geosynthetics the wide varietyevents.iitgn.ac.in/2016/geosynthetics2016/symposium... · 27-07-2016...

27-07-2016

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Geosynthetics –The wide variety

Geotextiles –Woven, non-woven Filter

fabrics

Geogrids

Geomembranes

Geocomposites

Geonets, Geofibres,mesh mattings,

Geopipes

GVRao, IITDelhi

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Woven Geotextile Non-woven Geotextile

GVRao, IITDelhi

Extruded Geogrids

GVRao, IITDelhi

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Geosynthetics – the functions

Separation

Drainage

Filtration

Reinforcement

Moisture barrier

Cushion

GVRao, IITDelhi

Reinforcement function

GVRao, IITDelhi

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GVRao, IITDelhi

Reinforcement function

Main Elements of Reinforced Soil Walls

Reinforced soil walls consist of three main elements:

• Soil

• Reinforcement

• Facing

GVRao, IITDelhi

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– The Fill

A wide range of soil/fill can be used.

The ideal fill is a cohesionless material with a high shearing resistance

GVRao, IITDelhi

Main Elements

of Reinforced Soil

Walls-

The Reinforcement

GVRao, IITDelhi

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GVRao, IITDelhi


– Facia Elements

GVRao, IITDelhi

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Basic Principles of Reinforced Soil

For reinforced soil to work, the soil and reinforcement must STRAIN

In a stable structure the strain in the soil and reinforcement are equal (i.e. There is strain compatibility)

The strain in the reinforced soil is influenced by:

• The stiffness of the reinforcement

• Properties of the soil

• The stress state of the soil

GVRao, IITDelhi

The Forces

GVRao, IITDelhi

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Design of Reinforced Soil Walls

Failure mechanisms of reinforced soil can be identified as limit modes covering:

(a) Sliding failure

(b) Bearing failure

(c) Reinforcement rupture

(d) Reinforcement pullout

(e) Slip/wedge failure

(f) Rotation and settlement

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Slope Failure

GVRao, IITDelhi

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Reinforced Slopes

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Basal Reinforcement

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Geosynthetic Basal Mattress

GVRao, IITDelhi

The structures that resist earthquake best are those :

• That deform while dissipating energy

• They are constructed of materials that resist shear and

tension and are flexible

• They are simple and regular in shape

• Their individual members are joined to form continuous

systems that promote redistribution of earthquake forces.

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Reinforced Soil structures have all of these properties.

This is the reason for high degree of acceptance of

Reinforced Soil technology in seismically sensitive

regions.


Over the years the strict application of

seismic design and construction codes

has made it possible to

reduce the extent of damage and loss in

reinforced soil structures.

Gemona – Italy 1976 -- 6.4 Richter

Japan 1983 -- 7.7 Richter

Belgium 1983 -- 1500 m long 17 m ht

Mexico city 1985

Bay of Plenty NZ 1987 -- 6.3 Richter

San Francisco 1989 – 2.69 g

Northridge, USA 1994 – 1.15 g/ 1.93 g

23 walls – some > 10 m ht

Many structures throughout the world were

actually subjected to seismic excitation .

The experiences gained are very revealing.

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CASE STUDY

- Northridge,

USA 1994

23 Reinforced Earth structures within the affected area.

35% less than 5 m height,

40% between 5 m and 10 m height and

25% greater than 10 m height.

The distance from earthquake epicenter - 13 to 83 km. Three

structures located within 14 km of epicenter.

All the structures are fully intact and structurally sound.

GVRao, IITDelhi

CASE STUDY -

Northridge, USA 1994

One wall is at 16 km from the

epicenter at Lyons avenue.

Nearby on the freeway three

bridges had collapsed. The

Reinforced Earth wall suffered

only some superficial damage of

some of the lowermost panels.

. The building next to the wall

suffered severe structural

damage and was certified

“Unsafe” to enter after the

earthquake.

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CASE STUDY –Great

Hanshin - Japan

17 Jan. 1995

• Measured 7.2 on the Richter scale, with its epicenter at the

north tip of Awaji island at a depth of about 14 km.

•Around 100 expressway bridge piers as well as 700 rail bridge

piers were fractured.

• The port of Kobe was extensively damaged by soil

liquefaction with most of its 186 berths rendered unusable.



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CASE STUDY - Great Hanshin, Japan 1995

812 Reinforced Soil structures were existing in the region.

124 were within 40 km radius from the epicenter .

Their height was 1.5 m to 16.5 m with 70% of them greater

than 5 m height and 13% higher than 10 m.

The structures in this region were not designed for

ground acceleration greater than 0.15g.

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CASE STUDY - Great Hanshin, Japan 1995 3 structures (2%) indicated some damage to the wall facing and movement of the wall and adjacent ground

7 structures (6%) indicated some damage to the wall facing and to the adjacent ground

22 structures (18%) indicated no damage to the wall despite some damage to the adjacent areas

92 structures (74%) indicated no wall or adjacent area damage

8 % structures which exhibited some damage, all of them were located in residential areas that suffered relatively heavy damage from the quake. In no case was the damage of Reinforced Soil structures was great enough to deprive them of their function.


