GeoTextile

The GeoTextile support type can be used to model the various types of slope reinforcement which are used in the form of meshes, grids, strips, etc. There are a wide variety of such support systems, referred to as geo-textiles or geo-fabrics, geo-grids, geo-synthetics etc.

Although the term "GeoTextile" actually refers to a specific category of reinforcing materials, for the purpose of the following discussion, the term GeoTextile will be used to refer to all forms of flexible planar reinforcement, which are used in the form of fabrics, meshes, grids, strips, membranes, etc, to reinforce slopes. This includes both synthetic (polymer) and metal (e.g. steel strip) reinforcement.

NOTE: make sure that you have read the Overview of Support Implementation topic, for important information which applies to all support models in Slide.

Strip Coverage

When a GeoTextile is used to reinforce a slope, the material is placed in strips of a certain width. The Strip Coverage refers to the spacing of these strips, in the Out of Plane direction (i.e. along the slope). If the strips are laid continuously beside each other, with no gaps between adjacent strips, then the Strip Coverage = 100%. If the strips are not laid continuously (i.e. there are spaces between adjacent strips), then the Strip Coverage will be less than 100%. For example, if 4 meter wide strips were laid with a 2 meter spacing between each strip, then the Strip Coverage would equal 67% (i.e. 4 / (4 + 2))

image\stripcover1.gif

Strip coverage = (a / (a + b) ) x 100 

Plan view of geotextile placement

Tensile Strength

The Tensile Strength refers to the Tensile Strength (maximum load capacity, in force units) per meter width of strip. This is also referred to as the Tear Strength, when dealing with geo-fabrics for example.

Force Application

For GeoTextile support, the default method of Force Application = Passive, since there is normally no significant initial loading or tensioning of the support (although light pre-tensioning is sometimes applied, to optimize the effectiveness of the support). See the Force Application topic for a discussion of the significance of Active and Passive support force application in Slide.

Force Orientation

Because of the flexible nature of a GeoTextile type of reinforcement, the orientation of the force which is applied to the slip surface, is NOT always assumed to be parallel to the reinforcement. When the support begins to take on load, due to displacements within the slope, the direction of the applied support force can be assumed to be:

image\support_forceorientation2.gif

Applied Force Orientation options for GeoTextile support

Anchorage

When GeoTextile support is used, the support strips are normally anchored to the slope face, using some type of anchoring system. In addition, the embedded end of the GeoTextile, may be anchored within the slope. The Anchorage option allows the user to select whether none, both or either ends of the GeoTextile are anchored (i.e. considered fixed and immovable).

The significance of the Anchorage option is as follows:

The potential failure modes of a GeoTextile are very important, because they are used to determine the Force Diagram for the GeoTextile, which in turn determines the magnitude of the applied support force at any point along the GeoTextile. This is described later in this topic.

NOTE: for a GeoTextile, an Anchorage Load Capacity is NOT specified, because it is assumed that the Anchorage Capacity is always greater than or equal to the Tensile Strength of the GeoTextile. Therefore the Anchorage Capacity is not considered in the overall load capacity of the GeoTextile, and does not affect the Force Diagram for the GeoTextile.

Pullout Strength

The parameters entered here (Adhesion and Friction Angle), will govern the pullout and / or stripping force, which can be generated by the support, if the Anchorage = None, Slope Face or Embedded End (see above). If Anchorage = Both Ends, then the Pullout Strength parameters will be disabled, since by definition, pullout or stripping cannot occur if both ends are anchored.

NOTE:

Adhesion

NOTE the units of Adhesion are Force / Area (i.e. the shear force which can be generated per unit area of the GeoTextile / Soil interface)

Friction Angle

Material Dependent Pullout Strength

The Material Dependent checkbox, allows the user to define the pullout strength of the GeoTextile, according to the material(s) which the support passes through. In order to use this option:

  1. Select the Material Dependent checkbox, and select the Define button.

  2. You will see the Pullout Strength dialog. In this dialog, select the Add Material button, for each material that you wish to define the pullout strength for.

  3. For each material, use the mouse to select a Material Name, and enter the Adhesion and Friction Angle for the material. (NOTE: the Material Names correspond to the materials you have defined in the Define Material Properties dialog).

