Workshop: TensarSoil Design theory and methods for ...iridexplastic.ro/Download/Tensar 2016/Romania 2016 - Retaining... · Workshop: TensarSoil Design theory and methods for reinforced

Tensar International B.V.Design Service Manager –continental Europe

Theo Huybregts

Workshop: TensarSoilDesign theory and methods for reinforced soil retaining walls with block facing

Walls

Steeper than 70 to the horizontal

Normally with concrete facing

Many published design guides

We will look at Bautechnik/2-part wedge

Types of structureDefinitions

Outline of the workshop

2 December 2015Reinforced soil structures design workshop2

Tensar RE500 uniaxial geogrids

Manufactured from HDPE

Strength required for

ULS static

SLS static

Seismic

Tensar uniaxial geogrids

Design parameters for reinforcement


Long term design tensile strength of polymer geogriddefined as

Pdes (long term design strength)

TB = Tensile (QC) strength

A1 (creep reduction factor, RFCR)

A2 (installation damage, RFID)

(factor of safety = 1.75)

Defining allowable design strength ULS staticBautechnik method



QC strength measured using test ISO 10319:1996Also used to define design strength for earthquake



0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Lo

ad

(kN

/m

)

Strain (%)

Tensile test ISO

10319:1996

Tensar RE560

Defining allowable design strengthLong term creep behaviour


HDPE geogrids are visco-elastic

Long term load strain behaviour is different to short term

This behaviour is determined using creep tests

Simple test: apply load & measure strain



Tests carried out at varying loads

Creep testing laboratories at various temperatures

10C

20C

30C

40C

50C


Defining allowable design strengthLong term creep behaviour


10

100

1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6 1E+7 1E+8 1E+9

Ru

ptu

re lo

ad

(%

of

ten

sil

e)

Time (hours)

Grade A

Grade B

Grade C

Grade D

Grade E

Grade F

120 years

Mean regression

Rupture load is plotted against time to rupture

Find rupture load for design life = Trup (%)

RFCR = 100/Trup


Defining allowable design strength for ULSLong term creep behaviour - determining A1 = RFCR

Trup

Design life

Tensar RE500 grids @ 20C

0

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8 10 12 14

Lo

ad

(%

)

Strain (%)

1 month 20 deg C

I month 30 deg C

120 yrs 30 deg C

120 yrs 40 deg C

120 yrs 50 deg C

1 month

120 yrs

1% criterion


Using load-strain data for one month of loading (730 hours) develop isochronous load-strain curve for 1 month

Develop a second isochronous load-strain curve for 120 years

Find load which gives 1% increase in strain from 1 month to 120 years

18.3%


Tensar RE580 @ 20C

Defining allowable design strength for SLSLong term creep behaviour - determining TCS

In accordance with BS 8006-1:2010 and BBA certificate No 99/R109, for retaining walls:

1 month = end of construction

120 years = design life

post-construction strain should be limited to 1%

TCS = 18.3% of QC for Tensar RE580 at 20C for retaining walls

0

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8 10 12 14

Lo

ad

(%

)

Strain (%)

1 month 20 deg C

I month 30 deg C

120 yrs 30 deg C

120 yrs 40 deg C

120 yrs 50 deg C

1 month

120 yrs

1% criterion


18.3%


Tensar RE580 @ 20C


In accordance with BS 8006-1:2010 and BBA certificate No 99/R109, for bridge abutments:

2 months = end of construction

120 years = design life

post-construction strain should be limited to 0.5%

TCS = 11.4% of QC for Tensar RE580 at 20C for bridge abutments

0

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8 10 12 14

Lo

ad

(%

)

Strain (%)

2 months 20 deg C

I month 30 deg C

120 yrs 30 deg C

120 yrs 40 deg C

120 yrs 50 deg C

2 months

120 yrs

1% criterion


11.4%


Tensar RE580 @ 20C


Determined from full scale site damage trials

Defining allowable design strengthInstallation damage factor - determining A2 = RFID




Fine fill





Medium fill





Coarse fill





5 series of site damage tests were carried out on the RE500 geogrids






0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Lo

ad

(%

)

Strain (%)

Control

Standard compaction

Refusal compaction

Tensar RE560


Based on comparing tensile strength of control and damaged material

RFID = T/TID

BS 8006-1:2010 Annex D: “Site damage test”

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Lo

ad

(%

)

Strain (%)

Control

Standard compaction

Refusal compaction

Tensar RE560



Control, TDamaged, TID


Durability normally investigated to determine resistance to:

Weathering

Microbiological degradation

Oxidation

Liquids (acids and alkalis)

