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Axial Stiffness of Geosynthetics

Geosynthetics are tensile reinforcing elements (geotextiles, geogrids) defined by their starting and end points and by the axial (normal) stiffness Jz [kN/m].

For nonwoven fabrics the axial stiffness is usually not considered since these elements typically serve as separating layers. Woven geotextiles experience for small deformations very low initial stiffness - in the small strain region (up to 5%) we encounter a considerable increase of deformations under constant load.

When designing geotextiles this property must be taken into account. We thus recognize both the long-term tension strength in dependence on partial reduction factors (reflecting damage of elements caused by installation, creep behavior of geosynthetics, biological and chemical effects) and initial normal stiffness in the small strain region in the interval of 0.5% to 2%.

To determine the minimum axial stiffness of georeinforcements it is possible to use the following expression where for the strength corresponding to the selected strain we accept maximally 10% deviation from a linear part of tension test:

where:

Tz

-

tensile strength at x% strain [kN/m]

ε

-

x% strain (relative extension) according to EN ISO 10 31 [%]

Tmax

-

maximal tensile strength according to EN ISO 10 319 [kN/m]

εmax

-

maximal strain (relative extension) according to EN ISO 10 319 [%]

Suppliers and producers of geotextiles typically provide the value of tensile strength at 2% strain. The expression then becomes:

The minimal (initial) axial stiffness of geotextiles from a short-term experiment (load rate according to EN ISO 10 319) for x-%-strain is given by:

where:

ε

-

x%-strain (relative extension) according to EN ISO 10 319 [-]

The maximum (theoretically attainable) axial stiffness of geotextiles for a short-term axial strength is determined as follows:

where:

εmax

-

maximum strain (relative extension) according to EN ISO 10 319 [-]

Intervals of recommended of values of axial (normal) stiffnesses of geosynthetics Jz [kN/m] are listed in the following table:

Variable description

Initial axial stiffness of geotextiles for

ε = 2%

Theoretical (maximal) axial stiffness of geotextile

Notation (unit)

Jε=x [kN/m]

Jεmax [kN/m]

Georeinforcements category

---

---

Non-woven geotextiles

-

-

Woven geotextiles

250 ÷ 500

1000

Unaxial geogrids

500 ÷ 1000

1500

Biaxial geogrids

100 ÷ 500

for ε = 0.5%

2500

Triaxial geogrids

250 ÷ 500

for ε = 0.5%

5000

Geomats

100 ÷ 500

1000

Drainage geocomposites

-

-

Composites

100 ÷ 500

1500

Geomeshes

-

-

Geocells

-

-

Literature:

GEOMAT ltd. (www.geomat.cz): Types of geotextiles and their function in civil engineering structures. Author: Martin Kašpar (kaspar@geomat.cz). In Czech.

HOLÝ, O., MIČA, L.: Determination of axial stiffness of geosynthetics for numerical modeling - part 1. TU Brno (paper in conference proceedings „Civil engineering structures in view of geomechanics“). In Czech.

EN ISO 10 319 (80 6125): Geotextiles - Tensile test on a wide strip. Czech standard institute, 2009. In Czech.