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TOPIC 5 SOIL BEHAVIOUR Course: S0705 – Soil Mechanic Year: 2008

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Bina Nusantara CONTENT SOIL STRENGTH (SESSION : F2F) STRESS – STRAIN RESPONSE (SESSION : OFC)

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Bina Nusantara SESSION SOIL STRENGTH

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Bina Nusantara SOIL STRENGTH DEFINITION The maximum or ultimate stress the material can sustain against the force of landslide, failure, etc. APPLICATION Soil Strength can be used for calculating : –Bearing Capacity of Soil –Slope Stability –Lateral Pressure

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Bina Nusantara VERTICAL SLOPE RETAINING EARTH WALL EMBANKMENT LANDSLIDE GLOBAL FAILURE OF SHALLOW FOUNDATION LOCAL FAILURE OF SHALLOW FOUNDATION SOIL STRENGTH

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Bina Nusantara SOIL STRENGTH FIELD INFLUENCE FACTOR –Soil Condition : void ratio, particle shape and size –Soil Type : Sand, Sandy, Clay etc –Water Content (especially for clay) –Type of Load and its Rate –Anisotropic Condition LABORATORY –Test Method –Sample Disturbing –Water Content –Strain Rate

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Bina Nusantara SHEAR STRENGTH OF SOIL PARAMETER –Cohesion (c) –Internal Friction Angle ( ) CONDITION –Total (c and ) –Effective (c’ and ’) GENERAL EQUATION (COULOMB) = c + n.tan

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Bina Nusantara SOIL TYPES COHESIVE SOIL –Has cohesion (c) –Example : Clay, Silt COHESIONLESS Soil –Only has internal friction angle ( ) ; c = 0 –Example : Sand, Gravel

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Bina Nusantara SHEAR STRENGTH PARAMETER COHESION (C) Sticking together of like materials. INTERNAL FRICTION ANGLE ( ) The stress-dependent component which is similar to sliding friction of two or more soil particles

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Bina Nusantara SHEAR STRENGTH PARAMETER UNDRAINED SHEAR STRENGTH Use for analysis of total stress Commonly = 0 and c = c u DRAINED SHEAR STRENGTH Use for analysis of effective stress, with parameter c’ and ’ ’ = c’ + ( n – u) tan ’

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Bina Nusantara MOHR COULOMB CONCEPT Mohr envelope line Mohr-Coulomb envelope line c 33 33 11 11 = c + .tan 1 = 3 +

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Bina Nusantara MOHR COULOMB CONCEPT 33 11 11 33 1 > 3 33 11 nn (1) (2)

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Bina Nusantara MOHR COULOMB CONCEPT = c + n.tan (1) and (2) The failure occurs when the value of 1 is minimum or the value of (0.5. Sin2 - Cos 2 . tan ) maximum

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Bina Nusantara MOHR COULOMB CONCEPT 22 c 33 11 nn Failure Envelope Line

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Bina Nusantara EXAMPLE Determine : - n -

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Bina Nusantara EXAMPLE Center of Circle = Radius of Circle =

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Bina Nusantara EXAMPLE

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Bina Nusantara EXAMPLE Determine : - -

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Bina Nusantara EXAMPLE

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Bina Nusantara SHEAR STRENGTH OF SOIL LABORATORY TESTS –Unconfined Compression Test –Direct Shear Test –Triaxial Test (UU, CU, CD) FIELD INVESTIGATION –Vane Shear Test PARAMETER CORRELATIONS –Cone Resistance (q c ) –N-SPT Value –California Bearing Capacity

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Bina Nusantara UNCONFINED COMPRESSION TEST

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Bina Nusantara UNCONFINED COMPRESSION TEST

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Bina Nusantara UNCONFINED COMPRESSION TEST

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Bina Nusantara DIRECT SHEAR TEST

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Bina Nusantara DIRECT SHEAR TEST Pasir Clay/Silt c

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Bina Nusantara 3 Conditions –Unconsolidated Undrained (UU) –Consolidated Undrained (CU) –Consolidated Drained (CD) TRIAXIAL TEST

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Bina Nusantara TRIAXIAL TEST Test ConditionStage 1Stage 2 Unconsolidated Undrained (UU) Apply confining pressure 3 while the drainage line from the specimen is kept closed (drainage is not permitted), then the initial pore water pressure (u=u o ) is not equal to zero Apply an added stress at axial direction. The drainage line from the specimen is still kept closed (drainage is not permitted) (u=u d 0). At failure state = f ; pore water pressure u=u f =u o +u d(f) Consolidated Undrained (CU) Apply confining pressure 3 while the drainage line from the specimen is opened (drainage is permitted), then the initial pore water pressure (u=u o ) is equal to zero Apply an added stress at axial direction. The drainage line from the specimen is kept closed (drainage is not permitted) (u=u d 0). At failure state = f ; pore water pressure u=u f =u o +u d(f) =u d(f) Consolidated Drained (CD) Apply confining pressure 3 while the drainage line from the specimen is opened (drainage is permitted), then the initial pore water pressure (u=u o ) is equal to zero Apply an added stress at axial direction. The drainage line from the specimen is opened (drainage is permitted) so the pore water pressure (u=u d ) is equal to zero. At failure state = f ; pore water pressure u=u f =u o +u d(f) =0 33 33 33 33

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Bina Nusantara TRIAXIAL TEST

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Bina Nusantara TRIAXIAL TEST

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Bina Nusantara TRIAXIAL TEST

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Bina Nusantara TRIAXIAL TEST

