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Technical paper: The notion of geotextiles as seperators in roads

By T S Ingold and P Crowcroft, Laing Design and Development Centre

This paper was first published in GE’s January 1984 issue

This is the second in a senes of articles on geotextiles and their applications. The first Geotextiles in
the UK
by D J Hoare, appeared in GE’s November 1983 issue



In certain uses a geotextile is often loosely referred to as a separator, a common example of this being the installation of a so-called separator at the sub-base/subgrade interface. This term may prove to be a misnomer since the state-of-the-art recognises four main functions for geotextiles, namely reinforcing, drainage, filtration and separation. The degree to which any of these functions is realised will depend on the nature of the soil-geotextile system and the loading regime. For example an extensible geotextile operating under dry conditions at the sub-base/subgrade interface of a lightly trafficked road constructed with competent sub-base over a competent granular subgrade would not be likely to invoke any of these functions.

Although a geotextile may be installed with the notion of it acting as a separator any resulting improvement may be due to any one, or any combination, of its four main functions. It is possible that the effectiveness of the pure separation function could be assessed by the use of a geotextile structured to suppress the other three functions. Such a material might be a strong, e> tensible smooth plastic membrane of low permeability. This would obviously not act as either filter or drain, and by virtue of its smoothness and extensibility would have little reinforcing effect. Installed beneath the granular fill forming a heavily trafficked haul road this material would, provided it did not rupture, separate the granular fill from the subgrade. However, it would almost certainly have a deleterious effect largely caused by slippage between granular fill and subgrade. This slip could be reduced by providing the membrane with a rough surface but this would then permit friction to develop between soil and membrane so giving it opportunity to generate the reinforcing mode. Even this would be no guarantee of enhanced performance since with a clay formation the subgrade/membrane shear stress developed could give rise to the development of high porewater pressures leading to premature failure. (Yabe & Kato, 1974; Ingold, 1980).

It is apparent that soil—geotextile interaction is a complex problem with the notion of a geotextile operating purely in the separator mode often being bogus. The complexity of this interaction is reflected in the many apparently conflicting reports of so-called separator performance.

The objective of this Paper is to briefly review a limited number of case histories of field trials and laboratory studies followed by a consideration of recent research with a view to differentiating between the effects attributable to separation and those allied with other geotextile functions. Although the separation mode might be manifested in static situations, such as the containment of drainage media in poor ground, or dynamic situations, for example marine structures or beneath railway ballast, there is a paucity of analytical and experimental data. Consequently consideration is limited to the more widely researched field of geotextiles in temporary and permanent pavements. 

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