Geotextile selection

Careful selection of a geotextile filter is required just as the stone size and number of layers were important for the traditional solution.

The textile must be sufficiently robust to withstand installation and service-life loads. It must have a suitable pore size and be capable of providing sustained permeability and filtration. It must be capable of passing water faster than the underlying soil. It must also possess the necessary extensibility to conform intimately with the profile of the rocks above it and to adapt to point loads in order to avoid puncture and tearing. Not all geotextiles offer this unique combination of properties. 

Filter properties
The quantity of soil that is able to migrate through a geotextile filter is dependent on: 

• the size, number and uniformity of the geotextile’s pores in relation to the soil particle sizes
• the magnitude of the forces (the higher the forces - the greater potential for soil migration)
• the structure and inter-particle bonding of the in-situ soil particles - the greater the soil density and the greater the interparticle forces, the less potential there is for the soil to migrate

For a geotextile to function effectively as a filter, it is essential that a condition of equilibrium is established close to the soil/geotextile interface as soon as possible following installation. Fine soil particles in the immediate vicinity of the geotextile migrate under the influence of seepage until a natural filter becomes established. This takes between one and four months. If soil particles are able to continually pass through the geotextile, the underlying foundation would eventually become unstable. 

The performance of a geotextile filter is dependent on a number of principal factors:

• the particle size distribution, structure and pH of the soil
• the characteristic pore size of the geotextile
• the permeability of the geotextile in relation to the soil

The universal approach to filtration using a geotextile is that the coefficient of permeability of the geotextile used should be 5 times greater than that of the underlying soil:

kg > 5-10ks
where: kg = coefficient of permeability for the geotextile
ks = coefficient of permeability for the soil

and:

O₉₀ < D₈₅
where: O₉₀ = apparent opening size for the geotextile
D₈₅ = particle size fraction for the soil

As with stone filters, it follows that the permeability of the geotextile must be greater than the underlying soil. The extent to which the geotextile’s permeability should exceed that of the soil will vary depending on site criteria. The relationship between the apparent opening size of geotextile (090) and the particle size of the underlying soil is also important and varies depending on the underlying soil e.g. whether or not it is cohesive. 

Table A shows the effect on permeability of increasing a soils proportion of fines and Table A shows some typical values for the hydraulic conductivity of different soil types.

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