GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE...Thickness (ASTM D5199) Geotextiles exhibit different thickness according to different pressures. The thickness is measured to an

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

Module - 3LECTURE- 10

Geosynthetic properties and test methods


Introduction Properties of geosynthetics Physical properties Mechanical properties Hydraulic properties Endurance properties

Geosynthetic clay liners (GCLs) properties and test methods Tensile strength of Nano material Ultimate versus allowable geosynthetics properties

OUTLINE


Geosynthetic properties and test methods


When to test geosynthetics?

Firstly for any project the design engineers must check therequired specifications of geosynthetic materials. Withoutknowing proper specifications, it is very difficult to select thecorrect geosynthetic for any specific project.

It is needed to conduct tests on geosynthetics from anindependent laboratory.

Generally, test results are supplied from manufacturer datasheet. These data sheets provide minimum average roll values(MARV) for quality control of geosynthetics.

Considering environmental impact, geosynthetics should becollected from the project site and sent to geosyntheticlaboratory for testing of their different properties.


Main strong reasons for exploring this exciting newengineering construction material in civil engineering areas follows: Excellent stress-strain behavior Good flexibility Excellent filtration characteristics High water permeability Excellent mechanical properties Can be welded together Does not form by-products High resistance to climate condition High resistance to chemical and biological attack Chemically ultraviolet stabilized Time, cost, rapid construction, environment friendly,

sustainability and durabilityProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Tests on geosynthetics are conducted in two different ways:

A) Index tests or in-isolation tests: Tests are performed only on geosynthetics itself

B) Performance tests: Tests are performed along with site-specific soil.

Physical Properties

Mechanical Properties

Hydraulic Properties

Endurance Properties

PROPERTIES OF GEOSYNTHETICS


Mass per unit area (Weight)

Thickness

Specific gravity

Stiffness or flexural rigidity or flexural stiffness

Physical Properties


Mass per unit area (ASTM D5261) Five test specimens are to be weighed in a weighingmachine (accuracy of 0.01 g) and average value is recorded. Test samples are of size 100 mm 100 mm Unit is expressed as g/m2

The cost of geotextile is directly related to the weight ofgeotextile.


Thickness (ASTM D5199)

Geotextiles exhibit different thickness according todifferent pressures.

The thickness is measured to an accuracy of 0.02 mmunder a specified pressure of 2.0 kPa.

Sample size is 200 mm 200 mm. The thickness isgenerally in the range of 0.25 to 8.5 mm.

The thickness of geogrids and geomembranes aremeasured under a normal stress of 20 kPa.


Thickness measurement of geotextile


Materials Sp. Gravity

Polypropylene (PP) 0.91

Polyethylene (PE) 0.9 to 0.96

Polyester (PET) 1.22 to 1.38

Polyvinyl chloride (PVC) 1.69

Nylon 1.05 to 1.14

Specific gravity (ASTM D 792 or D1505)

Specific gravity can be defined as ratio of the unit weight ofmaterial to the unit weight of distilled water at 4°C.

Specific gravity of different geosynthetic materials


The geotextile specimen is a 25 mm wide strip.

The geotextile is placed along the length of a horizontalplane and bends gravitationally under its own weight on ainclined plane making an angle of 41.5 degree with thehorizontal.

Stiffness or flexural rigidity or flexural stiffness (ASTM D1388)


Stiffness of the geotextile = (l/2)3 x W

l = length of overhang geotextile and bending length = l/2(cm), w = mass per unit area (mg/cm2)

The unit of stiffness is mg-cm.

The minimum stiffness of geotextile depends on the variousdegree of required workability (Haliburton et al., 1980)

The property is important in field workability requirementsfor installation of geotextile.

If the soil is very poor or California bearing ratio value isvery less, the stiffness of geotextile required is very high.


Mechanical Properties

Compressibility Tensile strength test of geosynthetics Burst strength test Puncture Resistance Test Penetration resistance test (drop test)/ drop cone

(impact strength)/ tear (impact) resistance Tensile behavior of geogrid Tensile strength of Gabions Direct shear test Pullout or anchorage resistance Tensile strength of geomembrane Tear resistance of Geomembrane


Compressibility Compressibility indicates the reduction in thickness underapplied pressure. Compressibility of geotextile depends on itsthickness and mass per unit area.

As the pressure increases, thickness of non-woven needle-punched (NW-NP) and resin bonded geotextiles gets reducedsignificantly and accordingly, the transmissivity gets reduced.

Compressibility of woven and non-woven heat bondedgeotextile (NW-HB) is low.

Compressibility of nonwoven needle-punched geotextileplays a very important role as most of the time we use thesetype of geotextiles to pass the liquid along their plane.


Variation in thickness of geosynthetics with change in pressure

It is clearly observed that nonwoven needle punchedgeosynthetics are more compressible.


Tensile strength

Wide width tensile strength

(ASTM D4595 and ISO 10319)

Very wide width tensile strength test

Narrow strip tensile strength (ASTM D4751)

Sewn seam strength of geotextile

(ASTM D4884 and ISO 13426)

Grab tensile strength (ASTM D4632)

Trapezoidal tear strength test


Wide width tensile strength (ASTM D4595)

Nonwoven geotextiles Woven jute geotextiles

Test result on a thermally bonded nonwoven geotextileProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

The machine strain rate is 10 ± 3 %.

The reason for the necessity of wide-width specimens isthat geotextiles (particularly non-woven) achieve highpoison’s ratio value from narrow strip test.

Tensile strength of geotextile (Tgeotextile) can be expressedas force per unit width.

