Zeynep ÇEKİNMEZ MSc Civil Engineer

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Middle East Technical University Civil Engineering Department. GENERAL INFORMATION ABOUT PLAXIS 2D and an EXAMPLE OF BRACED EXCAVATION in PLAXIS 2D. Zeynep ÇEKİNMEZ MSc Civil Engineer. OUTLINE. General Information about Plaxis General Information about Plaxis 2D Types of Modelling

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Middle East Technical University

Civil Engineering Department

and

an EXAMPLE OF BRACED EXCAVATION

in PLAXIS 2D

Zeynep ÇEKİNMEZ

MSc Civil Engineer

OUTLINE
• General Information about Plaxis 2D
• Types of Modelling
• Examples of Graphical Input of Geometry Models
• Elements used in Geometry Models
• Required Input Soil Parameters
• General Procedure of Analysis
• Flow Chart
• Step by Step Modelling an Example: “Assignment #6: Braced Excavation for Sewage Pipe System”
• Interpretation of results of the example
• Comparision of hand calculation and PLAXIS results
• Analysis in Plaxis Flow
• Flow Input
• Step by Step Modelling
• Software is from Delft/Netherlands
• Most widely-used software in geotechnical aspects:
• Researchers (MS and Phd students)
• Project Analysis
• Design Applications
• Synchronizing the real behavior of in-situ soil and the calculated one.
• User friendly, Finite Element Program (FE programe)
• Stresses developed at stress points
• Strains developed at nodes

By using STIFFNESS MATRIX [E]

• PLAXIS is a software analysing deformation, stresses and stability validation that occur in constructions in the field of geotechnical engineering by step by step constructionStaged Construction.
• PLAXIS software has various versions each analysis different type of structures/problems:
• PLAXIS 2D Foundation
• PLAXIS 2D Dynamics
• PLAXIS 2D Tunnel
• PLAXIS 2D Flow
• PLAXIS 3D Foundation
• PLAXIS 3D Tunnel
Types of Modelling

Plaxis 2D provides two different type of modelling:

1) Plane Strain

• Used for geometries with a (more or less) uniform cross section and corresponding stress state and loading scheme over a certain length perpendicular to the cross-section (z-Direction)
• Displacements and strains in z-direction are assumed to be zero. However normal stresses in z-direction are fully taken into account

y

Inputs and outputs are in unit/m

x

Types of Modelling

2) Axisymmetric model

• Used for circular structures with a (more or less) uniform radial cross section and loading scheme around to be identical in any radial direction.
• Note that for axisymmetric problems the x-coordinate represents the radius and the y-coordinate corresponds to the axial line of symmetry. Negative x-coordinates cannot be used.

y

x

Inputs and outputs are in unit/2π

Graphical input of geometry models

Node-to-Node Anchors-Struts

Surcharge

Plate Elements

Interfaces

Boundary Condition: ux=0, uy is free

Boundary Condition: ux=0, uy=0

Graphical input of geometry models

Node-to-Node Anchors-Tie Backs

Geogrid-Anchor Grout

Elements Used in Geometry Models

Used Elements

• Plates (foundations, walls, plates, piles etc.  structural members)
• Node-to-Node Anchors (struts, anchor tie-backs)
• Geogrids (Anchor grout, geosythetics)
• Interface (soil-structure interaction, flow conditions)
• Tunnel
• Hinge/Rotation (Connections of structural members)
• Prescribed Displacement
• Well
• Drain
Soil Models

Soil Models (stress-strain relation depending on constant/varying stiffness):

• Mohr-Coulomb (general)
• HS Small Model (small strains in hardening soils)
• Soft Soil Model (for clays, primary consolidation)
• Soft Soil Creep Model (for creep soils in soft clays, secondary consolidation)
• Joint Rock Model (for rock and its special characteristics)
Flow Chart

Geometry & Soil Properties & Element Properties

Mesh Generation

Initial Conditions

Pore Pressure Condition

K0-condition

Staged Construction

Geometry Configuration

Water Pressures

Update Mesh

Running

Results

Geometry & Soil Properties & Element Properties

Mesh Generation

Initial Conditions

K0-condition

Pore Pressure Condition

Geometry & Soil Properties & Element Properties

Mesh Generation

Initial Conditions

Pore Pressure Condition

K0-condition

Staged Construction

Geometry & Soil Properties & Element Properties

Mesh Generation

Initial Conditions

Staged Construction

Geometry Configuration

Water Pressures

Comparision: Summary of the Results
• Results of the analyse and the hand-calculations are different because of:
• “Staged Construction” in Plaxis
• Assumption of Trapezoidal Lateral Earth Pressure
• Total stress+pore pressure/effective stress+pore pressure Limitation of “Design Pressure Distributions”
• Plaxis based on E on elastic range acc. to MC yield criteria