CE-638Geosynthetics and Reinforced Soil Structures

Mid-Semester Exam = 25 Marks

Home Assignment + Attendance + Surprise Tests + etc = 15 Marks

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PROPERTIES OF GEOSYNTHETICS

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PROPERTIES OF GEOSYNTHETICS

Necessity

1. To determine the suitability of a material for specific application.

2. To control the quality of a product during production and use.

3. To verify the claimed characteristics by the manufacturer.

Pre-requisite

Before testing, the specimen is kept under controlled environment

for a specified period of time. This activity is called conditioning.

Temperature (27°C ± 2°C)

Humidity (50% ± 5%)

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CLASSIFICATION OF PROPERTIES

1. Physical Properties – properties describing the material itself

2. Mechanical Properties – properties describing strength parameters

3. Hydraulic Properties – properties describing flow of water through GS

4. Endurance Properties – properties describing durability

5. Degradation Properties – properties describing degradation process

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PHYSICAL PROPERTIES OF GEOTEXTILES

(i) Specific gravity

(ii) Mass per unit area (weight)

(iii) Thickness

(iv) Stiffness

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PHYSICAL PROPERTIES OF GEOTEXTILES

SPECIFIC GRAVITY

The ratio of the weight of a given volume of material (without any voids)

to the weight of an equal volume of distilled de-aired water at 27°C

Importance

Helps in identifying the base polymer (PP, PE, PET, PVC etc.)

Helps in calculating strength – weight and cost – weight ratios.

Plastics are available in the form of sheets, rods, tubes, molded items.

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PHYSICAL PROPERTIES OF GEOTEXTILES

DENSITY BOTTLE METHODOR

PYCNOMETER METHOD

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When plastics are available in the form of crystals

W1 = Weight of empty bottle

W2 = Weight of bottle + geosynthetic material

W3 = Weight of bottle + geosynthetic material + water

W4 = Weight of bottle + water

PHYSICAL PROPERTIES OF GEOTEXTILES

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When plastics are available in the form sheets, rods, tubes etc

Material Polypropylene (PP)

Polyethylene (PE)

Polyester (PET)

Polyvinyl Chloride(PVC)

SpecificGravity 0.90 – 0.91 0.91 – 0.96 1.22 – 1.38 1.3 – 1.5

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SPESIFIC GRAVITY OF COMMONLY USED POLYMERS

PHYSICAL PROPERTIES OF GEOTEXTILES

MASS PER UNIT AREA (WEIGHT)

Mass per unit area is the proper term for the weight of the geotextile.

It is usually given in units of gram per square meter (g/m2).

It is determined by weighing square test specimens(100mm×100mm)

Specimens are cut from various locations over the full width of the

laboratory sample.

Dimensions are measured without any tension in the specimen.

Normally the specimen is placed on the table during measurements.

The calculated values are then averaged to obtain the mean mass per

unit area of the laboratory sample. 10

PHYSICAL PROPERTIES OF GEOTEXTILES

MASS PER UNIT AREA (WEIGHT)

The results should be accurate to 0.01 g/m2.

The number of test specimens shall be a 5 – 10

The total geotextile sample should be the representative of the entire

roll width and a length such that the combined total minimum area

must not be less than 100000 mm2.

Each test specimen shall be equal in area not less than 10000 mm2.

Importance

Cost and mechanical properties such as tensile strength, tear strength,

puncture strength, etc., are directly related to mass per unit area. 11

PHYSICAL PROPERTIES OF GEOTEXTILES

THICKNESS

The thickness of a geosynthetic is the vertical distance between its

upper and lower surfaces.

A compressive pressure of 2 kPa is applied at the time of thickness

measurement.

It is measured by using a thickness-testing instrument to an accuracy

of at least 0.01 mm.

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PHYSICAL PROPERTIES OF GEOTEXTILES

THICKNESS

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PHYSICAL PROPERTIES OF GEOTEXTILES

STIFFNESS

It is the ability to resist flexure under its own weight.

Measured by making a cantilever with fixed deflection.

Half of the length is bending length.

