Issues to be addressed when modelling (polymer) reinforced ...indogeotek.com/wp-content/uploads/2018/08/PUM2018s-MIKE.pdf · 3rd Indonesian Plaxis User Meeting 8th August 2018 Mike

3rd Indonesian Plaxis User Meeting 8th August 2018

Mike Dobie (Tensar International) M4-1

Tensar International LimitedRegional Manager Asia Pacific

Michael Dobie CEng FICE

Issues to be addressed when modelling (polymer) reinforced soil structures using finite element method (FEM)

Indonesian Plaxis Users Meeting17th April 2014

Tensar International LimitedRegional Manager Asia Pacific

Presented by Michael Dobie CEng FICE

Progress made in modelling polymer geogrids using finite element analysis (FEA)

Indonesian Plaxis Users MeetingUpdated: 8th August 2018



Introduction

Reinforced or stabilised soil techniquesThree main applications where FEM might apply

17th April 2014Reinforced soil structures and FEM3

A foreword

The aim of this presentation is to outline some issues which arise when applying FEM to reinforced soil structures

The assumption is that the aim is to get as close as possible to what is likely to happen

As more assumptions and simplifications are used, the result is likely to be further from reality

BUT there can always be a result!!

Introduction



Road pavements or trafficked areas

Geogrid used to improve pavement performance



Walls and steepened slopes

Geogrid used to provide internal stability


Introduction


FEM ISSUES SPECIFIC TO RS STRUCTURES

Simple elastic soil models will not model the compaction procedure adequately (requires higher order models, and layer-by-layer “construction” with “compaction”)

The load-strain behaviour of polymer geogrid is time dependent (this is defined by isochronous load strain-curves, but the simple model in Plaxis must be “adapted”)

When the structure is completed, the first layer of geogrid placed will have reached a different isochronous load-strain condition compared to the last layer placed

The facing should be taken into account

BUT the aim is normally to predict the post-construction deformation

Concluding remarks

Reinforced soil structuresIs FEM a suitable method of calculation? Yes, but…




Geotechnical FEA taking into account the performance benefits of polymer geogrids

All “red” slides prepared & originally presented by Dr Andrew Lees

Senior Application Technology ManagerTensar International Limited (based in Cyprus)

Dr Andrew Lees

Joined Tensar about 3 years ago

Provides Tensar with expertise in the field of advanced geotechnical FEA

Based on the use of Plaxis



Development of TSSM“Tensar stabilised soil model”

Working platforms/ stabilisation

Geogrid applications

Paved & unpaved roads,

rail

Embankments & slopes

Structures“Stabilisation”



SmartRock (Liu et al, 2016)

Senses rotation and translation

TX190L at 0.25m below sleeper


TX190L geogrid reduced particle translation significantly.




Locked particle: restrained

against translation and rotation.

Restraint transferred through

soil by particle interlocking

forming stabilised soil.

TX190L geogrid reduced particle rotation even more significantly.

DEM rotational restraint (Huang et al, 2016)

0

5

10

15

20

25

30

35

0 10 20 30 40

φ′ (

°)

εa (%)

No rotational restraint

Full rotational restraint

DEM simulation of triaxial test on medium sand:

Rotational restraint at micro-scale provides significant additional shear strength at macro-scale.



• 1.0m high x 0.5m diameter

• Vacuum “cell pressure” up to 80 kPa

• Geogrid placed at mid-height

• 5 radial strain gauges

Large triaxial test

0

100

200

300

400

500

0 0.02 0.04 0.06 0.08 0.1 0.12

q(k

Pa

)

εa

no geogrid TriAx

Large triaxial test results

σ′3=10kPa

σ′3=43kPa

σ′3=75kPa

Greater ductility

Higher shear strength from

stabilisation



0

100

200

300

0 100 200 300 400 500 600

She

ar

stre

ss (

kPa

)

Normal effective stress (kPa)

Large triaxial test results

Unstabilised (no geogrid) peak failure envelope

TriAx-stabilised peak failure envelope

c′

c′

Tensar Stabilised Soil Model (TSSM)

Slope k 0

c 0

c t

σ 1

σ 3

Curvature a 0

Curvature a t

Outside influence extent of TriAxgeogrid

At TriAx geogrid elevation y t or z t

Linear interpolation of failure surface between TriAx geogrid plane and vertical influence extent Δy t or Δz t

Linear elastic perfectly-plastic




2D Axisymmetric FEA model

Horizonta

l fixity

Horiz. & vert. fixity

Applied confining stress

Horizontal fixityApplied vert. displacement

Geogrid elevation yt=0.5m

Triaxial test simulation

Δy: extent of

stabilisation

Δy: extent of

stabilisation


Well-graded crushed rock without geogrid

0

5

10

15

0 0.02 0.04 0.06 0.08

Ve

rtic

al l

oa

d (

kN/r

ad

ian

)

