No. 5 of 19Geosynthetics in Separation

ByProf. Alan McGown

University of Strathclyde

The information presented in this document has been reviewed by the Education Committee of the International Geosynthetics Society and is believed to fairly represent the current state of practice.

However, the International Geosynthetics Society does not accept any liability arising in any way from use of the information presented.

SeparationLecture Outline

• What is separation?• Geosynthetic properties related to the

separation function• Separation applications• Factors affecting design• Design codes

What is Separation?

Definition of the Separation Function

• A Geosynthetic placed at the interface between two dissimilar geotechnical materials functions as a Separator when it prevents these materials from mixing under the action of applied loads

The Influence of Permeability • Although the Geosynthetic acts primarily

as a physical barrier to the migration of solid particles, it may or may not allow the flow of water or gas between the two geotechnical materials depending on its permeability.

• When it is intended that the Geosynthetic is highly impermeable to water or gas, it is termed a Geomembrane.

• When the Geosynthetic exhibits significant permeability to water or gas, it is termed a Separator.

Geosynthetic Properties Related to the Separation Function

Important Properties• To ensure that a Geosynthetic

successfully performs as a Separator, the following properties must be considered:– Tensile strength and flexibility– Puncture and tearing strengths– Soil tightness and water/gas permeability

Tensile Strength and Flexibility

• In many situations, Separators are required to deform under load and to accommodate large tensile strains. Further they may be required to conform to highly irregular surfaces. In both cases they must do so whilst remaining intact.

Puncture and Tearing Strengths

• Sharp objects or angular aggregates above or below a Geosynthetic are likely to press into it under construction or operational loading.

• The Geosynthetic must possess sufficient puncture resistance and tear strength to resist these actions and remain intact or at least prevent propagation of any puncture holes or tears that are produced.

Soil Tightness and Water/Gas Permeability

• Waterborne fine particles should not pass through a Separator, however, water or gas should be allowed to pass through without the build-up of excess pore pressures.

• These two requirements are essentially conflicting hence in any given situation a compromise must be made regarding the Geosynthetic soil tightness and water/gas permeability.

Separation ApplicationsTypical Application of Geosynthetics as Separators

• At sub-grade/sub-base interfaces in temporary and permanent roads

• Between railroad ballast and a foundation soil

• Between embankment fill and soft foundation soil

Separation Geotextile Being Laid and Covered

Factors Affecting Designs

Important Site Specific Factors:

• Sub-grade/foundation soil size and grading

• Sub-grade/foundation soil strength• Fill particle size and grading• Thickness of the fill layer• Construction and operational loading

Sub-grade/Foundation Soil Size and Grading

• Sub-grades and Foundation Soils which are gap-graded or have a very high fines content may be problematic in terms of a Geosynthetic providing a high degree of soil tightness yet retaining sufficient water/gas permeability.

• Soils with large cobbles and boulders may also present problems as they cause puncturing and tearing of the Geosynthetic, particularly if the soil particles are angular rather than rounded. However, this may be largely avoided by careful preparation of the sub-grade/foundation, including removal of large stones and any other sharp or angular objects such as tree roots.

Sub-grade/Foundation Soil Strength

• The lower the strength of the Sub-grade/Foundation Soil, in terms of undrained shear strength, California Bearing Ratio (CBR), or other strength parameters, the greater will be the requirements for tensile strength and flexibility from the Geosynthetic.

• General and local deformations and strains are likely to increase as the strength of the Sub-grade/ Foundation decreases.

Fill Particle Size and Grading

• The maximum particle size, the grading and shape of the materials placed on the Geosynthetic must be considered in the design of Separators.

• The potential for damage to the Geosynthetic and the effect of abrasion are linked to these Fill properties.

Thickness of the Fill Layer• The purpose of the Fill layer placed over the

Geosynthetic acting as a Separator, is to spread loads over a sufficiently wide area such that the applied stress on the Sub-grade/Foundation Soil is less than the ultimate bearing capacity.

• The closer the applied stress is to this value of limiting stress then the more demands there will be on the Geosynthetic. Thus the thickness of the initial layer of Fill is a critical factor in the design.

Construction and Operational Loading• These loads result from the deposition of

Fill onto the Geosynthetic acting as a Separator during construction and then during operation.

• Very often the construction conditions are the most critical and must be taken into account during design.

Design Codes

• A wide variety of International and National Design Codes exist for Separators.

• In addition many Geosynthetic manufacturers provide Design Recommendations for Separators.

• These should be used as appropriate, taking into account the points previously made in this Lecture.