Tgeotextile = Fb/ W (kN/m)

Fb = Observed breaking force (kN), and

W = Specimen width (meter)


Very wide width tensile strength test

Size of sample for very wide width test

For design purpose, the very wide width tensile test is notrecommended.


Narrow strip tensile strength (ASTM D 1682)

Strain rate = 300 mm/ min

Tensile strength appearslow compared to wide widthtensile strength test.

Not recommended asdesign value.

Size of test sample with the test assembly


Tensile modulus:

Different tensile modulus from typical stress-strain curves


Tensile strength vs. strain curves for different geotextiles


Case 1:

For Sample - 1, Maximum tensile stress, Tmax = 30 kN/m Maximum strain = εg =45% Initial tangent modulus (E) = Initial slope of the curve

= 12/0.08 = 150 kN/m Toughness, TU = ½ × Tmax × εg = 1/2 × 30 × 0.45 = 6.75 kN/m

Case 2:

For sample - 2, Initial tangent modulus (E) = 8/ 0.50 = 16 kN/m Offset modulus = 20/ (0.31-0.20) = 181.8 kN/m Secant modulus at 40% strain, E40 = 34/ 0.40 = 85 kN/m


Sewn seam strength of geotextile (ASTM D4884 and ISO 13426)

(After Diaz, 1985)

Size of test sample for sewn seam strength

Strain rate =10 ± 3 %/ min, Unit in kN/mButterfly seam is recommended for sewing


The seam strength efficiency can be expressed as,

SE = (Tseam / Tg) x100 %

Where,

Tseam = wide width seam strength (ASTM D 4884), and

Tg = wide width geosynthetic strength without seam(ASTM D 4595)


Grab Tensile Test (ASTM D 1682)

The test relies on filament interaction in geotextile. Fornonwoven geotextile, the effects are more than wovengeotextile. The grab tensile test is reported in kN, not in kN/m.

As the sample is partially clamped, stress is not propagatedin entire width of the sample. It is an unusual test and widelymisused.

Grab tensile test specimen Laboratory Grab tensile test


Analytical analysis of grab tensile strength:

Grab tensile strength is required to design the geotextilesfor separation. When pressure is applied to the upperstone, it spreads the two lower stones laterally. As a result,tension is mobilized in the geotextile. It is analogous to thegrab tensile strength test.


222DDD

22 DD

D = Diameter of stoneli= Initial length of geotextile =

lf = final length of geotextile =

Without any stone breakage or slippage, maximum strain in geotextile can be expressed as,

%3331

23

22222

100

D

DDDDD

lll

i

if

Treqd = Ap (Dv)2 ε (Giroud,1984)

Treqd = required grab strength, Ap = applied pressure,Dv = maximum void diameter = 0.33Da, Da = average stone diameter


Example:

Tire inflation pressure = 650 kPa. Average stone diameter =50 mm. Assume the geotextile is placed beneath stone basecourse. Calculate required grab tensile strength of thegeotextile. Assume 50 % of total ultimate grab strain willmobilize.

Solution:We know that total ultimate grab strain = 33%.

So, the mobilized grab strain = 0.33 x 0.5 = 0.165

Hence, Treqd = 650 × (0.33 × 0.05)2 × 0.165 = 29.2 Newton


Trapezoidal tear strength test (ASTM D4533 and ISO 13434)

This test is done to tear the test specimen from the point ofincision.

Tear strength is important when the geosynthetic isdamaged.

Trapezoidal tear strengthis measured in N.


Burst strength test

Different types of burst strength test apparatus

Unit in kN/m2

(a) Mullen burst (ASTM D 3786)

(b) Ball burst (ASTM D 751)

(c) CBR burst (DIN 54307 E)


Analytical analysis of burst strength test:Burst strength is required to design the geotextiles forseparation. The geotextile may burst due to the appliedupward load.

Field model for burst resistance (Geotextile being forced up into voids of stone base due to traffic tire loads)

Laboratory simulation


gvgreqd DP21T

v

v

v

vg z2

wwz2

41

Theoretically, from the field concept Giroud (1984) developed a formula for required geotextile burst strength (Treqd).

Strain in geotextile (εg) depends on width of void (wv) anddeformation of the void (zv).


Mullen burst test can be analogous to the field conditions,

Experimental model set up forburst strength in laboratory

Geotextile is pushed upward and it forms hemispherical shapeas well as fails due to radial tension. So, the ultimate strength(Tult) of geotextile can be written as,

gbbult DP21T

Pb = Burst strength, Db = Diameter of burst equipment ≈ 30 mm, εg = Strain in geotextile


(C.R.F)factor reduction CumulativeTT ult

allowable

vg

bb

reqd

allow

DP.)F.R.C(DP

TT.S.F

If Dv = 0.33 Da, Db = 30 mm, C.R.F. = 1.5

ag

b

ag

b

DPP6.60

)D33.0(P)5.1(30P.S.F


Design chart for burst analysis of geotextile considering reduction factor = 1.5 and factor of safety = 2.0

The design chart reveals that size of stone aggregatepossess a significant impact on the burst strength of geotextile.


Example :

Let, tire inflation pressure (Pt) = Pg = 650 kPa and maximumsize of stone = 60 mm. Determine the required burststrength of geotextile using C.R.F = 1.5 and F.S. = 2.0.

ag

b

DPP6.60FS

kPa12876.60

6065026.60DPS.F

P agb

Solution:

We know, for C.R.F. = 1.5,

So, the required burst resistance is 1287 kPa.


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]


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GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE...Thickness (ASTM D5199) Geotextiles exhibit different thickness according to different pressures. The thickness is measured to an