CE-638
Geosynthetics 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 METHOD OR
PYCNOMETER METHOD
7 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
8 When plastics are available in the form sheets, rods, tubes etc
Material Polypropylene (PP)
Polyethylene (PE)
Polyester (PET)
Polyvinyl Chloride(PVC) Specific
Gravity 0.90 – 0.91 0.91 – 0.96 1.22 – 1.38 1.3 – 1.5
9 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.
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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.
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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
3 4
5 5
Applied Stress (kPa)
1 2 3 4 5
1
2
3 4
5
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MECHANICAL PROPERTIES OF GEOTEXTILES TENSILE STRENGTH
Most 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
50 100
Strain (%)
Tensile Stress (kN/m)
A
B C
D E
A Woven monofilament
B Woven slit-film monofilament C Woven multifilament
D Nonwoven heat bonded
E Nonwoven needle punched To 60 kN/m at 30% strain
0 0
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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
50 100
Strain (%)
Tensile Stress (kN/m)
A
B C
D E
A Woven monofilament
B Woven slit-film monofilament C Woven multifilament
D Nonwoven heat bonded E Nonwoven needle punched To 60 kN/m at 30% strain
0 0
INITIAL TANGENT MODULUS OFFSET TANGENT MODULUS SECANT TANGENT MODULUS
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ELASTIC MODULUS
Tensile Stress kN/m
Stress Strain Curve
Strain % Initial
Tangent Modulus
Stress Strain Curve
Offset Modulus
Offset Strain %
Tensile Stress kN/m Tensile Stress kN/m
Secant Modulus
Stress Strain Curve
Desired Strain
Strain %
INITIAL TANGENT MODULUS OFFSET TANGENT MODULUS SECANT TANGENT MODULUS
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FACTORS AFFECTING LAB RESULTS
Specimen width
Temperature
Mass per unit area
Aspect ratio (B/L) = 2
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FACTORS AFFECTING LAB RESULTS
Specimen width
Temperature
Mass per unit area
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FACTORS AFFECTING LAB RESULTS
Specimen width
Temperature
Mass 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.
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Fahmy et al. (1993)
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Fahmy et al. (1993) conducted tests on woven and nonwoven geotextile under confining pressures of 0, 35 , 70 and 140 kPa.
It was found that only the nonwoven needle punched geotextile shows significantly improved stress-strain behavior under confinement and the improvement was proportional to the confining pressure.
The possible reason for increased performance may be that the confining pressure holds the randomly oriented fibers in their original positions.
Thus, the low initial modulus response seen in Figure 2(a) [curve E] is eliminated.
For the other geotextiles tested under confining pressure the variation in results is not considerable but almost the same.
Therefore, confined tensile testing is not carried out on a routine basis.
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SEAM STRENGTH
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FATIGUE STRENGTH
36 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 2
Bursting Puncturing
Fine-grained soil Pressure
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
PUNCTURE TEST
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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 BETWEEN
CBR PUNCTURE RESISTANCE (F
p) AND
WIDE WIDTH TENSILE STRENGTH (T
f) Cazzuffi and Venesia (1986)
T
f= tensile force per unit width (kN/m) F
p= puncture/breaking force (kN)
r = radius of the puncturing rod (m)
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BURSTING STRENGTH
It 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
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SHEAR TEST
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Geosynthetic Reinforcement
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 to shear test, hence double of shear resistance obtained by shear test is taken as Pullout resistance.
56 For preliminary design or in the absence of specific geosynthetic test data, F may be conservatively taken as F = (2/3) tan ϕ for geotextiles and 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 = Porosity
m = Mass per unit area ρ = Density of polymer t = Thickness of geotextile
For a given geotextile’s weight and density, the porosity is directly related to thickness. Thickness in turn is related to the applied normal stress.
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 water
Immersion 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 water
Immersion of bucket with bottom gtx
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PERMEABILITY
The 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) will depend upon the thickness of the geotextile.
Permittivity is defined as the coefficient of permeability for water flow 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) m3 /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) will depend upon the thickness of the geotextile.
Transmissivity is defined as the product of coefficient of permeability 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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ABRASION
Wearing away of any part of a geotextile by rubbing it against a stationary abradant.
ABRASION RESISTANCE
The ability of a geosynthetic to resist wear due to rubbing is called abrasion resistance.
FIELD CONDITIONS
Geosynthetics used under pavements, railway tracks or in coastal erosion protection are subject to dynamic loading, which will lead to mechanical damage (abrasion).
77 Resistance to abrasion is expressed as the percentage loss of tensile strength or weight of the test specimen as a result of abrasion.
MEASUREMENT OF ABRASION
78 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.
