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MIDAS Geotechnical Training SeriesFully Coupled vs Consolidation Analysis

MIDAS Information Technology

Seongwan Bae

Angel F. Martinez



Project Overview

(Fully Coupled vs Consolidation Analysis)

GTS NX

3

Geometry Import

Geometry Works

Mesh generation

Boundary Conditions

Load Conditions

Define Water Level

STEP 01

STEP 02

STEP 03

STEP 04

STEP 05

STEP 06

Define Construction Sequence

Analysis Case

STEP 07

STEP 08

Perform analysis and check resultSTEP 09

Overview

GTS NX

Overview

1-4

01 Consolidation_by several methods

• In order to accelerate consolidation

without additional embankment and

potential shear failure of soil, a

uniform atmospheric pressure can be

applied to each depth of the soft

layer.

•This example is to compare the

results between Suction drain

method and Embankment with free

drainage by fully coupled and

consolidation methods

• Main objectives for this analysis are

as follows.

➢ Initial and long term settlements

during and after construction

➢ Compressibility

➢ Lateral ground flow

➢ Slope failure / reduction of bearing

capacity

➢ Differential settlement on adjacent

structures, services (road)

➢ Improvement methods – drainage,

solidification, compaction

GTS NX

1-5

02 Material for Soil and Structures

Name Embankment AS1 Ac AS2

Material Isotropic Isotropic Isotropic Isotropic

Model Type Elastic ElasticModified Cam

ClayElastic

General

Elastic Modulus (E) 10000 5000 3450 140000

Poisson’s Ratio(v) 0.33 0.33 0.35 0.33

Unit Weight(r) 17 18 16 19

Ko 0.5 0.5 0.5 0.5

Porous

Unit Weight(Saturated) 18 18 16 19

Initial Void Ratio(eo) 0 0 1.76 0

Permeability (kx,ky,kz) (m/day) 0.009 0.009 0.001 0.86

Non-Linear

OCR - - 2.05 -

Slope of Consol line (λ) - - 0.313 -

Slope of Over Consol Line (k) - - 0.063 -

Slope of critical State Line (M) - - 1.113 -

GTS NX

1-6

03 Soil Property (2D Plain Strain Element)

Name Embankment AS1 Ac AS2

Property 2D 2D 2D 2D

Model Type Plain Strain Plain Strain Plain Strain Plain Strain

Material Embankment AS1 Ac AS2

GTS NX

1-7

04 Construction Sequence (3 sets)

* Analysis Case 1 (Fully Coupled

Suction Drain)

1. Initial state of ground

2. Apply suction to each depth

at an interval of 5m (100

days)

3. Leave time (1000 days)

* Analysis Case 2 (Fully Coupled

Embankment)


2. Embankment (100 days)


* Analysis Case 3 (Consolidation

Embankment )


2. Embankment (100 days)


GTS NX

Stress→ Consider P.W.P for stress analysis

→ Time-Independent nonlinear static analysis

Drained: Long-term stability

Undrained: Short-term stability

Seepage

StressSeepage

Fully coupled analysis

→ Time-integrating method / Time-Dependent analysis

→ Changes in stress/stain/pore pressure with time

→ Governed by permeability of ground

[One way coupled]

[Two way coupled interaction]

Fully vs Semi Coupled Analysis

• Semi Coupled or Sequential analysis is the simplest way to consider the pore water pressure distribution by conducting seepage analysis beforehand, and reflecting it in the total stress/effective stress relationship equation of the stress analysis conducted in the following step. This method can be used to understand the static stress state of the given steady groundwater flow. However, since deformation due to stress analysis does not influence the seepage phenomenon inversely, there is no two-way coupling.

• Fully-coupled Stress-Seepage analysis is the two-way coupled analysis between seepage analysis and stress analysis. Both analyses are used to solve the coupled equation. It can display the pore water pressure, stress or deformation changes with time.

Semi coupled analysis

GTS NX

DOF for displacement and

excessive pore stress

Constant hydrostatic pressure

Simulate dissipation of excessive

p.w.p with time

Consolidation analysis

DOF for displacement and total

pore stress

Simulate changes in hydrostatic

pressure with time.

All seepage boundaries (head,

flux, review) are available.

