GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE 2.pdf · Bridges, Flyovers, Road Over Bridges,...

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Prof. J. N. Mandal

Department of Civil Engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

LECTURE- 2INTRODUCTION



RECAP of previous lecture…..

Contents of the course

References/ Journals/ Conferences/ Codes and DesignStandards

Target audiences

Why do we need to study this course? (Partly covered)

Some failure mechanisms: Geotechnical/ transportation/disaster management/ geo-environmental engineering

Global warming

Sustainability Using Geosynthetics

World wide levels of carbon dioxide have reached inhighest level in 3 million years (US Scientist).Carbondioxide was measured at 400 parts per million .(AtHawaii monitoring).Rise is a warning of large changesin climate and sea levels.We have failed miserably in tackling this problem(Pieter P Tans) (Times of India, May 12, 2013).



Melting Ice fields, increasing vector-borne diseasesand erratic weather patterns have been the direct resultof climate change.

Climate change is occurring 10 times faster than atany time in the past 65 million years.(Times of India.03-08-2013)

150 truck loads of clay = 1 Truck load of GCLs

Koerner et al., 2011

What is the role of Geosynthetics?


Green highways and streets construction (Lee et al., 2011)

Building environmentally and economically sustainable

transportation infrastructure highways (BE2ST in highways).

Recycled materials in a pavement can reduce

Global warming potential = 32%

Energy consumption = 28%

Water consumption = 29%, and

Hazard waste generation = 25%Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay


It is best to build the sustainable value addedenvironmental, economical infrastructures in whichtransference plays a very important role.

The proper use of materials can reduce hazardous wastegeneration, greenhouse gas emission, water and energyconsumption, global warming and cost of infrastructuresand increase service life.

MAJOR APPLICATIONS Reinforced soil walls/Slopes protection for approaches toBridges, Flyovers, Road Over Bridges, Underpasses,Highways/ Pavement roads, Railways and Airport runways

Pavement rehabilitation and strengthening

Land reclamation (Ground improvement)

Basal reinforcement/Piled embankment on soft soil

Pot hole/ Reflection cracking/ Asphalt overlays

Coastal/Shore protection, River training, Coastal erosionand Scour control


Rock fall protection/control

Landscaping, Buildings, Water Harvesting, BasementWater Proofing

Landfills and Cappings (Municipal solid waste, Industrialhazardous waste, Low level radio active waste)

Dewatering (Municipal and Industrial sludge, Marinesediment and Animal waste)

Separation, Filtration, Drainage, Erosion control andRevegetation


Tunnel, Dams, Water reservoirs, Canals, Ponds, TailingDams and Mine Rehabilitation

Sports fields and Golf courses

Land reclamation and Remediation

Landslides prevention


Lessons Learned from Failure and Success:

Poor design and /or design oversights

Poor foundation exploration

Poor field installation

Poor operation practices

Inadequate drainage system

Designs were all inadequate either underestimated the

seepage pressure or ignored them together.

No proper filling materialsProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

No design in seismically active area

No connection strength

No local stability of facing elements

Copycat and paste

In some cases there were simply no design whatever


Geosynthetics do not make MIRACLES and should not beexpected to make MIRACLES.

The geosynthetic should not automatically be consideredas the MAIN CULPRIT if failure occurs.

The geosynthetic should not be used as a SCAPEGOATwhen the design is flawed.

A failure may provide an opportunity to IMPROVE AFLAWED DESIGN (Giroud, J. P.)


WARNING: Catastrophic failures

Disastrous/ hazardous

Future generations

Bad name for technology

Good drainage and good compaction are mandatory

Set up proper technical guidance for QC and QA for thecurrent targeted applications.


Don’t you think there are enough reasonsto study this course?

Innovative unique design solutions Correct choice of products (QC/QA)

Real decision making and acquire practical skills

Proper installation techniques

Advice and right kind of testing

Saving in cost and time

Overall pleasing aesthetics

Products, software and equipments design and development

Environmental safety

KEY BENEFITS:


Before entering into the geosynthetic world, one shouldunderstand the basic concept of the reinforced earth system.