Vertical displacement (m)

Lab data

FEA TSSM

0

5

10

15

0 0.02 0.04 0.06 0.08

Ve

rtic

al l

oa

d (

kN/r

ad

ian

)


Lab data

FEA MC




Well-graded crushed rock with geogrid

0

5

10

15

20

0 0.02 0.04 0.06 0.08

Ve

rtic

al l

oa

d (

kN/r

ad

ian

)


Lab data

FEA

membrane

0

5

10

15

20

0 0.02 0.04 0.06 0.08

Ve

rtic

al l

oa

d (

kN/r

ad

ian

)


Lab data

FEA TSSM

Using realistic n-ε curves

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

he

igh

t (m

)

radial strain εr (%)

Lab data

FEA c' profile

FEA geogrid el.

Geogrid element

does not capture

confining effect

above and below

geogrid

Simulation of triaxial test:

Geogrid


Lab data

FEA - TSSM

FEA – geogrid EA



TSSM – BRE plate load tests

TSSM – BRE plate load tests

0

25

50

75

100

125

0 20 40 60 80 100

Ap

pli

ed

lo

ad

(kN

)

Vertical displacement (mm)

no grid (lab)

TX180 (lab)

FEA geogrid el.

FEA no grid

FEA c' profile

no geogrid (lab)

TX180 (lab)

FEA geogrid EA

FEA no geogrid

FEA TSSM

With tensile EA properties of the geogrid alone, performance at serviceable strains appears negligible.



TSSM – Applications in design

Already we have seen application in various forms of structure

Working platform design to support heavy plant

Railway ballast & sub-ballast

Stabilised gravel rafts to mitigate the effects of liquefaction

Summary of development of TSSM

In the stabilisation application, using tensile elements does not result in the same performance as observed in actual cases

The TSSM models the soil/geogrid as a composite, so that the geogrid itself is not part of the input

The benefit of stabilisation is modelled by an increased strength of the layer

This benefit reduces as you move further away from the geogrid level

Multiple layers of geogrid can be modelled

TSSM is now available for use in Plaxis

TSSM is currently being used for the design of “pavement” type structures such as: working platforms, rail track, gravel rafts over liquefiable soil

Can TSSM be used for structures? Not yet developed, but there may be potential in this application area too



FEA simulation of MSE walls

Working platforms/ stabilisation

Geogrid applications

Paved & unpaved roads,

rail

Embankments & slopes

Structures“Stabilisation”

“Reinforcement”




Some advice illustrated through simulation of a full-scale trial wall at…

Public Works Research Institute (PWRI), Tsukuba, Japan in 1995.

Silty sand backfill

Geogrid: Tensar SR55

Case study reference: Ling, Cardany, Sun & Hashimoto (2000) Finite element study of a geosynthetic-reinforced soil retaining wall with concrete block facing, Geosynthetics International 7(2), 137-162. Used to obtain monitoring data but disagree with some elements of FEA work presented therein.


Important aspects:

Backfill model and parameters

Geogrid model and stiffness

Interface elements

Wall elements

Construction sequence




ULS (strength reduction):


Comparison FEA v. lab data:

1

2

3

4

5

6

7

-10 0 10 20 30

Y (

m)

Horizontal deflection (mm)

4m fill lab

5m fill lab

6m fill lab

4m fill FEA

5m fill FEA

6m fill FEA

“Sum phase displacements”

Steps due to approximation of summing phase displacements



1

2

3

4

5

6

7

-10 0 10 20 30

Y (

m)

Horizontal deflection (mm)

4m fill lab

6m fill lab

4m fill FEA

6m fill FEA

4m fill FEA

6m fill FEA



“Sum phase displacements”

Total displacements

More appropriate output



Lateral earth pressure reduces as geogrid creeps

1

2

3

4

5

6

7

0 5 10 15 20

Y (

m)

Horizontal stress (kPa)

Lateral earth pressure on back of wall

lab 6m fill

FEA 6m fill

FEA 120 years





0

100

200

300

-1 0 1 2 3 4 5

Ve

rtic

al s

tre

ss (

kPa

)

Distance from back of wall (m)

Vertical stress at base of wall

Lab 6m fill

FEA 6m fill

Drop probably due to friction from side wall

A lot of stress transferred down blockwork wall


Backfill model and parameters:

Stress-dependent stiffness

High yield

Frictional strength

Hyperbolic model (e.g. Duncan & Chang, Hardening Soil (HS)).

