JANUARY 2023
INTRODUCTORY COURSE IN
PLAXIS 2D FOR GEOTECHNICAL
ANALYSIS & DESIGN
By : Mr. Yoang Sothoan
Let’s Learn
PLAXIS
FUNDAMENTAL OF
GEOTECHNICAL ENGINEERING
LECTURE NOTE 07
INITIAL STRESS &
STRENGTH REDUCTION METHOD
Initial stresses
PLAXIS 2D
07- Initial Stresses & C-Phi Reduction Analysis
Initial stresses represent the equilibrium state of the undisturbed soil and
consist of:
1. Soil Weight
2. Loading history
In PLAXIS, Two Possibilities exist:
•
K0 Procedure
•
Gravity loading
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INTRODUCTORY COURSE IN PLAXIS 2D
Initial stresses
PLAXIS 2D
07- Initial Stresses & C-Phi Reduction Analysis
K0 Procedure
1. Generation of initial stresses during input
2. Require coefficient for earth pressure at
rest K0
3. Advantage
:
No
displacements
are
generated, only stresses
4. Disadvantage : No equilibrium for inclined
surface, geometry
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INTRODUCTORY COURSE IN PLAXIS 2D
Initial stresses
PLAXIS 2D
07- Initial Stresses & C-Phi Reduction Analysis
Gravity loading
1. Calculation of initial stresses by weight loading
2. Advantage : Equilibrium satisfied in all cases
3. Disadvantage : Non-physical displacements
are created.
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INTRODUCTORY COURSE IN PLAXIS 2D
Initial stresses
PLAXIS 2D
07- Initial Stresses & C-Phi Reduction Analysis
Gravity loading
Recommended cases where Gravity loading should be used
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INTRODUCTORY COURSE IN PLAXIS 2D
Initial stresses
PLAXIS 2D
07- Initial Stresses & C-Phi Reduction Analysis
Gravity loading
Procedure:
1- Initial Phase
- Select K0 procedure, SMweight = 0
2- Phase 1
- Choose Plastic calculation, Total multipliers,
- Set weight multiplier SMweight = 1, this will reset all initial stresses to zero.
- Select Ignore undrained behavior to prevent generation of unrealistic excess
pore pressures
3- Phase 2
- Select Reset displacements to zero to discard all displacements from
raising the gravity
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INTRODUCTORY COURSE IN PLAXIS 2D
Initial stresses
PLAXIS 2D
07- Initial Stresses & C-Phi Reduction Analysis
Special cases
For complex initial situations, it may be needed to use several calculation phases
to model the current situation before starting the actual project.
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INTRODUCTORY COURSE IN PLAXIS 2D
C-Phi Reduction Analysis in Plaxis
PLAXIS 2D
07- Initial Stresses & C-Phi Reduction Analysis
Main advantages
• Requires no a-priori assumptions on the failure mechanism
• Critical surface is found automatically as slope failure occurs naturally through
the zones due to insufficient shear strength to resist shear stresses
• No requirement of assumptions on the inter-slice shear force distribution
• Application to complex conditions
• Information such as stresses, movements and pore pressures as for
deformation analysis
• Powerful approach
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INTRODUCTORY COURSE IN PLAXIS 2D
C-Phi Reduction Analysis in Plaxis
PLAXIS 2D
07- Initial Stresses & C-Phi Reduction Analysis
Safety factor
• PLAXIS = safety factor on soil resistance
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INTRODUCTORY COURSE IN PLAXIS 2D
C-Phi Reduction Analysis in Plaxis
PLAXIS 2D
07- Initial Stresses & C-Phi Reduction Analysis
Safety Factor Analysis
C-Phi reduction:
- Reduction of strength parameters c and tan(f) until failure is reached.
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INTRODUCTORY COURSE IN PLAXIS 2D
C-Phi Reduction Analysis in Plaxis
PLAXIS 2D
07- Initial Stresses & C-Phi Reduction Analysis
Safety Factor Analysis
Calculation procedure:
- Create a c/phi reduction phase
- Accept the default increment for Msf=0.1 from the multiplier tab sheet
- Calculate
- Carefully examine SMsf vs. displacement curve in plaxis curves.
Notes:
• Select control point within (expected) failure body
• Use sufficient number of load steps
• Use a sufficient fine mesh
• Limit the maximum structural forces by choosing elasto-plastic behavior for
walls, anchors, and geotectiles
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INTRODUCTORY COURSE IN PLAXIS 2D
C-Phi Reduction Analysis in Plaxis
PLAXIS 2D
07- Initial Stresses & C-Phi Reduction Analysis
Safety Factor Analysis
Number of load steps control:
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INTRODUCTORY COURSE IN PLAXIS 2D
C-Phi Reduction Analysis in Plaxis
PLAXIS 2D
07- Initial Stresses & C-Phi Reduction Analysis
Safety Factor Analysis
Failure mechanism inspection:
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INTRODUCTORY COURSE IN PLAXIS 2D