25 Soil PlasticityFurther classification within fine-grained soils (i.e. soil that passes #200 sieve) is done based on soil plasticity.Albert Atterberg, Swedish Soil Scientist ( )…..series of tests for evaluating soil plasticityArthur Casagrande adopted these tests for geotechnical engineering purposes
26 Atterberg Limits Shrinkage limit Plastic limit Liquid limit solid Consistency of fine-grained soil varies in proportion to the water contentShrinkage limitPlastic limitLiquid limitsolidsemi-solidplasticliquidPlasticityIndex(cheese)(pea soup)(pea nut butter)(hard candy)
27 Liquid Limit (LL or wL) Empirical Definition The moisture content at which a 2 mm-wide groove in a soil pat will close for a distance of 0.5 in when dropped 25 times in a standard brass cup falling 1 cm each time at a rate of 2 drops/sec in a standard liquid limit device
28 Engineering Characterization of Soils Soil Properties that Control its Engineering BehaviorParticle Sizecoarse-grainedfine-grainedParticle/Grain Size DistributionParticle ShapeSoil Plasticity
29 Clay Morphology Scanning Electron Microscope (SEM) Shows that clay particles consist of stacks of plate-like layers
30 Soil Consistency Limits Albert Atterberg ( ) Swedish Soil Scientist….. Developed series of tests for evaluating consistency limits of soil (1911)Arthur Casagrande ( )……Adopted these tests for geotechnical engineering purposes
31 Arthur Casagrande ( )Joined Karl Terzaghi at MIT in as his graduate studentResearch project funded by Bureau of Public RoadsAfter completion of Ph.D at MIT Casagrande initiated Geotechnical Engineering Program at HarvardSoil Plasticity and Soil Classification (1932)
44 Stresses in Soil Masses PXXArea = A = P/ASoil UnitAssume the soil is fully saturated, all voids are filled with water.
45 ′ = - u where, ′ = effective stress From the standpoint of the soil skeleton, the water carries some of the load. This has the effect of lowering the stress level for the soil.Therefore, we may defineeffective stress = total stress minus pore pressure′ = - u where, ′ = effective stress = total stressu = pore pressure
46 ′ = - u Effective Stress The effective stress is the force carried by the soil skeleton divided by the total area of the surface.The effective stress controls certain aspects of soil behavior, notably, compression & strength.
47 ′z = iHi - u Effective Stress Calculations where, H = layer thicknesssat = saturated unit weightU = pore pressure = w ZwWhen you encounter a groundwater table, you must use effective stress principles; i.e., subtract the pore pressure from the total stress.
49 Compressibility & Settlement Settlement requirements often control the design of foundationsThis chapter provides a general overview of principles involved in settlement analysisThe subject will be dealt with in greater detail in Chapter 7.
50 Increase in Vertical Effective Stress Due to a Placement of a fillDue to an external load
51 }z f′ }z f′ Consolidation z0′ z′ z0′ z0′ z0′ z′ Before After Hz0′}z f′z0′z′Voids eVv = eVsVv = (e - e)VsVoidsSolidsSolidsBeforeVsVsAfter
79 Transcosna Grain Elevator Canada (Oct. 18, 1913) West side of foundation sank 24-ft
80 Shear Strength of Soils Soil derives its shear strength from two sources:Cohesion between particles (stress independent component)Cementation between sand grainsElectrostatic attraction between clay particlesFrictional resistance between particles (stress dependent component)
81 Shear Strength of Soils; Cohesion Dry sand with no cementationDry sand with some cementationSoft clayStiff clay
98 Example: Direct Shear Test Given:A direct shear test conducted on a soil sample yielded the following results:Normal Stress, (psi)Max. Shear Stress, S (psi)10.06.525.011.040.017.5Required:Determine shear strength parameters of the soil
104 Soil Shear Strength under Drained and Undrained Conditions …. Drained conditions occur when rate at which loads are applied are slow compared to rates at which soil material can drainSands drain fast; therefore under most loading conditions drained conditions exist in sandsExceptions: pile driving, earthquake loading in fine sands
105 Soil Shear Strength under Drained and Undrained Conditions …. In clays, drainage does not occur quickly; therefore excess pore water pressure does not dissipate quicklyTherefore, in clays the short-term shear strength may correspond to undrained conditionsEven in clays, long-term shear strength is estimated assuming drained conditions
106 Shear Strength in terms of Total Stress Shear Strength in terms of effective stressShear strength in terms of total stressu at hydrostatic value
117 Example: Unconfined Compression Test Given:An unconfined compression test conducted on a soil sample yielded the results shown in the table.Required:Determine undrained shear strength, Su of the soil
120 Triaxial Compression Test Unconfined compression test is used when = 0 assumption is validTriaxial compression is a more generalized versionSample is first compressed isotropically and then sheared by axial loading13
121 Triaxial Compression Test 13Load applied in 2 stagesconfining pressure, 3dev. stress, = 1 - 3