A functional approach to geotextile specifications

Design by function

Specification of geotextiles has been made a little easier with the advent of Australian Standard test methods but it must be emphasised that these are merely test methods and are not by any means standard specifications for geotextiles.

Because geotextiles are man made materials they can be manufactured or ordered with characteristics to suit each application and on large projects custom geotextiles are a definite possibility. Adherence to principles developed for natural materials may tend to build in features or performance parameters that are unnecessary. A classic case occurs when substituting a granular drainage layer with a geotextile based layer. The granular layer will often have a thickness which is a practical minimum for construction and often considerably more than is needed for the drainage function. In moving to a geotextile it may be prudent to look at the functional drainage requirement from first principles rather than requiring an equivalent to the granular ‘standard’.

This sort of situation has lead to endorsement of the geotextile community of the ‘Design by Function’ principle where the geotextile is looked at in light of each of its functional requirements to evaluate the minimums required for each function.

This contrasts with a practice of selecting a particular product by the ‘what we used last time’ method and then quoting all of the Australian Standard test results for that particular product. It may suit that particular manufacturer but this method does not encourage competition” and the development of competitive alternates.

It may be useful to take a general look at the major categories of geotextile use and the parameters which are paramount to each when following a ‘Design by Function’ approach. The major functions of geotextiles are not mutually exclusive and often geotextiles are used to provide several different functional purposes. In these cases the parameters for each function need to be looked at to establish an across the board window of acceptability.

Separation is the term applied in roadworks and possibly other shallow foundation construction over poor subgrades. It involves using a geotextile to separate a good quality relatively high shear strength base course material from the usually wet subgrade with low shear strength. The separation action prevents mixing or contamination of the good material allowing it to develop its potential to support traffic or other loadings.

There is an element of truth in the argument that claims that this separation function could be effectively performed by soggy newspaper if only we could install it without damage. In practice we tend to look to separation fabric to provide other functions such as drainage and filtration as well.

Survival of the installation phase is a key element in the success of separation geotextiles and there has evolved two basic approaches of equal legitimacy:

• Survival by strength to resist the forces that would cause damage. This approach is typical of the relatively high strength/low elongation type of fabric which in Australia tends to be tape woven material. The important parameters for this approach are wide strip tensile strength and CBR burst strength with corresponding elongation results being of lesser importance.
• Survival by elongation to absorb the forces that would cause damage. This approach is typical of the lower strength/higher elongation non-woven type fabrics. A mix of strength and elongation capacity is required and the important parameters for this approach are wide strip tensile strength and elongation, CBR burst strength and elongation and drop cone puncture height.

Individual manufacturers have developed design guidelines to help select from their range of geotextiles. This information is usually based on the manufacturer’s research and field experience and can give a useful input to the selection process.

Specification by unit mass or other basic parameters is extremely dangerous unless the precise nature of the manufacturing process is also specified.

Filtration

The essential elements of drainage (one way) filtration design have been discussed by many others and it basically a matter of matching the geotextile nominal opening size (expressed as O₉₀ or O₉₅) to the soil particle size distribution such that enough of the larger soil particles will be retained to enable the formation of a ‘filter cake’ to retain the rest whilst maintaining the required permeability. If the opening size is too large insufficient particles are retained usually resulting in clogging downstream in the system and if the opening size is too small clogging will occur at the face of the geotextile.

It is often preferable to err to the too large side of this balance as provision can be usually be made for occasional flushing.

Filtration (two-way) under armour systems for revetments involves similar considerations but because the water movement is in two directions some degree of constant surface flushing may be expected to occur which may impede the development ofth~ ‘filter cake’.

Standard testing is carried out on uncontaminated materials and specification should therefore be based on a range of acceptable opening sizes in conjunction with acceptable levels of permeability prior to contamination.

Direct filtration testing can be readily carried out on candidate fabrics with the soils in question if there is any doubt about suitability and should be used on all work where the geotextile filter function is at all critical.

The filtration performance is only a part of the story as installation survival is equally critical. The comments regarding survival of separation fabrics are applicable with CBR (strength and elongation) and Drop Cone tests being paramount. Some of the geotextile manufacturers have developed criteria based on relationships between field tests with falling rocks and Drop Cone Tests. These are useful but must be approached with caution as European Drop Cone tests often use water or a similar substrate under the fabric whereas the Australian Standard is a ‘dry’ test with significantly different results.

Survival requirements may be reduced by adjustment to construction procedures such as placing a layer of finer material next to the geotextile or careful placement of the materials in order to avoid damage.

Reinforcement

There are two major considerations to geotextile reinforcement which are related to the need to generate sufficient reinforcing forces in the geotextile before the structure undergoes excessive deflection that may mitigate its usefulness.

The first is to provide adequate tensile strength at a small enough elongation or strain for it to be developed without excessive deflection of the structure. This can be defined by wide strip tensile test results for strength at appropriate elongations as well as the corresponding modulus.

The second is to provide adequate anchorage so that the tensile forces required can be developed. There are pull-out and other test methods available to define this but such a test method is not included in the Australian Standard Methods so reference must be made to ASTM or other overseas standards.

Beyond relatively simple or short term structures based on tensile force considerations with large safety factors there is a need to consider the long term behaviour of the geotextile including such considerations as creep and lateral soil restraint which can reduce or improve the effectiveness of the geotextile reinforcement.

Drainage

Many applications require the geotextile to provide a drainage path to allow escape of water. Occasionally this is a primary function but it is often a secondary function to a separation or reinforcement requirement.

The capacity of a geotextile to transmit water in its own plane is highly dependent on the normal or restraining pressure which tends to compress the material and diminish the flow capacity.

There are test methods available to directly measure the transmissivity of geotextiles and related products under various confining forces. The transmissivity is expressed in m²/sec per metre width of material and is determined in a longitudinal test arrangement (such as ASTM 4716 – 87) or a radial test arrangement (such as favoured by RILEM). Test results from the two arrangements are not interchangeable. This author prefers the ASTM longitudinal method.

Gas transmissivity may also be a desired property in landfill related work and an evaluation of this property was carried out by Koerner in 1984 and concluded that the transmissivity of gas in geotextiles was about 100 times that of water. Hence specification for gas flow function can be based on an extrapolation of the transmissivity requirement for water.

Once again there is also the question of construction survivabilty which again brings into play the considerations discussed under separation.

Protection

There is a growing use of geotextiles to provide cushioning support to a geomembrane liner in order to provide a composite with better puncture resistance than could be achieved by the same thickness of liner without the geotextile cushion.

This use of geotextiles also provides the added multi-function advantage of water and gas transmissivity in the geotextile if used under the liner.

Tests have been carried out evaluate this function and standard test methods are in development (GRI, ASTM) and it is generally accepted that the volume of fibre in the geotextile (as indicated by the unit mass) is of paramount importance. There is less agreement on the impact of the uncompressed density of the fibre with one school arguing that thicker (bulkier) is better and another arguing that the fibres are ultimately compressed by soil cover or contained materials and that initial density or thickness is immaterial.

Summary

In summary it may be seen that there is a continuing need across the range of geotextile functions for survival characteristics which will tend to provide a basic theme for geotextile specifications. Individual specifications can then be tailored to the functional requirements of the particular geotextile application.

The attached chart gives an insight into the functional requirements of the main types of geotextile application.

Functions of Geotextiles in Geotechnical Uses

Functional Specifications

Installation Specifications