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

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

Theo Huybregts

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

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 2 4 6 8 10 12 14

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

Defining allowable design strengthLong term creep behaviour

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

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

Design life

Tensar RE500 grids @ 20C

0 2 4 6 8 10 12 14

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

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 2 4 6 8 10 12 14

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

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 2 4 6 8 10 12 14

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

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 2 4 6 8 10 12 14

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 2 4 6 8 10 12 14

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

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

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 2 4 6 8 10 12 14

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 1 2 3 4 5 6

Carbon black (%)

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

VLS system

Mainly frictional

VLS system

Mainly frictionalConnection

Connection test

ASTM D6638-01N

VLS system

Mainly frictional

Results from connection tests

0 10 20 30 40 50 60 70 80

Normal load N (kN/m)

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 80

Keystone TW3 system

Mechanical connection

TW1 system

Mechanical connection Connection

Connection test

ASTM D6638-01

0 10 20 30 40 50 60 70 80

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 80

TuuACS

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

Basis for all design methods

Calculations

Check external stability to find L

GradeSpacing

Check internal stability to find layout (grade & spacing)

Surcharge, q

Specific design situations

Calculations

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

hi Coulomb

Inter-wedge boundary

2-part wedge method general procedure

Wedge 1

Wedge 2

Surcharge, q

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

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

Surcharge, q

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

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

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

Internal stability

Look at forces applied to Wedge 2

Surcharge, q

Calculations

Eagh = 0.5Kahi2

Eaph = Kaqhi

Eah = Eagh + Eaph

Eav = Eah tan( - b)

Internal stability

Surcharge, q

Calculations

( - i)

Z = Eah

– (G + ql + Eav)tan( - i)

Internal stability

Surcharge, q

Calculations

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

2-part wedge method general procedureDetermining the geogrid resistance

Calculations

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

2-part wedge method general procedureAdding resistance from the geogrid

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

Look at Geogrid 4 in isolation

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

F = ptan

End of geogrid

Connection strength

Tcon /FS

F = ptan Envelope of available resistance

End of geogrid

Connection strength

Tcon /FS

F = ptan Envelope of available resistance

Wedge 1

Wedge 2

Surcharge, q

Internal stability

Which two-part wedge is critical?

How do we choose?

Surcharge, q

Internal stability

Check for various values of i

Wedges are checked at 3 intervals

Surcharge, q

Internal stability

Check for various values of Hi

Wedges are checked at every grid level

Surcharge, q

Internal stability

Sliding between geogrid layers

Sliding between grids is checked at every level

Surcharge, q

Internal stability

Sliding over geogrid layers

Sliding over grids is checked at every level

Surcharge, q Internal stability

Summary

Calculations

Check sliding over geogrid

Check sliding between geogrid

Check families of wedges

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

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

Internal stability

2-part wedge method general procedureAdding earthquake forces

Distribution of seismic forces

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

PdynhPdynv

Calculations

( - i)

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

Internal stability

Additional forces applied to Wedge 2

Surcharge, q

Calculations

Internal stability

Comparison with static case

( - i)

Surcharge, q

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

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)

Surcharge, q External stability

Similar definitions for internal stability are used

Factors

Bautechnik design methodLoad combinations which determine the use of LL

Likely to be critical for

Bearing capacity

Steeper wedges

LC A maximum overturning

Factors

Bearing capacity (only used for external stability)

Factors

External sliding

Sliding on geogrid

Sliding between geogrid

Less steep wedges

External stability

Factors

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?

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

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