ENV2 E 1 Y Fluvial Geomorphology 2004 5

ENV-2 E 1 Y: 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

ENV-2 E 1 Y: 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

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

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)

Section 4 - Shear Strength of Soils • Resolution of Forces P 1 At Equilibrium: Resolve forces parallel to P 1 : P 1 = P 2 cos 2 + P 3 cos 3. . . 4. 1 3 2 P 3 Similarly at right angles to P 1 P 2 sin 2 = P 3 sin 3. . . 4. 2

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

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? N F F F = N tan . . . 4. 3 is the angle of internal friction 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 N F Block will fail F F = C + N tan C is the cohesion . . . 4. 4 Block is stable C N

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 • = tan – cohesive materials - i. e. = 0 (wet clays) • = c – materials with both cohesion and friction • = c + tan (sands)

Section 4 - Shear Strength of Soils • Stress Point at B - stable F • Stress Point at A F F A - stable only if cohesion is present G G • if failure line changes, then failure may occur. B N

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

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

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

Section 4 - Shear Strength of Soils Dense Test Loose Test displacement V V Medium Dense displacement

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

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

Section 4 - Shear Strength of Soils • Distance stress point is from failure line is a measure of stability. b m lo r h o u o -C M +ve pwp Moves point closer to failure line less stability A -ve pwp moves stress point to right Moves point further from failure line • Greater distance > greater stability Slopes near Hadleigh Essex are greater stability only stable because of -ve pwp

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.

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

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

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.

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

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

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 45 o to the horizontal, • Drained test – orientation will be at (45 + /2), - often not as well defined.

Section 4 - Shear Strength of Soils -ve pwp +ve pwp Water squeezed out e Water sucked in Critical State Line log • 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.

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.