Stiffness = cube of bending length × weight per unit area

Automatic Method

41.5°l

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MECHANICAL PROPERTIES OF GEOTEXTILES

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MECHANICAL PROPERTIES OF GEOTEXTILES

Compressibility

Tensile Strength

Fatigue Strength

Burst Strength

Tear Strength

Impact Tests

Puncture Tests

Friction Behaviour

Pullout (Anchorage) Tests

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MECHANICAL PROPERTIES OF GEOTEXTILES

COMPRESSIBILITY

Rate of decrease of thickness due to increased normal stresses.

Determined by observing the change in thickness of geotextile at

varying applied normal stresses.

Woven and nonwoven heat bonded geotextiles have very low

compressibility

Little direct consequence as far as design is concerned.

Nonwoven needle punched geotextiles have more compressibility.

It will affect transmissivity adversely because nonwoven needle

punched geotextiles are used for conveying water. 17

COMPRESSIBILITY OF DIFFERENT TYPES OF GEOTEXTILES 18

1

2

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5

5

Applied Stress (kPa)

12345

1

2

3

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MECHANICAL PROPERTIES OF GEOTEXTILES

TENSILE STRENGTHMost important property because tensile

strength is needed for all the functions e.g.

reinforcement, separation, filtration, drainage.

Maximum tensile stress at the time of failure.

kN/mm2 (not used) irregular thickness

kN/m (used) maximum load per unit length

Determined by wide strip tensile test

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WOVEN

NON-WOVEN

SAMPLE RESULTS OF TENSILE TESTS

10

30

20

40

50 100Strain (%)

Tens

ile S

tress

(kN/

m)

A

B

C

D

E

A Woven monofilamentB Woven slit-film monofilamentC Woven multifilamentD Nonwoven heat bondedE Nonwoven needle punched

To 60 kN/m at 30% strain

00

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TYPES OF TENSILE TESTS

Wide-strip tensile test

Narrow strip tensile test

Grab tensile test

Very wide strip tensile test

Depending upon nature of test specimen

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The test provides following four parameters:

1. Maximum tensile stress (referred to as the geotextile’s strength)

2. Strain at failure (generally referred to as maximum elongation or

simply elongation

3.Toughness (absorbed energy, work done per unit volume before

failure, usually taken as the area under the stress-strain curve)

4. Modulus of elasticity (which is the slope of the initial portion of the

stress-strain curve)

TENSILE STRENGTH

Parameters obtained by stress-strain curve

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ELASTIC MODULUS

10

30

20

40

50 100Strain (%)

Tens

ile S

tress

(kN/

m)

A

B

C

D

E

A Woven monofilamentB Woven slit-film monofilamentC Woven multifilamentD Nonwoven heat bondedE Nonwoven needle punched

To 60 kN/m at 30% strain

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INITIAL TANGENT MODULUSOFFSET TANGENT MODULUSSECANT TANGENT MODULUS

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ELASTIC MODULUSTe

nsile

Stre

ss k

N/m

Stress Strain Curve

Strain %

InitialTangentModulus

Stress Strain Curve

OffsetModulus

Offset Strain %

Tens

ile S

tress

kN

/m

Tens

ile S

tress

kN

/m

SecantModulus

Stress Strain Curve

Desired Strain

Strain %

INITIAL TANGENT MODULUS OFFSET TANGENT MODULUS SECANT TANGENT MODULUS

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FACTORS AFFECTING LAB RESULTS

Specimen widthTemperatureMass per unit area

Aspect ratio (B/L) = 2

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FACTORS AFFECTING LAB RESULTS

Specimen widthTemperatureMass per unit area

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FACTORS AFFECTING LAB RESULTS

Specimen widthTemperatureMass per unit area

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CONFINING TENSILE STRENGTH

The geosynthetics in field are not provided in isolation.

They are subjected to some confining pressure.

Confining pressure interlocks of soil particles with the geosynthetic

structure and may increase the frictional force.

It may have a significant effect on the stress–strain properties.

The confined modulus may be more than the isolation modulus.

McGown et al. (1982) developed the mechanism for holding the

geotextile specimen for confined tensile strength.

It is a boxlike part in which specimen is sandwiched between

lubricated membrane and sand layer under lateral pressure.30

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Fahmy et al. (1993)

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Fahmy et al. (1993) conducted tests on woven and nonwovengeotextile under confining pressures of 0, 35 , 70 and 140 kPa.