Investigated using

Screening tests (relatively quick)

Long term exposure tests (normally accelerated but may take a long time)



Defining allowable design strengthDurability - determining RFD



Defining allowable design strengthDurability - determining RFD

Degradation Test standard

Weathering resistance

BS EN 12224:2000: “Geotextiles and geotextile-related products. Determination of the resistance to weathering”

Resistance to microbiological degradation

BS EN 12225:2000: “Geotextiles and geotextile-related products. Method for determining the microbiological resistance by a soil burial test”

Resistance to oxidation

BS EN ISO 13438:2004: “Geotextiles and geotextile-related products. Screening test method for determining the resistance to oxidation” (56 days at 100C)

Resistance to liquids

BS EN 14030:2001: “Geotextiles and geotextile-related products. Screening test method for determining the resistance to acid and alkaline liquids”

Durability screening tests to ISO Standards

Results in terms of retained strength and failure strain



Defining allowable design strengthDurability - determining RFD (results for RE500 grids)

Degradation % retained strength % retained failure strain

Weathering resistance

98.5 100.7 99.0 123.2 109.4 120.1

Resistance to microbiological degradation

99.1 102.6 103.1 101.5

Resistance to oxidation

104.1 101.4102.1

102.5 107.1 104.0111.0

110.9

Resistance to liquids- acid

100.4 99.9 104.3 104.8 106.2 109.6

Resistance to liquids- alkali

99.4 99.7 103.1 105.8 107.1 106.8


Exposure of geogrid to UV

During temporary storage on site


Defining allowable design strengthResistance to weathering (exposure)



Major importance in tropical countries





During installation





During early part of service life



Some results from 10 year UV exposure trial at Albury, Australia

Load versus strain curves before exposure

and after 10 years of exposure

Protection provided by minimum 2% carbon black

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10 12 14

Lo

ad

(%

)

Strain (%)

Exposed

Control


Tensar 55RE


Defining allowable design strengthResistance to weathering (exposure) - evidence

Importance of using a minimum of 2% carbon black

For carbon black content less than 2% geosynthetics still “look” black

BUT protection rapidly reduces as carbon black content reduces

0

20

40

60

80

100

120

0 1 2 3 4 5 6

% lif

e w

ith

2%

carb

on

bla

ck

Carbon black (%)

HDPE


Range of carbon black content used in TensarRE500 geogrids


Defining allowable design strengthResistance to weathering (exposure) - protection

Based on extensive testing (EP9090 in US), HDPE is inert to all aqueous solutions of acids, alkalis, salts normally found in soil

Very important in tropical climate



Defining allowable design strengthResistance to chemicals - HDPE preferred

Connection

Modular block systems connection strength

Design parameters for connection to facing


ConnectionConnection test

NCMA method NCMA SRWU-1

ASTM D6638-01N

T




VLS system




VLS system

Mainly frictional




VLS system

Mainly frictionalConnection

Connection test


ASTM D6638-01N

T




VLS system

Mainly frictional

Results from connection tests



0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80

Co

nn

ecti

on

lo

ad

Tco

n(kN

/m

)

Normal load N (kN/m)

VLS


VLS system

Mainly frictional

Results from connection tests

acs represents mechanical component

cs represents frictional component

Tcmax is maximum

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80

Co

nn

ecti

on

lo

ad

Tco

n(kN

/m

)


VLS

ACS

CS

Tcmax




Keystone TW3 system




Keystone TW3 system

Mechanical connection




TW1 system




TW1 system





TW1 system

Mechanical connection Connection

N

T



Connection test


ASTM D6638-01


0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80

Co

nn

ecti

on

lo

ad

Tco

n(kN

/m

)


VLS

TW1

ACS

CS

Tcmax

TW1 system


Results of connection tests

For mechanical connection Acs Tcmax and cs = 0




TW1 system


Results of connection tests

For mechanical connection Acs Tcmax and cs = 0

TUU is geogrid strength using same procedure as connection test

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80

Co

nn

ecti

on

lo

ad

Tco

n(kN

/m

)


VLS

TW1

TuuACS

CS

Tcmax

TUU

Tuu is geogrid strength using same procedure as connection test





Tensar International B.V.Design Service Manager –Continental Europe

Theo Huybregts

Workshop: TensarSoilDesign theory and methods for German Institute für Bautechnik method

External stability

Internal stability

Adding earthquake loads

Serviceability

Elements of reinforced soil design

TensarSoil workshopMethod of calculation


Is this method new?