Large stress change




Backfill model and parameters:

1

2

3

4

5

6

7

0 10 20 30 40

Y (

m)

Wall horizontal deflection (mm)

Predicted wall deflection at 6m fill

original

x0.5

x2

Backfill stiffness

1

2

3

4

5

6

7

0 20 40 60 80Y

(m

)



original

phi 35

phi 40

phi 45

Backfill strength

Predicted wall deflection sensitive to both backfill stiffness and strength.


Geogrid model and stiffness:

Geogrid stiffness expressed as an axial stiffness EA (kN/m)

It depends on…

0.01 1 100 10000 1000000

EA

(k

N/m

)

Hours

RE540 at 20°C

10 19 26 32 35 39Axial load (kN/m)

The geogrid productIn-service temperature

Time

Load levelEA Generator in development.In the meantime, IB available or contact Tensar.



0

10

20

30

40

50

0 500 1000 1500 2000

Ma

x.

wa

ll d

efl

ect

ion

(m

m)

EA (short-term) (kN/m)



1

2

3

4

5

6

7

0 10 20 30 40 50

Y (

m)



original

x10

x5

x2

x0.5

Geogrid stiffness

Predicted wall deflection sensitive to geogrid stiffness, but over limited range.

Approx. range of Tensar RE500 series



Taking account of creep

Linear elastic model Visco-elastic model

EA

EAST

EALT

Dashpot, tR

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1 100 10000 1000000

Str

ain

%

Hours

RE580-10%-10degC

test data

Plaxis

EAST (1 hour)EALT (1M hours)tR 300 days

Difficult to get full match.Must tailor parameters to required output.

Geogrid tensile test simulation



1

2

3

4

5

6

7

0 10 20 30 40

Y (

m)

Wall deflection (mm)


v-e 6m fill

v-e 120yr

EA short-term

EA ST to LT



1

2

3

4

5

6

7

0 10 20 30 40

Y (

m)



v-e 6m fill

v-e 120yr

EA long-term

Linear elastic geogrid model: using long-term EA value provides approximate long-term prediction.

Linear elastic geogrid model: using short-term EA value provides good short-term prediction.

But changing to long-term value has no effect.

A visco-elastic model provides the most accurate long-term predictions.


Interface elements:

Deformed mesh x5 Close-up

Back of wall

Between blocks

Soil-geogrid interface?No, interlock with geogrid apertures provides full-strength interface (at serviceable strains)

Geogrid-block connection




Interface elements:

1

2

3

4

5

6

7

0 10 20 30 40 50

Y (

m)



original

wall

joints

back of

wall

none

Interface element locations

Predicted wall deflections are sensitive to interface element inclusion


Wall elements:

Area/ volume elements?

Or line/ surface elements?



1

2

3

4

5

6

7

0 10 20 30 40

Y (

m)



area

plate

plate base hinge

plate hinge joints


Wall elements:

Line (plate) elements were only reasonably accurate in this case with a hinge placed at the base, but that might not always work.

Wall elements

1

2

3

4

5

6

7

0 10 20 30 40

Y (

m)



orig. 0.35m

0.2m

0.5m


Wall elements:

Effect of wall thickness

Thicker blocks have a significant beneficial effect on wall deflections.

Wall thickness




Wall elements:

Well-known effect of wall thickness

Continuum elements

and reality

Non-continuum

elements

Interface

friction

Interface

friction

M ≠ 0 M = 0


Construction sequence:

All in one go? Or simulate layered construction?




Construction sequence:

Some simplification may be possible, e.g. 2 or 3 layers per stage (depending on overall height), but taking no account of construction sequence is likely to give inaccurate outputs.

1

2

3

4

5

6

7

0 10 20 30 40

Y (

m)



original

one go

Two layers added per analysis stage

All layers added in one analysis stage

Summary of FEA simulation of MSE walls

Good agreement between FEA and monitored data

Use displacement output following element activation

HS/Duncan & Chang type model good for backfill

Wall deflection sensitive to backfill strength and stiffness

Geogrid stiffness depends on product, temperature, load level and time since loading

Wall deflection sensitive to geogrid stiffness (to a point)

Improved long term predictions with visco-elastic geogrid model and simulating construction sequence

Interface elements needed at back of wall and block joints, but not at backfill/geogrid interface

Area/volume elements better than line/surface elements for wall

Thicker walls reduce deflection



Geotechnical FEA taking into account the performance benefits of polymer geogrids

Thank you for your attention

Documents

Issues to be addressed when modelling (polymer) reinforced ...indogeotek.com/wp-content/uploads/2018/08/PUM2018s-MIKE.pdf · 3rd Indonesian Plaxis User Meeting 8th August 2018 Mike