# Course : S0705 – Soil Mechanic

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

CONTENT SOIL STRENGTH (SESSION 17-18 : F2F)
STRESS – STRAIN RESPONSE (SESSION : OFC) Bina Nusantara

SESSION 17-18 SOIL STRENGTH
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 Bina Nusantara

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

SOIL STRENGTH FIELD INFLUENCE FACTOR LABORATORY
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 Bina Nusantara

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

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

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

 = c + .tan MOHR COULOMB CONCEPT c  3 1   1 = 3 + 
Mohr-Coulomb envelope line c 3 1 Mohr envelope line 1 = 3 +  Bina Nusantara

MOHR COULOMB CONCEPT n  1 > 3  1 3 3  1 (1) (2)
Bina Nusantara

 = c + n.tan MOHR COULOMB CONCEPT (1) and (2)
The failure occurs when the value of 1 is minimum or the value of (0.5 . Sin2 - Cos2 . tan) maximum Bina Nusantara

MOHR COULOMB CONCEPT 2 c   3 1 n   Failure Envelope Line
Bina Nusantara

EXAMPLE Determine : - n -  Bina Nusantara

EXAMPLE Center of Circle = Radius of Circle = Bina Nusantara

EXAMPLE Bina Nusantara

EXAMPLE Determine : -  -  Bina Nusantara

EXAMPLE Bina Nusantara

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

UNCONFINED COMPRESSION TEST
Bina Nusantara

UNCONFINED COMPRESSION TEST
Bina Nusantara

UNCONFINED COMPRESSION TEST
Bina Nusantara

DIRECT SHEAR TEST Bina Nusantara

DIRECT SHEAR TEST Clay/Silt Pasir c Bina Nusantara

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

TRIAXIAL TEST Test Condition Stage 1 Stage 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=uo) 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=ud0). At failure state =f ; pore water pressure u=uf=uo+ud(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=uo) 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=ud0). At failure state =f ; pore water pressure u=uf=uo+ud(f)=ud(f) Consolidated Drained (CD) Apply an added stress  at axial direction. The drainage line from the specimen is opened (drainage is permitted) so the pore water pressure (u=ud) is equal to zero. At failure state =f ; pore water pressure u=uf=uo+ud(f)=0  3 3 Bina Nusantara

TRIAXIAL TEST Bina Nusantara

TRIAXIAL TEST Bina Nusantara

TRIAXIAL TEST Bina Nusantara

TRIAXIAL TEST Bina Nusantara

Type of tests and shear strength
SHEAR STRENGTH OF SOIL SELECTION OF TRIAXIAL TEST Soil type Type of construction Type of tests and shear strength Cohesive Short term (end of construction time) Triaxial UU or CU for Undrained Strength with appropriate level of insitu strength Staging Construction Triaxial CU for Undrained Strength with appropriate level of insitu strength Long term Triaxial CU with pore water pressure measurement or Triaxial CD for effective shear strength parameter Granular All Strength parameter ’ which is got from field investigation or direct shear test Material c- Long Term Bina Nusantara

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

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

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

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

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

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

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

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

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

SESSION 19-20 STRESS-STRAIN RESPONSE
Bina Nusantara

STRESS-STRAIN MODELS Non-Linear and Elastic Linear and Elastic
Bina Nusantara

STRESS-STRAIN MODELS Elastic Perfectly Plastic Elasto-Plastic
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. s dx t dz H gxz gxz = dx/H ez = - dz/H = ev Bina Nusantara

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

SAND BEHAVIOUR 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 Bina Nusantara

SAND BEHAVIOUR 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) 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 e log s’ CSL swelling line NCL - normal consolidation line It is found that NCL and CSL have the same slope in e-log s’ Bina Nusantara

CLAY BEHAVIOUR – DRAINED CONDITION
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. Bina Nusantara

CLAY BEHAVIOUR – UNDRAINED CONDITION
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, s’ plot. Once the critical state has been determined in the e, s’ 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. 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 Bina Nusantara

DIFFERENCES BETWEEN SAND AND CLAY
All soils are essentially frictional materials but different parameters are used for sands (Id) and clays (OCR) e Clay Loose Sand Dense NCL NCL log s’ (MPa) Bina Nusantara 0.1 1 10 100

APPLICATION Bina Nusantara

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