It was found that only the nonwoven needle punched geotextileshows significantly improved stress-strain behavior underconfinement and the improvement was proportional to theconfining pressure.

The possible reason for increased performance may be that theconfining pressure holds the randomly oriented fibers in theiroriginal positions.

Thus, the low initial modulus response seen in Figure 2(a) [curveE] is eliminated.

For the other geotextiles tested under confining pressure thevariation in results is not considerable but almost the same.

Therefore, confined tensile testing is not carried out on a routinebasis. 34

SEAM STRENGTH

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FATIGUE STRENGTH

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Field Conditions

Wave loading on offshore structures

Seismic loading

Rail/road loading

What is the similarity among all these loads?

All are of repeated nature.

Strength of geotextile will reduce after each load application.

Fatigue strength may be defined as the ability of a geotextile to

withstand repetitive loading before undergoing failure.

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Fatigue strength is determined through conducting wide strip tensile

test by applying a predetermined load (less than the failure load) and

then reducing it to zero.

The load is again applied and then relaxed.

This cycling is repeated till failure takes place.

The resulting cyclic stress – strain response is used to calculate the

cyclic modulus.

As expected, lower the stress level, the larger the number of cycles

required before failure.

FATIGUE STRENGTH

TEAR STRENGTH

Tear

(split, scratch, damage, injury, cut)

Tear may present in the supplied geotextile

Tear may take place during construction (transporting, handling, placing)

The cut propagates when subjected to tension.

TEAR STRENGTH

The ability of geotextile to resist the stresses causing propagation of tear.

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TRAPEZOIDAL TEAR TEST (ASTM D4533)

Machine : as used in case of grab test

Specimen size: 200 mm × 76 mm

A trapezoidal section is drawn as follows:

Strain rate 300 mm per minute

TEAR STRENGTH

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TEAR STRENGTH

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IMPACT RESISTANCE

The ability of a geosynthetic to withstand stresses generated by the

sudden impact of falling objects such as coarse aggregates, tools,

and other construction items during installation process.

Impact strength/dynamic puncture strength/dynamic perforation

strength

CONE DROP TEST METHOD

This test involves the determination of the diameter of the

punctured hole made by dropping a standard brass or stainless steel

cone weighing 1 kg from a specified height onto the surface of a

circular geosynthetic specimen gripped between clamping rings.

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MANNUAL METHOD

MECHANICAL METHOD

Height = 500 mm or

Diameter = 50 mm

IMPACT RESISTANCE

The geosynthetic may be supported by water or soil to simulate the

field conditions.

The diameter of the punctured hole, measured using a penetration

measuring cone, in combination with the drop height, gives a

measure of impact resistance.

The smaller the diameter of the hole, the greater the impact

resistance of the geosynthetic to damage during installation.

The impact resistance (strength) can be expressed as either the

diameter of the hole at a standard drop height of 500 mm or drop

height that will produce a hole of diameter 50 mm.

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PUNCTURE RESISTANCE

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PUNCTURING

BURSTING

SOURCES OF PUNCTURING

FALLEN POINTED STONES ETC IMPACT RESISTANCE

STATICALLY LOADED POINTED STONES ETC PUNCTURE RESISTANCE

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PUNCTURE RESISTANCE

1 2Bursting Puncturing

Fine-grained soilPressure

Stone bed

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PUNCTURE RESISTANCE

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Cone penetration test – dynamic loading – impact resistance

Damage of geotextiles due to objects such as stones, ballast, etc.

under static condition.

Thus, the puncture strength is the ability of geotextile to withstand

the localized stresses generated by penetrating or puncturing objects

e.g. aggregate etc.

PUNCTURE RESISTANCE

Dia. of steel rod = 8 mm

Dia. Of empty cylinder = 45 mm

Depth of cylinder = 100 mm

Strain rate = 50 mm per minute

PUNCTURETEST

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Large size, CBR apparatus gives more satisfactory results.

Rod dia. = 50 mm (beveled at 45° for 2.5 mm radius)

Mould diameter = 150 mm

Strain rate = 50 mm per minute

Puncture strength in force units.

