Presentation on theme: "Lecture-8 Shear Strength of Soils"— Presentation transcript:
1 Lecture-8 Shear Strength of Soils Dr. Attaullah Shah
2 Strength of different materials SoilShear strengthPresence of pore waterComplexbehaviorSteelTensile strengthConcreteCompressive strength
3 What is Shear Strength?Shear strength in soils is the resistance to movement between particles due to physical bonds from:Particle interlockingAtoms sharing electrons at surface contact pointsChemical bonds (cementation) such as crystallized calcium carbonate
4 Influencing Factors on Shear Strength The shearing strength, is affected by:soil composition: mineralogy, grain size and grain size distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid.Initial state: State can be describe by terms such as: loose, dense, over-consolidated, normally consolidated, stiff, soft, etc.Structure: Refers to the arrangement of particles within the soil mass; the manner in which the particles are packed or distributed. Features such as layers, voids, pockets, cementation, etc, are part of the structure.
5 Shear Strength of SoilIn reality, a complete shear strength formulation would account for all previously stated factors.Soil behavior is quite complex due to the possible variables stated.Laboratory tests commonly used:Direct Shear TestUnconfined Compression Testing.
6 Soil Failure and shear strength. Soil failure usually occurs in the form of “shearing” along internal surface within the soil.Thus, structural strength is primarily a function of shear strength.Shear strength is a soils’ ability to resist sliding along internal surfaces within the soil mass.
7 Slope Stability: Failure is an Example of Shearing Along Internal Surface
11 Shear failure Soils generally fail in shear embankmentmobilized shear resistancestrip footingfailure surfaceAt failure, shear stress along the failure surface reaches the shear strength.
12 Shear failurefailure surfaceThe soil grains slide over each other along the failure surface.No crushing of individual grains.
13 Shear failure mechanism At failure, shear stress along the failure surface () reaches the shear strength (f).
14 Shear failure of soilsSoils generally fail in shearRetaining wall
15 Shear failure of soils Soils generally fail in shear Failure surfaceMobilized shear resistanceRetaining wallAt failure, shear stress along the failure surface (mobilized shear resistance) reaches the shear strength.
16 Mohr-Coulomb Failure Criterion failure envelopeffriction anglecohesioncf is the maximum shear stress the soil can take without failure, under normal stress of .
17 Mohr-Coulomb Failure Criterion (in terms of total stresses) cfailure envelopeCohesionFriction angleff is the maximum shear stress the soil can take without failure, under normal stress of .
18 Mohr-Coulomb Failure Criterion (in terms of effective stresses) ’c’’failure envelopeEffective cohesionEffectivefriction anglefu = pore water pressuref is the maximum shear stress the soil can take without failure, under normal effective stress of ’.
19 Mohr-Coulomb Failure Criterion Shear strength consists of two components: cohesive and frictional.’ff’'c’’f tan ’frictional componentc’cohesive component
20 Mohr-Coulomb Failure Criterion Shear strength consists of two components: cohesive and frictional.cfrictional componentcohesive componentfff tan cc and are measures of shear strength.Higher the values, higher the shear strength.
24 Determination of shear strength parameters of soils (c, f or c’, f’) Laboratory tests on specimens taken from representative undisturbed samplesField testsVane shear testTorvanePocket penetrometerFall conePressuremeterStatic cone penetrometerStandard penetration testMost common laboratory tests to determine the shear strength parameters are,Direct shear testTriaxial shear testOther laboratory tests include,Direct simple shear test, torsional ring shear test, plane strain triaxial test, laboratory vane shear test, laboratory fall cone test
25 Laboratory tests z svc + Ds shc After and during construction Field conditionszsvcshcBefore constructionA representative soil sample
26 Laboratory tests shc Simulating field conditions in the laboratory svc + DsshcTraxial testLaboratory testsSimulating field conditions in the laboratoryRepresentative soil sample taken from the siteStep 1Set the specimen in the apparatus and apply the initial stress conditionsvcshcsvctDirect shear testStep 2Apply the corresponding field stress conditions
27 Schematic diagram of the direct shear apparatus Direct shear testSchematic diagram of the direct shear apparatus
28 Preparation of a sand specimen Direct shear testDirect shear test is most suitable for consolidated drained tests specially on granular soils (e.g.: sand) or stiff claysPreparation of a sand specimenComponents of the shear boxPreparation of a sand specimenPorous plates
29 Preparation of a sand specimen Direct shear testSpecimen preparation completedPressure platePreparation of a sand specimenLeveling the top surface of specimen
30 Direct shear test Steel ball P Test procedure Pressure plate Step 1: Apply a vertical load to the specimen and wait for consolidationPPressure platePorous platesProving ring to measure shear forceS
31 Direct shear test Steel ball P Test procedure Pressure plate Step 1: Apply a vertical load to the specimen and wait for consolidationPTest procedurePressure plateSteel ballProving ring to measure shear forceSPorous platesStep 2: Lower box is subjected to a horizontal displacement at a constant rate
32 Direct shear test Shear box Loading frame to apply vertical load Dial gauge to measure vertical displacementShear boxProving ring to measure shear forceDial gauge to measure horizontal displacementLoading frame to apply vertical load
33 Analysis of test results Direct shear testAnalysis of test resultsNote: Cross-sectional area of the sample changes with the horizontal displacement
34 Direct shear tests on sands Stress-strain relationshipShear stress, tShear displacementDense sand/ OC claytfLoose sand/ NC claytfChange in height of the sampleExpansionCompressionShear displacementDense sand/OC ClayLoose sand/NC Clay
35 Direct shear tests on sands How to determine strength parameters c and fShear stress, tShear displacementtf3Normal stress = s3tf2Normal stress = s2tf1Normal stress = s1Shear stress at failure, tfNormal stress, sfMohr – Coulomb failure envelope
36 Direct shear tests on sands Some important facts on strength parameters c and f of sandDirect shear tests are drained and pore water pressures are dissipated, hence u = 0Sand is cohesionless hence c = 0Therefore,f’ = f and c’ = c = 0
37 Direct shear tests on clays In case of clay, horizontal displacement should be applied at a very slow rate to allow dissipation of pore water pressure (therefore, one test would take several days to finish)Failure envelopes for clay from drained direct shear testsShear stress at failure, tfNormal force, sf’Normally consolidated clay (c’ = 0)Overconsolidated clay (c’ ≠ 0)
38 Interface tests on direct shear apparatus In many foundation design problems and retaining wall problems, it is required to determine the angle of internal friction between soil and the structural material (concrete, steel or wood)Where,ca = adhesion,d = angle of internal friction