Module-1INTRODUCTION TO REINFORCED EARTH

OUTLINE

Introduction and historical back ground

Basic concepts and mechanisms of reinforced earth

Basic design of reinforced earth wall


Reinforced earth may be defined as a constructionmaterial composed primarily of soil whose performance hasbeen improved by the introduction of small quantities ofother material in the form of solid plates, perforated platesor fibers or fibrous membrane.

The materials resist tensile forces and interact with soilthrough friction and/or adhesion.

What is reinforced earth ?

Introduction of reinforcement in soil for improving thesoil structure is not a new idea.


Many birds and animals build their nests/habitations usingbranches of straw, sticks and soil as shown in Figure 1. Manypeople used sticks and soils to reinforce mud dwellings asshown in Figure 2.

Figure 1 Bird nest Figure 2 Reinforced soil houseProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

As I said on many occasions, the oldest profession in theworld is not the one you may believe, but it is geotechnicalengineering. Many many years ago, when some monkeysdid not want to live anymore in the trees but wanted to liveon the ground to become men, the first thing they did wasto send one of them on the ground to evaluate its bearingcapacity. So, the first geotechnical engineer was a monkey.From that day on, geotechnical engineer was slowlyevolved.

Dr. J. P. GiroudPast presidentInternational Geosynthetic Society (IGS)


One of the greatest examples of reinforced earth is the‘Great Wall of China’. Mixtures of gravel and clay along withTamarisk branches as reinforcement were used asconstruction materials (Department of Transport, 1977).

We add straw or rice husk with the soil for strengtheningthe bricks (Old Testament).

The use of bamboo mats and coconut piles for buildingcore walls or bunds is familiar in Kerala, India.

The wood branches have been used along the ‘Yellowriver’ in China to form the revetments.


The Great Wall of China, built more than 2000 yearsago, contains some sections where clay and gravel werereinforced with tamarisk branches.

(http://www.polyfabrics.com.au/pdf/tenax_tt_slopes.pdf)


This concept is very ancient: 3000 years ago theBabylonians used intertwined palm branches to reinforcetheir "ziggurat“. The Aqar-Quf Ziggurat, in the actual Iraq,was made of clay bricks reinforced with woven mats of reedlaid horizontally on a layer of sand and gravel at verticalcenters between 0.5 m and 2.0 m. This structure wasoriginally over 80 m high.

Aqar-Quf Ziggurat (Iraq)(http://www.polyfabrics.com.au/pdf/tenax_tt_slopes.pdf)


Gravels encased with steel wire meshes or bambooshave been utilized to build the revetments in China andTaiwan. (Design manual).

Brushwoods were used in England to repair landslidesand for erosion control (Doran, 1948).

Wooden beams can be used as reinforcement materialsto construct vertical retaining walls (Munster, 1930).

In 1880’s, brushwoods were placed for stabilization ofthe soil along the bank of the Mississippi river (Hass andWeller, 1952).


In India, bamboos, straws, gunny bags made of jute andcoir, woods, palms, sisal, grass, sugar cane, plant leaf, pineapple etc. have exclusively been used for the construction ofshelters, bricks, roads and for flood protection particularly inrural areas for many many years. We do not know that thistechnology is called “Reinforced Earth”.

‘Reinforced earth’ is the most emerging and promisingalternative design technique come up into the market. It isalso cost effective with respect to the traditional constructionmaterials.


Historical background of reinforced earth:Henri Vidal (1966), French architect and engineer, is thepioneer of reinforced earth systems.

Henri VidalInventor of Reinforced Earth


Figure 3 Concept of reinforced earth


Basic Concepts and Mechanisms of Reinforced Earth


Basic concepts

Embankments

Interaction Polymer properties

Applications

Steep slops

Soil mechanics

Reinforced fill applicationsSoft soil applications

Unpaved roads Retaining walls

(Short term reinforcement strength required) (Long term reinforcement strength required)


Let us consider two soil samples, one unreinforced and theother reinforced as shown in Figure 4.