PUNCTURE RESISTANCE

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RELATION BETWEENCBR PUNCTURE RESISTANCE (Fp)

ANDWIDE WIDTH TENSILE STRENGTH (Tf)

Cazzuffi and Venesia (1986)

Tf = tensile force per unit width (kN/m)

Fp = puncture/breaking force (kN)

r = radius of the puncturing rod (m)50

BURSTING STRENGTHIt is multi-axial tensile test.

Specimen is clamped with sufficient rubber membrane below it.

Air pressure is applied till failure (bursting).

Air pressure at failure is called bursting strength.

This test is more important in case of geomembranes.

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SOIL-GEOSYNTHETIC INTERFACE CHARACTERISTICS

52SHEAR TEST

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GeosyntheticReinforcement

Steep Slope

Stable Foundation Soil

SHEAR TEST

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Soil

Soil

Soil

Soil

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PULLOUT RESISTANCE

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PULLOUT RESISTANCE

where, Le = embedment length of the test specimen; W = width of the test specimen;ef = effective normal stress at the soil–test specimen interfaces;Ci = coefficient of interaction (a scale effect correction factor)depending on the geosynthetic type, soil type and normal load applied;F = pullout resistance (or friction bearing interaction) factor.

Since this test is similar toshear test, hence double ofshear resistance obtainedby shear test is taken asPullout resistance.

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For preliminary design or in the absence of specific geosynthetic testdata, F may be conservatively taken as F = (2/3) tan ϕ for geotextilesand F = 0.8 tan ϕ for geogrids.

INFLUENCE OF SPECIMEN EMBEDMENT LENGTH

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INFLUENCE OF NORMAL STRESS

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HYDRAULIC PROPERTIES

POROSITY

PERCENT OPEN AREA

APPARENT OPENING SIZE

PERMEABILITY

FILTERATION

FIELD APPLICATIONS

DRAINAGE

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POROSITY

n = Porositym = Mass per unit areaρ = Density of polymert = Thickness of geotextile

For a given geotextile’s weight anddensity, the porosity is directlyrelated to thickness. Thickness inturn is related to the applied normalstress.

PERCENT OPEN AREA

This property is applicable only for woven geotextiles, and even then

only for woven monofilament geotextiles.

Higher POA indicates more number of openings per unit area.

For filter applications of a geotextiles, its POA should be higher.

Higher POA avoids clogging throughout the design life of a filter.

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Project a light through the geotextile onto a large poster sized piece

of a cardboard background that is crosshatched like graph paper.

Squares are counted and summed up for the open area.

Measure total area (yarns plus voids) on cardboard.

This test is not applicable to nonwovens, since the overlapping

yarns block any light from passing directly through the geotextile.

MEASUREMENT OF PERCENT OPEN AREA

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Cardboard

Sometimes pore spaces are measured by Image

analysis, a technique used for the direct

measurement of pore spaces within a cross-

sectional plane of the geotextile with the help of

a microscope.

IMAGE ANALYSER (MICROSCOPE)

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APPARENT OPENING SIZE OR

EQUIVALENT OPENING SIZE

Pores in a geotextile are not of one size but are of a range of sizes.

Pore size distribution is to be determined.

Pore size distribution of a geotextile is represented similar to the

particle size distribution for a soil.

The geotextile is used as a sieve, of unknown sizes, and the

particles (glass beads) of different known sizes are passed through

the geotextile as a sieve.

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The pore size (or opening size), at which 95% of the pores in the

geotextile are finer, termed the equivalent opening size (EOS)

designated as O95.

EQUIVALENT OPENIG SIZE

DRY SIEVING

Filed condition

WET SIEVING

Continuous spray of waterImmersion of bucket with bottom gtx

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95% of geotextile pores are 300 µm or smaller.

95% of particles with a diameter of 300µm are retained on the

geotextile during sieving.

5% of particles with a diameter of 300µm are passed through the

geotextile during sieving.

If a geotextile has an O95 value of 300µm, it means ………………..

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AOS or EOS is, in fact, considered as the property that indicates the

approximately largest particle that would effectively pass through

the geo-textile and thus reflects the approximately largest opening

dimension available in the geotextile for soil to pass through.