Fully coupled analysis Fully coupled

Consolidation

<Difference>

<Common>

→ Two way coupled interaction

→ Time dependent behavior based on quasi-static analysis

Consolidation vs Fully Coupled

[Relation b/w two analyses]

• The consolidation analysis begins with the assumption that steady state pore water pressure can be maintained, and is used to see the changes in excess pore water pressure. In other words, this analysis is used to simulate the phenomenon of how excess pore water pressure changes with the changes in load/boundary conditions.

• Fully-coupled Stress-Seepage analysis does not follow assumption that steady state pore water pressure is maintained. Hence, it is suitable for simulating the transient seepage phenomenon, stress analysis and stability in abnormal condition in a fully coupled form. Unlike the consolidation analysis, it is possible to define the changes in seepage boundary conditions with time, boundary flow rate etc. In other words, for Fully-coupled Stress-Seepage analysis, it is possible to use all the transient seepage boundary conditions, structural load and boundary conditions.

GTS NX

1-10

General

Water Level

[Line(2D), Surface(3D)]

Water Level for Mesh Set

Seepage / Fully Coupled Analysis

Nodal Head

Nodal Flux

Surface Flux

Review

Consolidation Analysis

Draining Condition

Non Consolidation

Key Boundary Conditions

Used to simulate the domain where the excess pore pressure is 0 (drain). The Drainage condition is used as a boundary condition for Consolidation analysis.

The elements have an additional pore pressure degree of freedom, as well as displacement degree of freedom, at the nodes. Used to model non-consolidation layers to turn off PP degree of freedom.

Input the head of the model. Both the constant head value for steady state analysis and the changing head value for Transient analysis can be entered by applying the Seepage boundary condition function. The Nodal head is used as a boundary condition for Seepage/Consolidation analysis (Fully-Coupled).

Create a changing groundwater level by selecting a geometry shape on the work screen.

GTS NXConcepts of Coupled Analysis

Semi-Coupled Consolidation Fully-Coupled

Nodal head(total head) O X O

Nodal head(pressure head) O X O

Boundary review O X O

Drainage condition X OO

(total head=0)

Non-consolidated condition X O O

Node/surface flow O X O

<List of applicable load/boundary conditions for each coupled analysis type>

< Outline of coupled analysis>

Fully Drained



Modeling Procedure

(Fully Coupled vs Consolidation Analysis)

GTS NX

Procedure

1-13

01 Create New Project

11

Main Menu > New

Analysis Setting > Model Type >

2D

Unit System > kN-m-day

2

2

GTS NX

Procedure

1-14

02

1

1

Main Menu > Import > DXF 2D

(Wireframe)

Select AutoCAD DXF File >

Embankment Consolidation vs

Fully Coupled CAD.dxf Open

Select OK

2

2

3

3

3

Geometry works (Create or Import from AutoCAD)

GTS NX

Procedure

1-15

03 Define Material / Property for Soil & Structures

1 Mesh > Prop. > Property >

Import

Select “Embankment

Consolidation vs Fully Coupled

Final.gts” file

Select All

Select OK

* Can Import from the existing model

file (Refer to Project Overview >

Material / Property)

2

3

1

2

3

1

GTS NX

Procedure

1-16

04 Generate Mesh (2D Element)

1 Mesh > Generate > 2D > Auto -

Area

Select Edge(s) > Select edges

for “Embankment” as

highlighted in the figure.

Input element Size : 1

(1m between two nodes)

Select Property : Embankment

Input Mesh Set Name :

Embankment

Select Apply

(Follow the same procedure

for below soil layers)

2

2

2

3

4

5

3

4

5

It is possible to input different size of element for each area. All nodes at each boundary line

will be connected automatically. This is to control Mesh Quality and the number of elements.

6

6

1

GTS NX

Procedure

1-17

04 Generate Mesh (2D Element)

1 Mesh > Generate > 2D > Auto -

Area

Select Edge(s) > Select edges

for “AS1” as highlighted in the

figure.

Input element Size : 1

(1m between two nodes)

Select Property : AS1

Input Mesh Set Name : AS1

Select Apply

(Follow the same procedure

for below soil layers)

2

2

2

3

4

5

3

4

5

6

6

After creating mesh for all area at once, it is possible to change “Property”.