Tatatsuoka procedure for GRS-RW construction


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Cross-section of the GRS-RW at Tanata



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Japan GRS RW

During the Earthquake, GRS full height

concrete facing and a total length of 2 km

performed very well.

In contrast conventional Retaining walls

failed miserably.


Goegrid-reinforced soil RW along JR Kobe Line

(1992)

27 July 2016 36

OUR EARTH G V

RAO

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Goegrid-reinforced soil RW along JR Kobe Line

(1995)

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OUR EARTH G V

RAO

Post earthquake view of the Tanata GRS-RW wall


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No Comple

tion of

Constru

ction

Facing Geogrid Reinforce

ment

length

(m)

height of

the wall

(m)

Inclination

Internal

friction

angle (deg)

Observation

1 Jun.

1990

Steel

frame

SR55 4.8 6.5 1:0.1 30 No damage

2 Feb.

1991

Concrete

block

SR55 3.0 4.0 1:0.3 30 No damage

3 Aug.

1991

Sandbag SR55 55 6.2 1:0.5 30 No damage

4 Feb.

1992

Sandbag SR55 5.0 6.6 1:0.3 30 No damage

5 Mar.

1993

Concrete

block

SR55 4.5 5.25 1:0.5 30 Small gaps between

concrete blocks and a few

centimeters settlement

S Dec.

1993


7 Jul.

1994

Steel

frame

SR55 2.0 5.0 1:0.5 35 No damage

S Aug.

1994

Sandbag SR35 4.0 5.5 1:0.2 30 A crack ran parallel to wall

face on the crest because

of unstable foundation

9 Sep.

1994


10 Dec.

1994


Structure of the investigated geogrid-reinforced soil walls

CASE STUDY - Izmit, Turkey 1999

On August 17, 1999 a magnitude MW 7.4 earthquake stuck the

province of Kocaeli in western Turkey. The epicenter was near

the densely populated town of Izmit.


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CASE STUDY - Izmit, Turkey 1999

A pair of Reinforced Soil walls retaining a bridge approach was

severely tested by the earthquake.

The primary fault rupture was only a few meters from the walls

and passed beneath the bridge structure which collapsed.


Reinforced slope as designed

and constructed


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Reinforced slope failure during

Chichi earth quake


Collapse of Reinforced Fill Soil at Chi-

Nan in Taiwan –CHICHI Earthquake, 1999

Peak ground acceleration – 1.0 g

2,500 Fatalities

Near the site ah = 0.6 g ; av = 0.28 g

Designed for clay-gravel fill

c’ = 20 kPa ; = 35o

Actual values – (Back analysis)

c’ = 10 kPa – 120 kPa

= 20o – 50o

Still stable for 3 years before Earthquake

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Reinforced Fill at NAN-HUA, 1998

50 m high, 6 tiers of 8 m height, 2:1 face slope

Clay-gravel fill

c’ = 0, = 30o

Polyester geogrids

Good drainage Measures

GVRao, IITi Gandhi Nagar

Earthquake of 6.4, Oct 19, 1999

No Damage

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Seismic Analysis

Pseudo-static Analysis

Mononabe-Okabe Analysis

Japanese Method/French Method/AASHTO *Partial Load Factors

Basic hor seismic cofficient, regional,ground corrections

*Partial Material Factors

*FOS for Internal Stability

*FOS for External Stability

Results of direct shear tests on dry Ottawa sand

(after Schimming and Saxe 1964)

No influence of rate of loading on shear strength

of dry cohesionless soil

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Role of Extensible Geosynthetic During seismic conditions an increase in the

characteristic strength of polymeric materials is acceptable.

Influence of strain rate on monotonic load-extension behaviour of typical

geogrid reinforcement products (after Bathurst and Cai 1994)

Seismic stability of Reinforced soil walls

For evaluation of external stability, a reinforced wall is treated much like a gravity wall. The reinforced zone is assumed to be acted on by its own weight, W, and the static soil thrust, PA. Earthquake loading is represented pseudo statically by the dynamic soil thrust, PAE, and the inertial force on the reinforced zone, PIR

a) Geometry and notation for reinforced soil walls; (b) static and pseudostatic forces

acting on reinforced zone

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External Stability

Determine the peak horizontal ground surface acceleration,

amax.

Calculate the dynamic soil thrust from

Calculate the dynamic soil thrust from

where (b) is the unit weight of the backfill soil.

maxmax1.45c

aa a

g

( ) 2

0.375b

cAE

a HP

g

The external stability of a particular wall design can

be evaluated by the following procedure

Calculate the internal force acting on the reinforced zone from

where (r) is the unit weight of the reinforced zone

Add PAE and 50% of PIR to the static forces acting on the

reinforced zone and check sliding and overturning stability (the

reduced value of PIR is allowed to account for the fact that the

maximum values of PAE and PIR are unlikely to occur at the same

time).