  4. Any material which you have NOT included in the Pullout Strength dialog, will use the Pullout Strength parameters which you have entered in the Define Support Properties dialog.

  5. When you are finished customizing the Pullout Strength parameters for all the required materials, select OK in the Pullout Strength dialog.

Shear Strength Model – Linear

If the Shear Strength Model = Linear, this means that the Shear Strength (i.e. Pullout Strength) of the support is determined from the Mohr-Coulomb relationship for the strength of an interface:

          Eqn. 1

where = interface adhesion, = interface friction angle, entered as the Pullout Strength parameters.

image\linear_mc.gif

Linear Mohr-Coulomb shear strength envelope

For analyses involving pore pressure, the normal stress takes into account the pore pressure. Therefore the Adhesion and Friction Angle can be considered effective stress parameters.

See the additional important note below, regarding the interpretation of the pullout strength parameters, and the normal stress, for a GeoTextile.

NOTE: if the Anchorage = Both Ends (see above), then the Shear Strength Model option will be disabled, since Pullout Strength will not be applicable, and the only possible failure mode will be Tensile.

Shear Strength Model – Hyperbolic

When the Shear Strength Model = Hyperbolic, the Shear Strength (i.e. Pullout Strength) of the support, will be determined from the following equation:

          Eqn. 2

This equation was derived from a study of the constitutive behaviour of geosynthetic interfaces [ Esterhuizen, Filz & Duncan (2001) ], and has been found to characterize the shear strength of soil / geo-synthetic interfaces, and other types of interfaces. It is important to note the definition of the parameters and in Equation 2. The following figure illustrates a hyperbolic shear strength envelope.

image\hyperbolic2.gif

Hyperbolic shear strength envelope

Adhesion is defined as the shear strength at .

Friction angle is defined as the friction angle at .

NOTE that the definition of Adhesion and Friction Angle for the Hyperbolic shear strength model, are different from those which define a linear, Mohr-Coulomb strength envelope. The Adhesion for a Hyperbolic shear strength envelope, is actually the limiting, maximum shear strength, for high normal stress.

If the Strength Model = Hyperbolic, the Adhesion and Friction Angle which you enter as the Pullout Strength parameters, must correspond to these definitions.

For analyses involving pore pressure, the normal stress takes into account the pore pressure. Therefore the Adhesion and Friction Angle can be considered effective stress parameters.

See the additional important note below, regarding the interpretation of the pullout strength parameters, and the normal stress, for a GeoTextile.

Interpretation of GeoTextile Shear Strength Parameters

In order to correctly interpret the definition of the Adhesion and Friction angle parameters, used to define the Pullout Strength of a GeoTextile in Slide, it is important to understand the following:

For example, consider a unit width of GeoTextile, with Length = L and Pullout Force = P.

image\support_geo3b.gif

Pullout force and shear strength of a GeoTextile, Strength Model = Linear (Mohr-Coulomb)

The relationship between the pullout FORCE (P), and the SHEAR STRENGTH (), is therefore:

(or )

When the pullout and stripping forces are calculated for a GeoTextile in Slide, a factor of 2 is included in the force calculation, in order to account for the shear strength of BOTH sides of the GeoTextile (see below for equations). NOTE that this applies to both Linear or Hyperbolic shear strength models in Slide.

This interpretation of GeoTextile pullout strength is applicable to geo-textiles, geo-fabrics, metal strips, etc., which have a 100% contact area with the soil (i.e. there are no "holes" in the material, and the GeoTextile / Soil interface is continuous).

When dealing with Geo-Grids and similar products, which have a significant area of soil-soil contact, through the holes in the Geo-Grid, this relationship is a simplification of the true failure mode.

However, for Geo-grids, the "apparent" interface shear strength parameters (Adhesion and Friction Angle) can be calculated, and these are sufficient for the purposes of modeling in Slide.

Normal Stress

In general, the normal stress, which is used in the shear strength equations (Eqn.1 or Eqn.2) will vary along the length of a GeoTextile. The normal stress is determined as follows:

Further Reading

Further information regarding GeoTextile uses and properties, can be found in Jewell (1996) . In particular, Chapter 4 of this reference provides insight into the behaviour of GeoTextiles and GeoGrids in reinforced slopes, and the significance of the "bond coefficients", which are commonly used to describe the shear strength of the GeoTextile / Soil interface.