Go to “Mesh > Element > Parameters > 2D”

1

GTS NX

Procedure

1-18

05 Define Boundary Condition (Water Level)

1 Static / Slope Analysis >

Boundary > Water Level > Edge

Select Target Edge(s) > Select

edges to define water level as


Interval : 1 (m)

Input Name : Water Level (1)

Select OK

1* 1 Water Level Function will be

defined for initial water level.

In the works tree, there are 1

water level functions created

automatically.

2

3

4

5

2

3

4

5

Initial Water Level

GTS NX

Procedure

1-19

05 Define Boundary Condition (Suction Drain)

1 Seepage/Consolidation

Analysis > Boundary > Nodal

Head

Select Object(s) > Select 9 edges

for “Suction drain” as


Value : -5

Select Type : Pressure

Boundary Set Name : Suction

1

2

2

3

4

5

3

4

5

Practical suction pressure is within

the range of 50~100kPa,

depending on the field condition.

-5 = -5 x 9.8kN/m2 = 49kPa

GTS NX

Procedure

1-20

05 Define Boundary Condition (Drain by Total Head)


Analysis > Boundary > Total

Head

Select Object(s) > Select 10

edges as shown

Value 2m Pressure

Boundary Set Name Draining

Total Head

Repeat for bottom layer with

12m Pressure

1

2

2

3

4

3

4

Pore pressure set to given values

5

5

GTS NX

Procedure

1-21

05 Define Boundary Condition (Consolidation)


Analysis > Boundary > Draining

Condition

Type Edge

Select Object(s) > Select 20

edges as shown

Boundary Set Name Draining

Condition

1

2

2

3

4

3

4 Pore pressure goes to 0 on

selected nodes.

GTS NX

Procedure

1-22

05 Define Boundary Condition (Consolidation)


Analysis > Boundary > Non

Consolidation

Type Element

Select Embankment

Boundary Set Non Consolidation

1

2

2

3

4

3

4 Pore pressure degree of freedom

turned off.

GTS NX

Procedure

1-23

05 Define Boundary Condition (Ground)


Analysis > Boundary >

Constraint > Auto

Boundary Set Name : Ground

Boundary

2

2

1

GTS NX

Procedure

1-24

06 Load Condition (Self Weight)

1


Analysis > Load > Self Weight

Load Set Name : S/W2

2

GTS NX

Procedure

1-25

07 Define Construction Stage (Create 2 Stage Set)

1Seepage/Consolidation

Analysis > Construction Stage >

Stage Set

Stage Type > Consolidation

Set Name : Consolidation

Embankment

Select Add

Stage Type > Fully Coupled

Stress Seepage

Set Name : Suction Drain

Select Add

Repeat for Fully Coupled

Embankment Set

Select Suction Set and Click

Define CS…

* 3 Construction stage will be defined

for each stage set.

1

2

4

7

8

3

4

2

3

5

6

5

6

7

8

9

9

GTS NX

Procedure

1-26

08-1 Define Construction Stage (Suction Drain)

1 Stage 1 : Initial state

Time Step : Initial value (1)

Activated Data : Mesh Sets for

initial state of ground (2D

elements only except

“Embankment”), Ground

Boundary, Self Weight

Check “Define Water Level for

Global”

Input 1m, Select “Water Level

(1)”

Check “Clear Displacement”

Select Save

Select New

3

1

2

3

4

4

Drag & Drop

5

5

6

7

7

2

6

2

GTS NX

Procedure

1-27

1 Stage 2 : Suction

Time Step : 100 days / 10 steps

Activated Data : Suction

Select Save

Select New

3

1

2

3

4

Drag & Drop

5

5

2

4

2


GTS NX

Procedure

1-28

1 Stage 3 : Leave Time


Deactivated Data : Suction

Select Save

Select Close

*Create other 2 sets by

same procedure or by

making copies of sets and

modifying boundaries

applied.

