluvial Geomorphology: 2004 - 5 • Introduction • Seepage and Water Flow through Soils • Consolidation of Soils • Shear Strength~ 1 lecture • Slope Stability ~ 4 lectures • River Bank Stability~ 2 lectures • Special Topics • Decompaction of consolidated Quaternary deposits • Landslide Warning Systems • Slope Classification • Microfabric 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 object • a shear load or force is one which acts along the plane of the surface of an object • the 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 EQUILIBRIUM • There are three conditions: • the net effect of all forces parallel to one direction must be zero • the net effect of all forces orthogonal (at right angles) to the above direction must be zero • the sum of the moments of the forces must be zero • the first two conditions can be checked by resolving forces (e.g. see Fig. 4.1)

5. P1 3 2 P2 P3 Section 4 - Shear Strength of Soils At Equilibrium: Resolve forces parallel to P1 :- P1 = P2 cos 2 + P3 cos 3 ...........4.1 Similarly at right angles to P1 P2 sin 2 = P3 sin 3 ...........4.2 • Resolution of Forces

6. Section 4 - Shear Strength of Soils Coulomb: a French Military Engineer Problem: Why do Military Fortifications Fail?

7. N F F N Section 4 - Shear Strength of Soils 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 

8. N F F  C N Section 4 - Shear Strength of Soils 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

9. Section 4 - Shear Strength of Soils F = C + N tan ......4.4 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. F A B N Section 4 - Shear Strength 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

11. N N N N N N N N F - F F  Displacement N Section 4 - Shear Strength of Soils dense loose Peak in dense test is reached at around 1 - 3% strain

12.  displacement Section 4 - Shear Strength of Soils Increasing normal stress dense / loose Displacement Normalising curves to normal stress leads to a unique set of curves for each soil.

13. N N N N N N F BANG! Displacement Section 4 - Shear Strength of Soils • Types of Shear Test • Stress controlled test • Strain controlled test (as done in practical) Failure in stress controlled test Readings cannot be taken after peak in a stress controlled test

14.   displacement displacement V V displacement displacement Section 4 - Shear Strength of Soils Dense Test Loose Test Medium Dense

15. Void Ratio displacement Section 4 - Shear Strength of Soils Plot volume changes as Void Ratio loose Critical void ratio • All tests eventually come to same Void Ratio medium dense

16. Section 4 - Shear Strength of Soils Effects of Water Pressure •  = c +  tan  • Does not allow for water pressure. • Principal of Effective Stress • From Consolidation Total Stress = effective stress + pore water pressure • or ’ =  - u • In terms of stresses involved water cannot take shear • so  = c + (  - u ) tan  • or  = c + ’ tan  • Mohr - Coulomb failure criterion • if pore water pressure = 0 then original equation applies

17.  A  Section 4 - Shear Strength of Soils • Distance stress point is from failure line is a measure of stability. • Greater distance > greater stability Mohr - Coulomb -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 Slopes near Hadleigh Essex are only stable because of -ve pwp

18. Section 4 - Shear Strength of Soils The Triaxial Test • Problems with Standard Shear Box • Shear zone is complex • Difficult to get undisturbed samples which are square • Difficult to do undrained or partially drained tests • sands - always will be drained • clays - may be partially drained - depends of strain rate.

19. Section 4 - Shear Strength of Soils The Triaxial Test Load Cell Pressure Sample in rubber membrane Porous stone

20. Section 4 - Shear Strength of Soils The Triaxial Test • Cell pressure can be varied to match that in ground • cylindrical samples can be obtained • sample can be sealed to prevent drainage or to allow partial drainage • can perform both undrained and drained tests

21. Section 4 - Shear Strength of Soils • Drained Test • allow 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 Test • no drainage is allowed. • measure the pore water pressures during the test.

22. Section 4 - Shear Strength of Soils • Drained Test • response to load and volume change is similar to standard shear box. • Undrained Test • burette 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

23. +ve +ve water pressure water pressure displacement displacement -ve -ve Section 4 - Shear Strength of Soils Dense Loose • In undrained dense tests pwp goes negative • In drained dense tests volume increases

24. Section 4 - Shear Strength of Soils • 4.8 Failure modes in the Triaxial Test. • Loading • its length will shorten as the strain increases • some 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 visible • usually numerous micro failure zones criss-crossing the bulging region. • Undrained test • orientation of the failure zone is at 45o to the horizontal, • Drained test • orientation will be at (45 + /2), - often not as well defined.

25. -ve pwp +ve pwp Water squeezed out e Water sucked in Critical State Line log  Section 4 - Shear Strength of Soils • Diagram 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. • Drained • Some soils expand • Some soils contract • Depends on initial compaction. • Undrained • Some samples +ve pwp develop • Some samples -ve pwp develop • All samples move towards Critical State Line (CSL) • What happens if sample has OCR consistent with CSL? • sample shears with no volume change in dense test • or no pore water change in undrained test.