No, it has been in use for more than 16 years

Some of the highest reinforced structures in the world have been designed using the two-part wedge

Design using 2-part wedge methodFinding layout of reinforcement

Method of calculation


Basis of design method

External stability analysis uses the same basic approaches in tie-back wedge and two-part wedge methods

Differences are in the internal stability calculations

Aim: to minimise assumptions required to reach a satisfactory design

Any mechanism used should be admissable and complete (ie. include all forces which are involved)

Original reference: Deutsches Institut für BautechnikCertificate No Z20.1-102




Surcharge, q

H

Basis for all design methods

Calculations

L

Check external stability to find L

GradeSpacing

Check internal stability to find layout (grade & spacing)




Surcharge, q

H

Specific design situations

Calculations

L

GradeSpacing

Additional design forces due to earthquakes

High temperature at facing resulting in lower design strength


Connection strength between facing and reinforcement



Wedge 1

Wedge 2

Surcharge, q

L

hi Coulomb

Inter-wedge boundary

2-part wedge method general procedure


Hi

i


Wedge 1

Wedge 2

Surcharge, q

L



Hi

For full analysis this is done by checking wedges at 0.1 intervals

This takes into account complex geometry, c, and isolated surcharge rigorously

i


Surcharge, q

H

G

l

Eah

EavWedge 2

Wedge 1

External stability

Calculations


External stability check Defined by special case when Hi = H, i = 0

Check sliding on base

Check bearing capacity

L Find L

Check eccentricity of resultant


Surcharge, q

H

L

G

l

Eah

EavWedge 2

Wedge 1

Additional sliding check

Calculations

Check sliding on base

Based on different backs of RSB


External stability check Defined by special case when Hi = H, i = 0


External stability results

External failure

General overall stability failure

Check with slope stability program

External stabilityFinding size of the reinforced soil block



Surcharge, q

L

hi



Hi

Eah

Eav

G

l

R

Z

= for internal stability is set as default, but may be adjusted

i

Internal stability

Look at forces applied to Wedge 2


Surcharge, q

L

hi



Hi

Eah

Eav

G

l

R

Z

Calculations

Eagh = 0.5Kahi2

Eaph = Kaqhi

Eah = Eagh + Eaph

Eav = Eah tan( - b)

b

i

Internal stability



Surcharge, q

L

hi



Hi

i

Eah

Eav

G

l

R

Z

Calculations

b

Eah

Eav

G

ql

Z

R

( - i)

Z = Eah

– (G + ql + Eav)tan( - i)

Internal stability



Surcharge, q

L

hi



Hi

i

Z

Calculations

T3

T4

T5

T6

Add possible resistance from facing

T3 + T4 + T5 + T6

Resistance to Z is provided by four layers of geogrid

So for satisfactory design

Ti = R > Z

Plus any resistance from the facing

Internal stability



Surcharge, q

L

hi

2-part wedge method general procedureDetermining the geogrid resistance


Hi

i

Z

Calculations

T3

T4

T5

T6

Add possible resistance from facing

T3 + T4 + T5 + T6



Ti = R > Z


Internal stability



Surcharge, q Wedge stability


2-part wedge method general procedureVisualising the mode of wedge failure













Pull-out from facing

Failure through facing

Pull-out from backfill

Rupture of geogrid








T3

T4

T5

T6

2-part wedge method general procedureAdding resistance from the geogrid


T4

2-part wedge method general procedureAdding resistance from the geogrid - T4


Look at Geogrid 4 in isolation


T

x

End of geogrid

Slope given byT = 2xvF/FS

Design strength Tal /FS

Connection strength

Tcon /FS


Envelope of available resistance

2-part wedge method general procedureDeveloping the envelope of available resistance


T4

F = ptan

x’


T

x

End of geogrid



Connection strength

Tcon /FS



T3


F = ptan Envelope of available resistance

x’


T

x

End of geogrid



Connection strength

Tcon /FS



T6


F = ptan Envelope of available resistance

x’


Wedge 1

Wedge 2

Surcharge, q

Hi

i

Internal stability

hi

Which two-part wedge is critical?

How do we choose?