2

3

4

2

1

4

2

5

Drag & Drop

5

3


GTS NX

Procedure

1-29

08-2 Define Construction Stage (Embankment)









Global”


(1)”


Select Save

Select New

3

1

2

3

4

4

Drag & Drop

5

5

6

7

7

2

6

2

GTS NX

Procedure

1-30

1 Stage 2 : Embankment


Activated Data : Embankment

And Draining Total Head

Select Save

Select New

3

1

2

3

4

Drag & Drop

5

5

2

4

2


GTS NX

Procedure

1-31



Select Save

Select Close

2

3

4

2

1

2

3 4


GTS NX

Procedure

1-32

08-3 Define Construction Stage (Consolidation)









Global”


(1)”


Select Save

Select New

3

1

2

3

4

4

Drag & Drop

5

5

6

7

7

2

6

2

GTS NX

Procedure

1-33

1 Stage 2 : Embankment


Activated Data : Embankment,

Drainage Condition and Non-

Consolidation

Select Save

Select New

3

1

2

3

4

Drag & Drop5

5

2

4

2


GTS NX

Procedure

1-34



Select Save

Select Close

2

3

4

2

1

2

3 4


GTS NX

Procedure

1-35

09 Analysis Case

1

Analysis > Analysis Case >

General

Title : Case Name

Solution Type : Construction

Stage

Select Construction Stage Set :

Suction Drain

Analysis Control : Check each

option in Red Box

Select OK

Select OK

(Create 2nd and 3rd Analysis case

for Embankment stage by fully

coupled method and

consolidation method set with the

same options.

1

2

3

4

5

2

3

4

5

6

7

7

5

5

6

GTS NX

▪ Analysis > Analysis Case > General > Analysis Control

▪ Consider geometric nonlinear effects in stress, fully coupled and slope stability analysis.

▪ Analysis can take into account load nonlinearity. Reflecting the effects of follower loads, where the load direction changes with the deformation.

▪ In case of large deformation analysis, the user can check more reasonable behavior.

▪ In case of embankment, “Initial Configuration” option in General tab is applicable to check more realistic behavior.

Normal pressure Pressure in specified direction

[Directional change of pressure load due to the large deformation]

[Staged Consolidation analysis for embankment]

[Geometric nonlinearity + Initial configuration option]

[Linear geometry without option][Result comparison]

09 Review of Analysis Control (Geometric Nonlinearity)

GTS NX

Procedure

1-37

10 Perform Analysis and Check Results

1 Analysis > Analysis > Perform

Select OK

1

2

2

GTS NX

Procedure

1-38

11 Post Processing (Pore Pressure)

1

Consolidation method.

From the RESULTS WORK TREE

Check Pore Pressure Head for

the last step in LEAVE TIME for

each of the analysis cases.

Fully Coupled Suction

Fully Coupled Embankment

GTS NX

Procedure

1-39

11 Post Processing (Settlement with time)

1 From the RESULTS WORK TREE

Check Total Displacements for

the last step in LEAVE TIME for

each of the analysis cases.

2

Consolidation method.

Fully Coupled Suction

Fully Coupled Embankment

GTS NX

Procedure

1-40

11 Post Processing (Settlement with time)

1 Result > Advanced > Extract

Select Analysis Set : Suction

Drain

Select Result Type :

Displacements

Select Results : TY

TRANSLATION(V) (vertical

displacement)

Select Nodal Results Extraction

Select Table

Select Two Column Export to

Excel

Repeat process for other cases

and graph settlement

comparison

1

2

3

4

5

6

2

3

4

3

5

6

6

GTS NX

1-41

- Suction drain: -50 kPa (60days)

[Pore pressure distribution with suction drain] Stage 2 (Embankment step 1)

<Fully Coupled Suction drain>

Results Comparison

<Consolidation>

[Excess Pore Pressure distribution with suction drain] Stage 2 (Embankment step 1)

-0.6

-0.4

-0.2

0.0

0 50 100 150 200 250 300

Max displacement

Suction drain period

[Maximum displacement vs. time]

Suction drain removed

Residual displacement

[Excess Pore Pressure vs Time

GTS NX

1-42

<Excavation - dewatering>

- Comparison of sequential and fully-coupled analysis

[Full Coupled Vertical displacement after 22 days]

Initial water level

layer1

layer2


[Semi Coupled Vertical displacement after 22 days]

GTS NX

1-43

<Excavation - dewatering>

- Comparison of sequential and fully-coupled analysis

[Full Coupled Pore Pressure Head after 22 days]


[Semi Coupled Pore Pressure Head after 22 days]

-6

-5

-4

-3

-2

-1

0

-1 1 3 5 7 9 11 13 15

Pre

ssure

Head

Time (day)

Pore Pressure vs Time



END

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