Interface Friction

Geomembrane Interface Friction

Geomembrane interface friction is a key design factor in backfilled liner applications. This Tech Note discusses the interface friction performance of geomembranes.

Cover material over a geomembrane liner system can provide protection against ultraviolet degradation, oxidation, mechanical damage, wind uplift, and temperature fluctuations. The cover material normally used is a thin layer of soil which tends to slide down slope due to gravity if not carefully designed. The purpose of this Tech Note is to provide guidance on predicting soil / geomembrane interface stability. Since every project site has a different set of conditions, this Tech Note should not be used in the final design of a gemembrane system. In critical applications, proper laboratory testing must be carried out along with detailed analysis to ensure all stability issues have been properly addressed.

Importance of the Critical Interface

Often in geomembrane systems there is more than one interface that needs to be considered. For example, if a geotextile is used to cover a geomembrane prior to placing backfill then the designer must consider not only the soil / geomembrane interface, but also the geomembrane / geotextile and geotextile / soil veneer interfaces for stability. It is important to recognize the critical interface when examining slope stability in this context.

Stability Issues

The following is a list of the most common considerations facing the designer when addressing the stability of a soil veneer over a geomembrane liner system:
- drainage conditions
- cover geometry and anchorage
- geomembrane material
- angle of soil / geomembrane interface

Drainage Conditions

Once a liquid is introduced to the soil / geomembrane interface, stability of the soil veneer can be affected significantly. Proper drainage of the soil cover should be considered to ensure maximum stability. Drainage of the soil veneer may be accomplished through the use of a geotextile, a sand layer, or a geonet at the critical interface. Three critical items necessary to consider when examining a drainage layer include (but are not limited to): flow rate (or transmissivity), normal stress over the drainage layer, and the hydraulic gradient along the drainage layer. It is also important to note that if the drainage layer is in intimate contact with the geomembrane, this interface must be examined along with the other interfaces critical to slope stability. A collection system at the base of the slope should also be incorporated into the drainage design, for example, a perforated subdrain at the toe of the slope could act as a header to collect the runoff from the drainage system and safely drain the collected water away from the slope.

Cover Geometry and Anchorage

The geometry of the cover and anchorage will have a significant impact on slope stability.

The anchor trench at the top of the slope should be designed to hold the geosynthetic layer (or layers) to a point where it’s strength is fully mobilized (ie: at its tensile strength at yield, at supporting scrim break, or at an allowable strain). Please refer to the Layfield Design Guide for common geosynthetic anchor trench geometries.

The geometry of the soil cover over the geomembrane can also greatly influence stability. The designer may choose geometries such as a benched slope, or a wedge to increase slope stability. In some cases, such as a landfill application, a buttress at the toe of the slope may add to the stability of the soil cover over the geomembrane.

Cover Soil Type

Examination of the table below will show that different soils have varying effects on interface stability with geomembranes. Particle size, moisture content, and transmissivity are three important characteristics to consider when examining the soil to be used against a geomembrane. Also, any angular stone, sticks, or other deleterious material in the soil may result in damage to the geomembrane. Please refer to the Layfield Tech Note on Liner Backfill for more information on suitable materials to be used as backfill.

The designer may wish to consider the use of a reinforcement geosynthetic such as a geogrid, or woven geotextile to provide additional strength in the soil veneer over a geomembrane.

Geomembrane Material

As illustrated in the table below, softer geomembranes (such as PVC) tend to have higher friction angles than do harder, materials (such as HDPE). Texturing of the geomembrane surface (available on some materials) tends to provide a higher friction angle with soil covers but not necessarily with nonwoven geotextiles. The softer, more flexible geomembrane materials tend to have higher friction angles for the following reasons:
- the softer material tends to allow the soil particles of the cover soil to embed themselves within the surface of the geomembrane.
- a more flexible material tends to allow for more intimate contact between the geomembrane and the under and overlying soil

Angle of Soil / Geomembrane Interface

Generally, a slope of 4:1 or 14 degrees is the maximum slope that will maintain stability of a soil veneer over most geomembranes. Note that site conditions will dictate stability in all circumstances. There are many technical papers detailing laboratory results of interface friction angles between geomembranes and various other surfaces available. The table below illustrates the interface friction of PVC and HDPE geomembranes with various soil types and a nonwoven geotextile.