IMPORTANT

DRY SIEVING

Filed condition

WET SIEVING

Continuous spray of waterImmersion of bucket with bottom gtx

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PERMEABILITYThe ability of geosynthetic to transmit a fluid.

Water flow direction

Geosynthetic strip

Geosynthetic strip

Water flow direction

Permeability across the plane of geotextile

Permeability along the plane of geotextile

(Permittivity)

(Transmissivity)

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PERMITTIVITY (ψ)Water flow direction

Geosynthetic strip

Geotextiles deform under applied loads (overburden).

Permeability across the plane of geotextile (Permittivity) willdepend upon the thickness of the geotextile.

Permittivity is defined as the coefficient of permeability for waterflow normal to its plane divided by its thickness.

This is quite useful in filter applications.

ψ = permittivity (sec-1)

kn = permeability normal to the geotextile (m/sec)

t = thickness of the geotextile (m)

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q = flow rate (m3/sec)

i = hydraulic gradient (dimensionless)

Δh = total head loss (m)

A = total area of specimen (m2)

CONSTANT HEAD

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FALLING HEAD

Flow

rate

(q) m

3 /sec

(Δh × A) (m3)

Permittivity (ψ)

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When the flowing fluid is not water (leachate or waste oil) then

ρf

ρw

Permittivity of the fluid under consideration

= Permittivity using water

= Density of the fluid

= Density of water

= Viscosity of the fluid

= Viscosity of water

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TRANSMISSIVITY (θ)

Geosynthetic strip

Water flow direction

Similar to permittivity

Geotextiles deform under applied loads (overburden).

Permeability along the plane of geotextile (Transmissivity) willdepend upon the thickness of the geotextile.

Transmissivity is defined as the product of coefficient ofpermeability for water flow along its plane and its thickness.

This is quite useful in drainage applications.

θ = transmissivity (m2/sec)

kp = permeability along the geotextile (m/sec)

t = thickness of the geotextile (m)

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θ = transmissivity of geotextile (m2/sec)q = flow rate (m3/sec)B = width of the geotextile test specimen (m)Δh = total head loss (m)L = length of the geotextile (m)

TRANSMISSIVITY (θ)

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RADIAL TRANSMISSIVITY (θ)

where

q = flow rate of liquid (m3/sec)

r2 = outer radius of the geotextile test specimen (m)

r1 = inner radius of the geotextile test specimen (m)

Δh = head loss between r1 and r2 (m)

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DURABILITY

Ability to maintain requisite properties against environmental or

other influences over the selected design life.

Assessed on the basis of mechanical property test results.

Assessed in terms of percentage retained tensile strength (RT)

and/or percentage retained strain (Rε).

Te = Mean tensile strength of the exposed geosynthetic specimen.

Tu = Mean tensile strength of the unexposed geosynthetic specimen.

εe = Mean strain at max. load of the exposed geosynthetic specimen.

εu = Mean strain at max. load of the unexposed geosynthetic specimen.

Temperature Degradation, Oxidation Degrad, Hydrolytic Degrad,Chemical Degrad, Mechanical Degrad, Biological Degrad,Radioactive Degrad, Sunlight (Ultraviolet) Degrad.

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ABRASIONWearing away of any part of a geotextile by rubbing it against astationary abradant.

ABRASION RESISTANCEThe ability of a geosynthetic to resist wear due to rubbing is calledabrasion resistance.

FIELD CONDITIONSGeosynthetics used under pavements, railway tracks or in coastalerosion protection are subject to dynamic loading, which will lead tomechanical damage (abrasion).

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Resistance to abrasion is expressed as the percentage loss of tensilestrength or weight of the test specimen as a result of abrasion.

MEASUREMENT OF ABRASION

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ULTRAVIOLET RESISTANCE

Sunlight is an important cause of degradation of the polymers from

which geosynthetics are made.

Spectrum of Sunlight

Infrared, with wavelengths longer than 760 nm

Visible, with wavelengths between 760 and 400 nm

Ultraviolet, with wavelengths shorter than 400 nm

The ultraviolet (UV) region is further subdivided into the following:

UV-A (400 to 315 nm), which causes some polymer damage;

UV-B (315 to 280 nm), which causes severe polymer damage;

UV-C (280 to 100 nm), which is only found in outer space.