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Bina Nusantara SHEAR STRENGTH OF SOIL SELECTION OF TRIAXIAL TEST Soil typeType of constructionType of tests and shear strength CohesiveShort term (end of construction time) Triaxial UU or CU for Undrained Strength with appropriate level of insitu strength Staging ConstructionTriaxial CU for Undrained Strength with appropriate level of insitu strength Long termTriaxial CU with pore water pressure measurement or Triaxial CD for effective shear strength parameter GranularAll Strength parameter ’ which is got from field investigation or direct shear test Material c- Long TermTriaxial CU with pore water pressure measurement or Triaxial CD for effective shear strength parameter

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Bina Nusantara EXAMPLE USE OF UU STRENGTH IN ENGINEERING PRACTICE Embankment constructed rapidly over a soft clay deposit

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Bina Nusantara EXAMPLE USE OF UU STRENGTH IN ENGINEERING PRACTICE Large earth dam constructed rapidly with no change in water content of clay core

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Bina Nusantara EXAMPLE USE OF UU STRENGTH IN ENGINEERING PRACTICE Footing placed rapidly on clay deposit

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Bina Nusantara EXAMPLE USE OF CU STRENGTH IN ENGINEERING PRACTICE Embankment raised (2) subsequent to consolidation under its original height (1)

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Bina Nusantara EXAMPLE USE OF CU STRENGTH IN ENGINEERING PRACTICE Rapid drawdown behind an earth dam No drainage of the core. Reservoir level falls from 1 2

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Bina Nusantara EXAMPLE USE OF CU STRENGTH IN ENGINEERING PRACTICE Rapid construction of an embankment on a natural slope

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Bina Nusantara EXAMPLE USE OF CD STRENGTH IN ENGINEERING PRACTICE Embankment constructed very slowly, in layers, over a soft clay deposit

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Bina Nusantara EXAMPLE USE OF CD STRENGTH IN ENGINEERING PRACTICE Earth dam with steady-state seepage

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Bina Nusantara EXAMPLE USE OF CD STRENGTH IN ENGINEERING PRACTICE Excavation or natural slope in clay

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Bina Nusantara SELECTION OF SHEAR STRENGTH PARAMETER CU with pore water pressure measurement

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Bina Nusantara SESSION STRESS-STRAIN RESPONSE

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Bina Nusantara STRESS-STRAIN MODELS Stress, Linear and Elastic Stress, Strain, Non-Linear and Elastic Strain,

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Bina Nusantara STRESS-STRAIN MODELS Strain, Stress, Elasto-Plastic Stress, Strain, Elastic Perfectly Plastic

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Bina Nusantara STRESS-STRAIN RESPONSE OF SOILS Triaxial tests are the standard means of investigating the stress-strain-strength response of soils. To simplify, only simple shear tests will be considered. The simple shear test is an improved shear box test which imposes more uniform stresses and strains. dx H dz xz xz = dx/H z = - dz/H = v

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Bina Nusantara SAND BEHAVIOUR Depends on: Mean Effective stress (Normal effective stress in simple shear) Relative density, I d

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Bina Nusantara SAND BEHAVIOUR

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Bina Nusantara SAND BEHAVIOUR For tests performed with the same normal stress All samples approach the same ultimate shear stress and void ratio, irrespective of the initial relative density Initially dense samples attain higher peak angles of friction Initially dense soils expand (dilate) when sheared Initially loose soils compress when sheared

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Bina Nusantara SAND BEHAVIOUR

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Bina Nusantara SAND BEHAVIOUR The ultimate values of shear stress and void ratio depend on the applied normal stress The ultimate stress ratio and angle of friction are independent of density and stress level Initially dense samples attain higher peak angles of friction, but the peak friction angle decreases as the stress increases Initially dense soils expand and initially loose soils compress when sheared. Increasing the normal stress causes less dilation (more compression)

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Bina Nusantara CLAY BEHAVIOUR Essentially the same as sands. However, data presented as a function of OCR rather than relative density. OCR is defined as NCL - normal consolidation line e log s’ CSL swelling line It is found that NCL and CSL have the same slope in e-log s’

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Bina Nusantara CLAY BEHAVIOUR – DRAINED CONDITION

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Bina Nusantara CLAY BEHAVIOUR – DRAINED CONDITION In drained loading the change in effective stress is identical to the change in total stress. In a shear box (or simple shear) test the normal stress is usually kept constant, and hence the response is fixed in the t, s’ plot. The soil heads towards a critical state when sheared, and this ultimate (or critical) state can be determined from the t, s’ plot. The change in void ratio can then be determined. Knowing the sign of the volume change enables the likely stress-strain response to be estimated.

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Bina Nusantara CLAY BEHAVIOUR – UNDRAINED CONDITION

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Bina Nusantara CLAY BEHAVIOUR – UNDRAINED CONDITION In undrained loading the void ratio (moisture content) must stay constant. The soil must head towards a critical state when sheared, and knowing e the critical state can be determined from the e, ’ plot. Once the critical state has been determined in the e, ’ plot the ultimate shear stress is also fixed. The ultimate shear stress is related to the undrained strength. This relation can be obtained by considering a Mohr’s circle.

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Bina Nusantara CLAY BEHAVIOUR – UNDRAINED CONDITION In undrained loading the effective stresses are fixed because void ratio (moisture content) must stay constant. The total stresses are controlled by the external loads, and the pore pressure is simply the difference between the total stress and effective stress. The CSL provides an explanation for the existence of cohesion (undrained strength) in frictional soils From the CSL it can also be seen that changes in moisture content (void ratio) will lead to different undrained strengths

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Bina Nusantara DIFFERENCES BETWEEN SAND AND CLAY All soils are essentially frictional materials but different parameters are used for sands (Id) and clays (OCR) log ’ (MPa) e NCL Loose Dense Clay Sand

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Bina Nusantara APPLICATION

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