The designer should note that geosynthetic properties are often proprietary and subject to change. Geosynthetic materials generally must maintain a certain minimum specification however characteristics such as texturing can vary greatly between manufacturers.

Simplified Design Example

Problem: What type of soil is required to cover a 30mil smooth PVC geomembrane on a 3:1 slope using a factor of safety of 1.5?

Solution: Tan-1 (1/3) =18.4 deg. Going to the design curves (chart on previous page) allows us to interpolate the FS = 1.5 curve to yield a required friction angle of 28 deg. From the table below we find that a 30mil smooth PVC geomembrane with a backfill of fine sand would be suitable in this application.

Laboratory Testing

Geomembrane friction angles are so sensitive to site-specific variables that laboratory testing is always recommended in critical stability calculations. It is important that the designer specifies his or her requirements for testing clearly to the lab prior to commencing with the tests.

Some site-specific critical testing parameters are as follows:
- geomembrane material type
- type and gradation of the soil
- soil density and moisture content
- moisture condition of the soil during the test
- normal stress to be applied
- strain rate to be applied during shear
- total strain to be evaluated during the test

Available Test Methods

There are five test methods available to the designer, they include: Direct Shear Box (ASTM D3080), Large Scale Shear Box (ASTM D5321), Tilt Table (no ASTM standard), Torsional Ring Shear Device (no ASTM standard), Double Interface Shear Device (no ASTM standard). The two most common are the ASTM D3080 and ASTM D5321.

The decision on which test should be used is left to the design engineer considering site specific conditions. The Direct Shear Box test method has the advantages of being inexpensive, simple, and has a large experience base. The Large Scale Direct Shear Box (300mm x 300mm box size) test method allows for larger displacements of the interface being tested which helps to minimize boundary effects experienced in the direct shear which has a smaller box size (100mm x 100mm). Disadvantages of this test include the fact that the sample changes in area as it is being tested, a correction factor should be applied, also this test is more expensive that the direct shear.

Your Obligation to Test

It is the responsibility of the owner, or owner’s representative to determine the suitability of the geomembrane in the environment in which it will be used.

Layfield recommends that in all applications where friction angle will be an important part of the design that a test be performed. This test should include the actual subgrade and backfill materials planned for use at the site, and the actual geosynthetics proposed for the project. Layfield can assist the designer by providing geosynthetic samples for testing and can recommend suitable testing labs in your area. A typical direct shear box test test is usually under $1,000. In relation to the cost of failure an interface friction test is invaluable.

In cases where there are multiple layers of geosynthetics it is very difficult to predict where the failure plane will occur. Layfield strongly recommends that a friction angle test be performed on all multiple layer geosynthetics projects.

As noted previously, the purpose of this Tech Note is to provide guidance on predicting soil / geomembrane interface stability. Since every project site has a different set of conditions, this Tech Note should not be used in the final design of a gemembrane system. In critical applications, proper laboratory testing must be carried out along with detailed analysis to ensure all stability issues have been properly addressed.

Additional manufacturer’s test data and design assistance are available from Layfield Plastics. If you have any additional questions on friction angles of geomembranes, please do not hesitate to call your local technical sales representative.

References

1.Designing with Geosynthetics 3rd Edition, Dr. R.M. Koerner, Ph.D.,P.E. 1994
2. PGI Technical Bulletin, March, 1997
3. Interface Friction/Direct Shear Testing & Slope Stability Issues Short Course Notes, Dr. Gilbert & Sam Allan, April 27, 1999, Geosynthetics 1999 Conference, Boston, MA.

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