BS8661 Geotextiles: Guidance for specification for basic separation and filtration functions

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Article provided by Ian Fraser, Chair – BSI Committee B553: Geosynthetics. Note: the views expressed above are those of the author and not BSI

BSI has recently published an important new British Standard BS 8661 – Geotextiles: Guidance for specification for basic separation and filtration functions. As the title implies, this document gives designers and specifiers some simple guidance on the selection of a suitable grade of separation and filtration geotextile for their next project. The standard focuses on the lighter and most basic end of the market and recognizes that more stringent design and specification considerations will be required for more onerous applications. The types of geotextiles addressed would generally be utilised, for example, as separation layers under roads, pathways, car parks and other trafficked areas. Although these might be considered the most straightforward applications it is still easy to make simple mistakes that may compromise the integrity of the construction in the future.  Furthermore it should be noted that, by volume, these are easily the most prevalent applications of separation geotextiles and therefore responsible for the greatest share of geotextiles used.

This type of standard has been long overdue in the UK, which has seen a gradual decline in the level and quality of geotextiles used in basic separation and filtration over the last 10 to 20 years.  This decline has taken us to a point where the UK’s standards are lagging behind many other developed countries like the US, Germany, South Africa and the Scandinavian states. The irony is that much of the early development of geotextile manufacture and their application was undertaken in the UK and it could be argued that the use of geotextiles as separation layers was born here.  The decline in standards is mostly due to a fairly poor level of understanding and therefore specification but – before steam starts shooting from your ears – I am not blaming specifiers and designers for that so please read on.

The main practical area of concern expressed by geosynthetics experts is the ability of some materials to withstand installation stresses and continue to function as intended.  I am sure you are reading that last sentence and thinking ‘that’s obvious, of course they must be robust enough to survive installation’. Obvious or not, it is a factor that is all too frequently ignored when specifying and selecting geotextiles in the UK and it is no accident that this is a key issue addressed by most other international separation geotextile classification systems.  The sorts of laboratory tests that we undertake on geotextiles to obtain the important values for parameters that you see on datasheets, like their effective opening size and permeability, for example, are extremely useful for determining a geotextile’s likely performance on site.  However these tests are undertaken on pristine samples in a laboratory, and to be confident that these materials will perform satisfactorily in a real application we must be reassured that the installation stresses will not significantly change their performance.  In simple terms, if the placement and compaction of aggregate over the geotextile is going to tear a hole in the material then clearly the characteristic opening size given on the data sheet will be somewhat academic and its ability to separate is clearly going to be significantly impaired.  Don’t despair because the solution is very simple.  We just need to specify geotextiles that are robust enough, for a range of given conditions, to not be significantly damaged by installation or subsequent application. We can therefore be confident that they will retain their performance characteristics when applied on site.

Whilst the solution is obvious the reality is that specifiers in the UK have not, until now, been provided with a helpful tool to allow them to make an informed choice about the grade of geotextile they need to employ for their given project conditions.  In particular little guidance has been provided on robustness and what we like to call ‘construction or installation survivability’. Without that tool a reasonably detailed level of geosynthetic expertise is required in order to make a choice and being somewhat of a niche area, that expertise is thin on the ground. In reality very few civil engineering undergraduate courses contain much, if any, information on geosynthetics and post graduate geotechnical courses don’t fare much better. To my point above, the lack of guidance and a suitable tool is not the fault of specifiers and designers. Given the current educational environment outlined above it is therefore incumbent upon the geosynthetics industry and its associated experts to help designers by plugging that gap by providing some simple guidance to help them navigate towards the correct decisions. BS 8661 sets out to do just that and provide that tool.

The standard offers a choice of 3 basic grades of geotextile specification profile.  Guidance on the selection of the correct profile for your project is provided based on:

  • subsoil conditions – shear strength/CBR
  • construction conditions – compaction plant/site traffic/fill grain size/layer thickness
  • importance of the filtration function in the proposed application.

As a side note, the functions of separation and filtration often work hand in hand in many UK geotextile applications, mostly because the presence and passage of water is a key part of everyday geotechnical design in this country.  However, there may be instances where filtration is not a concern, for example where you are separating coarse materials or where no significant water passage is anticipated.

The required levels of the parameters quoted in the specification profiles in the standard are defined using current EN test methods (as per CE and UKCA Marking) and include consideration of manufacturing tolerances. These parameters are therefore those which are already tested and quoted by all European and most international manufacturers so are easy to obtain.  As indicated above, designers who are creating a specification for a project can simply run through the selection process and then ask in their specification for a geotextile to comply with, for example, BS8661 Specification Profile 2.  Of course designers can also use the standard to check that a geotextile which has perhaps been offered on a project by a contractor is suitable for the site and construction conditions.

It is encouraging to note that the development of BS8661 has been supported by National Highways, Network Rail and the Environment Agency.

I am sure that designers and other end users will find this to be a valuable tool that they can rely on to help them make more informed decisions and protect the integrity of their works. As with all British Standards, the document will be reviewed regularly so all and any suggestions for improvements will be gratefully received.  BS8661 is available now and if you are a designer I encourage you to obtain a copy and utilise it in your next project. If you are a geotextile manufacturer or supplier I would suggest that you check your product range against the specification profiles to be sure that you comply.