Narejo et al (1996) have proposed that the mass/unit area determines the cushioning efficiency of a nonwoven geotextile, while Jones et al (2000) propose CBR is the most appropriate parameter to use. The simple demonstration that follows shows that mass is relevant in geotextiles of a given type and structure, but that structural differences have a more significant effect on fundamental cushioning efficiency. In fact, lighter materials with a more rigid structure may produce more cushioning than heavier materials of a "looser" structure. Consider the fibers of a geotextile to be equivalent to a pile of loose scaffolding rods – the ability of a drainage stone to penetrate the loose rods is simply a function of their mass. Now take the same rods over the same area and assemble them into a rigid framework structure – this rigid structure (of the same mass) will provide far more protection against the penetrating stone.

Figure 1 shows a series of model geotextile "fibers" (construction toy units) and a large drainage "stone."  Figure 2 shows a loose layer of these "fibers" simulating a geotextile, and Figure 3 shows how the stone settles into and displaces some of the fibers.

Figure 1. "Fibers" and stone.

   

Figure 2. "Geotextile" (top left), stone (top right), and displaced "fibers" (bottom left).

Figure 3 shows similar tests with loosely assembled fibers. Note the essentially consistent height of the stone above the dish with respect to the background. All stone photographs were taken from the same camera location.

Figure 3. Several loose fiber tests; initial geotextile on left, settled stone on right.

When a simulated geogrid structure is placed on top of the loose fibers (Figure 4) or in the mid-plane of the loose fibers (Figure 5), the stone is held higher off the protected surface – the geomembrane.

Figure 4. "Geogrid" on top of loose fibers.

Figure 5. Geogrid in mid-plane of loose fibers.

A three dimensional structure of fibers supports the stone even further above the "geomembrane," as shown in Figure 6. Even without the loose fibers, the much lighter weight of the structured fibers provides the same protection, as shown in Figure 7. So, mass is not the significant parameter.

Figure 6. Structured fibers within geotextile provide increased protection.

Figure 7. Structured fibers only (lower mass) provide the same increased protection.

An even lower mass, oriented structure (Figure 8) will provide the same degree of protection as shown in Figures 6 and 7 as does the oriented structure of the "normal" mass in Figure 9.  In the latter case, the loose fibers may help stabilize the oriented structure to make it more effective.

Figure 8. Low mass oriented structure provides good protection.

Figure 9. Normal mass oriented structure provides same protection as Figure 8.

Therefore, it is apparent that the structure of the geotextile is the primary determinant of geomembrane puncture protection.  However, for any given structure, mass per unit area is a secondary determinant.

"Structure" can be obtained from changing needling density, fiber moduli, fiber material, melt bonding fiber contacts, thermally fusing surfaces, incorporating geogrids, lamination, etc.

In the USA, adequate puncture protection is considered to be provided by geotextiles with a mass between 12 and 24 osy (400 and 800 g/m²).   In Germany, geotextiles with masses exceeding 2500 g/m² were initially thought necessary, but these have now been replaced with a composite of over 1200 g/m² geotextile and a sand blanket layer. It appears that adequate protection could be given by intermediate mass structured products.

One final example. Figure 10 shows the drape of two geotextiles with the same mass per unit area – won’t the stiffer one provide more puncture protection?

Figure 10. Drape of geotextiles with same mass per unit area.

The notion that CBR is a method of evaluating the puncture protection capabilities of geotextiles will be confirmed in a paper by different authors to be presented at the 7ICG conference in Nice.

References.

Jones, D.R.V., Shercliff, D.A., & Dixon, N., 2000, "Difficulties Associated with the Specification of Protection Geotextiles Using Only Unit Weight,”  Proc. 2^nd European Geosynthetics Conference, Patron Editore, Bologna, pp 551-555.

Narejo, D., Koerner, R.M., & Wilson-Fahmy, R.F., 1996, "Puncture Protection of Geomembranes Part II: Experimental," Geosynthetics International, Vol. 3, No. 5, IFAI, pp 629-653.