Figure 4 Difference in slope and settlement after loadingProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Vidal (1966 and 1969) developed the fundamental conceptsand mechanisms of reinforced earth. He introducedhorizontal steel strip reinforcement of width ‘b’ tounreinforced soil mass as shown in Figure 5. Reinforcementis placed perpendicular to the direction of applied verticalstress (σ1).

Figure 5Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

tanb2TTT 112

Where,T1 =Tensile strength on left sideT2 = Tensile strength on right sideδT = Change in tensile strengthb = Width of strip reinforcementδl = length of strip under normal pressuretanδ = Coefficient of friction between soil and reinforcement

From Figure 5,

No failure by slippage will occur between soil andreinforcement if the following condition is satisfied,

tanb2T

1


Figure 6(a) shows unreinforced soil mass under vertical stress(σ1). There is development of axial compression (δv) andlateral expansion (1/2 δh) occurs on both sides.

Now, a reinforced soil mass is constructed by introducinghorizontal layers of reinforcements as shown in Figure 6(b)and subjected to same vertical stress (σ1).

Behaviour of reinforced soil:


Due to application of reinforcement, there will bedevelopment of friction or adhesion between the soil andreinforcement. The Young’s modulus of reinforcement (Er) is much higherthan the Young’s modulus of soil (Es). Therefore, lateral strainin the reinforced soil mass will be very small, almost negligiblecompared to that of the unreinforced soil. Therefore, inreinforced condition with higher reinforcement modulus, even inactive condition, the soil mass will behave as if in at restcondition.The soil mass is in active state, but δh = 0. The stress circle will be within the failure envelope. Failurewill not occur until the reinforcement may fail by either pulloutor breakage.


Let us assume a cubical reinforced soil mass of unit volume atdepth ‘z’ from the ground surface as shown in Figure 7.

At active condition,Vertical stress on the soil mass, σ1 = .zHorizontal stress on the soil mass, σ3 = ko. σ1 = ko. . ZTherefore, total horizontal force on the unit soil mass= σ3 x (1x1) = σ3 = ko. σ1

Figure 7


The lateral force is transferred from soil to the reinforcement.

The lateral stress per unit area of reinforcement = (ko. σ1)/Ar

If Er = Young’s Modulus of reinforcement, and Ar = Crosssectional area of reinforcement,

The lateral strain (εr) in the reinforcement or soil along thereinforcement = (ko. σ1)/(Er. Ar)

As the stiffness of reinforcement (Er. Ar) is higher, lateralstrain (εr) tends to zero.

It should be noted that if the stiffness of reinforcementdecreases, the lateral strain (εr) increases and earth pressurecoefficient (k0) tends to ka.


Let us consider a semi infinite soil mass. As the soil is semiinfinite, the lateral deformation is zero.

Let us make a vertical cut in the unreinforced soil. There will be change in lateral stress condition.

At a depth ‘z’ from ground surface,

Vertical stress σ1 = γ. Z.

Lateral stress, σ3 = ko. σ1

Therefore, it is required to apply sufficient hydrostaticpressure (P = ko. γ. Z) along the vertical cut to maintainequilibrium.


If horizontal layers of reinforcements are placed in the soilmass along the vertical cut as shown in Figure 8, bond orinteraction between soil and reinforcements will occur.

Figure 8

Tensile force induced into the reinforcements developshorizontal shearing stress between soil and reinforcement.


Due to reinforcement, the lateral strain in soil mass remainsunaltered though the soil is not at rest condition. The stressstate in the soil becomes quite higher and close to failureenvelope as shown in Figure 9, but failure does not occur.

Figure 9Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Please let us hear from you

Any question?


Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]


GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE 2.pdf · Bridges, Flyovers, Road Over Bridges,...

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