L


Surcharge, q

Hi

Internal stability

Check for various values of i

Wedges are checked at 3 intervals



L


Surcharge, q

Hi

Internal stability

Check for various values of Hi

Wedges are checked at every grid level



L


Surcharge, q

Hi

Internal stability

Sliding between geogrid layers

Sliding between grids is checked at every level



L


Surcharge, q

Hi

Internal stability

Sliding over geogrid layers

Sliding over grids is checked at every level



L


Surcharge, q Internal stability

Summary

H

Calculations

Check sliding over geogrid

Check sliding between geogrid

Check families of wedges

Aim is to find layout (grade and spacing) of geogrid



L


Internal stability results

Assumptions to be considered are:

seismic coefficients to calculate inertia and earth pressure

distribution of inertia and earth pressure

presence of temporary surcharge at the same time as EQ

factors of safety to be used

Pseudo-static design method requires the inclusion of additional static forces to model seismic condition

2-part wedge method general procedureAdding earthquake loads



Establishing PGALocal seismic codes


Slovakia seismic hazard

PGA (peak ground acceleration) is the normal starting point for reinforced soil design

Modified as outlined in the following slides


Within the structure Ah & Av

may be amplified or attenuated depending on the mechanism

kh(int)

kv(int)

Ah & AvAccelerations at the base of the structure Ah & Av (up & down)

kh(ext)

kv(ext)




Sliding on inclined plane between grid

Sliding on grid

Sliding on base

Bearing capacity

Mechanisms which do not cut reinforcement

Use reduced acceleration




Sliding within reinforced soil block

Two-part wedge

Mechanisms which do cut reinforcement




For mechanisms which do cut reinforcement displacement would not be acceptable as it would result in rupture or massive distortion of the grid

For these mechanisms some amplification of Ah and Av is allowed for as follows

kh(int) = (1.45 - Ah)Ah for Ah < 0.45

kh(int) = Ah for Ah > 0.45

kv(int) = (1.45 - Av)Av for Av < 0.45

kv(int) = Av for Av > 0.45




Surcharge, q

L


Hi

G

l

R

Z

Internal stability


2-part wedge method general procedureAdding earthquake forces

Eah

Eav



0.5H

H

Distribution of seismic forces


khG*

kvG*

Inertia forces are applied to front 0.5H of reinforced soil block

Only 50% of additional seismic earth pressure forces act

PdynhPdynv

Take 0 or 50% live load


Surcharge, q

L


Hi

Eah

Eav

G

l

R

Z*

khG*

kvG*

PdynhPdynv

Calculations

Eah

Eav

G

ql

Z*

R*

( - i)

Pdynh

Pdynv

khG*

kvG*

Z* = Eah + khG* + Pdynh – (G + ql+ Eav + Pdynv + kvG*)tan( - i)


Internal stability

Additional forces applied to Wedge 2


Surcharge, q

L


Hi

G

l

R

Z

Calculations


Internal stability

Comparison with static case

Eah

Eav

G

ql

Z

R

( - i)

Z*

Eah

Eav


Surcharge, q

L


Hi

Z*

Calculations


Internal stability

Additional forces applied to Wedge 2

T3* + T4* + T5* + T6*




Ti* = R* > Z*

*For seismic design geogrid strength is based on short term tensile strength

T3*

T4*

T5*

T6*


View results for different cases

View results for different cases

Surcharge, q For modular block facings

Also gabions

Improving modelling of contribution to stability from the reinforcement depending on facing type

Includes shear resistance

through facing

Resistance from 3 geogrids



Aim is an economical design but

with adequate margin against

failure and must be serviceable

Factors

Bautechnik design methodOther factors which define the method

Wall friction on back of reinforced soil block (RSB)

= for internal mechanisms

= 2/3 for external mechanisms

Soil strength definition is constant volume for fills (ccv = 0 normally, and cv)

In foundation stability calculation, the inclination factor is included in calculating bearing capacity

Two different load cases (or “load combinations”) are used which affect the distribution of live load (temporary surcharge)

LC A

LC B


Surcharge, q External stability

Similar definitions for internal stability are used

LC A

Factors

Bautechnik design methodLoad combinations which determine the use of LL

Eqh

Eqv

Eh

EvG

Q2

Likely to be critical for

Bearing capacity

Steeper wedges




LC A maximum overturning

Factors


Eqh

Eqv

Eh

EvG

Q2


Bearing capacity (only used for external stability)




LC B

Factors


Eqh

Eqv

Eh

EvG


External sliding

Sliding on geogrid

Sliding between geogrid

Less steep wedges


External stability


LC C

Factors


Eqh

Eqv

Eh

EvG

Used for serviceability

Post-construction strain check


19m high wall supporting 63m high slope

Designed using two-part wedge method

Thank you!!

Two-part wedge - thinking outside the box


Any questions?

Documents

Workshop: TensarSoil Design theory and methods for ...iridexplastic.ro/Download/Tensar 2016/Romania 2016 - Retaining... · Workshop: TensarSoil Design theory and methods for reinforced