Implementation of GeoTextile Support in Slide

The GeoTextile properties which are entered in the Define Support Properties dialog, are used to determine a Force Diagram for a GeoTextile, as described below.

Consider a GeoTextile which intersects a slip surface as shown below.

image\support_geo.gif

Li  = length of geotextile within sliding mass

Lo  =  length of geotextile embedded beyond slip surface

Geotextile parameters

A  =  Strip Coverage (%)

 =  Interface Friction Angle (deg)

a  =  Interface Adhesion (force / area)

T  =  Tensile Strength (force / width)

For GeoTextile support in Slide, three failure modes are possible, depending on the setting of the Anchorage option.

Pullout

For a geotextile, pullout can only occur if the embedded end is NOT anchored (i.e. Anchorage = Slope Face or Anchorage = None). The maximum pullout force which can be mobilized, PER UNIT WIDTH OF SLOPE, is given by:

         Eqn. 3

where the shear strength is given by Eqn.1 if the Strength Model = Linear (Mohr-Coulomb), or Eqn.2 if the Strength Model = Hyperbolic.

NOTE: the factor of "2" in the Pullout force equation (Eqn. 3) is used to account for the shear strength of the GeoTextile / Soil interface, on both sides of the GeoTextile. This is discussed above.

Material Dependent Shear Strength

If the GeoTextile Pullout Strength is specified as Material Dependent, then the Pullout Force is determined by integrating along the length of the GeoTextile, to determine the force contributed by each segment of the GeoTextile which passes through different materials. In this case, Equation 3 will use the appropriate Adhesion and Friction Angle for each material.

Tensile Failure

Tensile Failure of a geotextile is ALWAYS a possible failure mode, regardless of the setting of the Anchorage option. At any point along the geotextile, the maximum tensile load, PER UNIT WIDTH OF SLOPE, which can be mobilized by the geotextile, is simply given by:

           Eqn. 4

NOTE that if Anchorage = Both Ends, then Tensile Failure will be the ONLY failure mode possible for the geotextile.

Stripping

Stripping is the term used in Slide, when support failure occurs by the sliding mass "stripping" off of the support (i.e. the support remains embedded in the slope). NOTE that for geotextile support in Slide, stripping is only possible if Anchorage = None or Anchorage = Embedded End only (i.e. the geotextile is NOT anchored to the slope face). In most cases, geotextiles are anchored to the Slope Face, therefore stripping should not normally occur with a geotextile. However, if this is a possible failure mode for a geotextile, then the force required for the sliding mass to "strip" off of the support, is given by:

          Eqn. 5

where the shear strength is given by Eqn.1 if the Strength Model = Linear (Mohr-Coulomb), or Eqn.2 if the Strength Model = Hyperbolic.

Force Diagram

At any point along the length of a geotextile, the magnitude of the force which will be applied to a slip surface which intersects the GeoTextile, is given by the minimum of the Tensile, Pullout (if applicable), and Stripping (if applicable) forces. This depends on the setting of the Anchorage option, as follows:

Anchorage = None  Applied Force = min (F1, F2, F3)

Anchorage = Slope Face  Applied Force = min (F1, F2)

Anchorage = Embedded End Applied Force = min (F2, F3)

Anchorage = Both Ends  Applied Force = F2

In most cases, a GeoTextile will be anchored to the slope face, but not anchored within the slope (i.e. Anchorage = Slope Face). In this case, the Force Diagram for a GeoTextile will resemble the following figure, in which the only failure modes are Tensile and Pullout.

image\support_geo2.gif

Force diagram for GeoTextile support (Anchorage = Slope Face).

References

  1. Esterhuizen, J., Filz, G.M. and Duncan, J.M., Constitutive Behaviour of Geosynthetic Interfaces, Journal of GeoTechnical and Geoenvironmental Engineering, October 2001, pp 834-840.

  2. Forsman, J. and Slunga, E., The Interface Friction and Anchor Capacity of Synthetic Georeinforcements, Fifth International Conference on Geotextiles, Geomembranes and Related Products, Singapore, September 1994.

  3. Jewell, R.A., Soil Reinforcement With Geotextiles, Special Publication 123, Construction Industry Research and Information Association, London, 1996.