For seismic design, factors of safety against sliding and

overturning should be greater than or equal to 75% of the

minimum acceptable factors of safety for static loading.

( )r

cIR

a HLP

g

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Internal Stability

Determine the pseudo-static inertial force acting on the potentially unstable internal failure zone,

where WA is the weight of the failure mass

Internal stability for seismic conditions can be

evaluated in the following steps:

c AIA

aWP

g

Critical potential failure surfaces for evaluation of internal seismic stability of reinforced

soil walls: (a) inextensible reinforcement; (b) extensible reinforcement

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• Distribute PIA to each reinforcement layer in proportion to its resistant area (the area of reinforcement that extends beyond the potential internal failure surface). This process produces a dynamic component of tensile force for each layer of reinforcement.

• Add the dynamic components of tensile force to the static components of tensile force to obtain the total tensile force for each layer of reinforcement.

• Check to see that the allowable tensile strength of the reinforcement is at least 75 % of the total tensile force in each layer of reinforcement. ;

• Check to see that each layer of reinforcement extends far enough beyond the potential internal failure surface to avoid pullout failure with a factor of safety not less than 75 % of the minimum static factor of safety when the total tensile force is applied.

Influence of Water

• The presence of water on either side of a retaining wall strongly influences the seismic behaviour of the wall

• Water on the outboard side of the wall can exert dynamic, in addition to hydrostatic, pressures on the face of the wall

• Water within the backfill can influence the inertial forces acting on the wall and can develop hydrodynamic or excess pore water pressures.

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Alters inertial forces within the backfill

Inertial forces on the relative movement between the backfill soil particles and the pore water

If ‘k’ small – pore water moves with the soil during Earthquake (no relative movement of soil & water)

» inertial forces total unit weight of soil

If ‘k’ is very high, pore water may remain stationary essentially, while soil skelton moves back & forth

» inertial forces submerged unit weight of soil

Also Hydrodynamic water pressures can develop – to be added to soil & hydrostatic pressure

In addition, there is an F S in the design strength

to account for long term creep

Extensibility of the geosynthetic reinforcement

effects the overall stiffness of the reinforced soil

mass.

As the overall stiffness reduces, damping

should increase.

Role of Extensible Geosynthetic

contd…

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The superior performance of Reinforced Soil structures

during earthquakes is due to:

Reinforced Soil being a flexible structure, allows

significant differential movement to occur within the

reinforced soil mass.

The Reinforced Soil granular backfill serves as an

excellent damping medium.

CONCLUSIONS

THANK YOU

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References:

• Bathurst, R.J., and Alfaro,M.C., (1997), “Review of seismic design, analysis and performance of geosynthetic reinforced walls, slopes and embankments”, Proc. Earth Reinforcement, pp 887-918.

• Kobayashi, K., Tabata, H., and Boyd, M., (1996), “The performance of reinforced earth structures in the vicinity of Kobe during the Great Hanshin Earthquake”, Proc. Earth Reinforcement, pp 395-400.

• Kramer, S.L., (1996), “Geotechnical Earthquake Engineering”, Pearson Education Inc.

• Tatsuoka, F., Koseki, J., and Tateyama. M., (1997), “Performance of reinforced soil structures during 1995 Hyogo-ken Nanbu Earthquake”, Proc. Earth Reinforcement, pp 973-1008.

• White, D.M., and Holtz, R.D., (1997), “Performance of geosynthetic-reinforced slopes and walls during the Northridge, California Earthquake of January 17, 1994”, Proc. Earth Reinforcement, pp 965-972

Mononobe – Okabe (M-O) Method

Is a direct extension of static Coulomb theory

To pseudo static conditions

Pseudo static accelerations ah = kh.g ; av = kv.g

Total active Thrust

Dynamic active Earth Pressure coefficient

211

2AE AE VP k H K

2

2

2

cos

sin sincos cos cos 1

cos cos

AEk

a) Forces acting on active wedge in M-O analysis; (b) force polygon illustrating equilibrium

of forces acting on active wedge

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Passive Pressure where

211

2PE PE VP K H k

2

2

2

cos

sin sincos cos cos 1

cos cos

AEk

Dynamic component acts opposite in direction to passive component

a) Forces acting on wedge in M-O analysis; (b) force polygon

illustrating equilibrium of forces acting on active wedge

Calculation of total earth pressure distribution due to soil self-weight

(Bathurst and Cai 1995)

211

2AE V AEP k K H

1

AE A dyn

V AE A dyn

P P P

or

k K K K

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geosynthetics the wide varietyevents.iitgn.ac.in/2016/geosynthetics2016/symposium... · 27-07-2016...

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