CBR Strength (Puncture) of Geosynthetics

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Courtesy of ASTM International

While there exist many types of mechanical tests for use on geosynthetics, most seem to be more suitable for one type of system or another. This paper focuses on a test which may have widespread applicability to assess the strength of all types of geosynthetics. It is the forcing of a 50-mm (2.0-in.) diameter plunger through a horizontally fixed geosynthetic of 150-mm (6.0-in.) diameter. Such dimensions are common to the California bearing ratio (CBR) test apparatus used in assessing soil strength for road subgrades. In actuality, the test is not a puncture test, but is an axisymmetric strength test and should be considered as such. The unsupported geosynthetic is indeed in a state of tension. The stress conditions are well defined, which allow for stress and strain to be calculated and compared to the actual value as obtained in a plane strain (wide width) tension test.

This approach has been followed in this study wherein a wide range of geosynthetics (geotextiles, geomembranes, geonets, and geocomposites) have been tested for CBR (puncture) strength and then calculations performed to determine stress and strain. These calculated values were then compared to those obtained in wide width tests. Results are interesting in that the average values of percent variation between wide width tensile strength and CBR calculated tensile strength for those geosynthetics tested are as follows: (1) geotextiles ±12%, (2) geomembranes ±33%, (3) geonets ±17%, and (4) geocomposites ±10%. It is felt that this type of test (which is basically an index test) is helpful in a number of situations, not the least of which is in assessing strength changes before and after exposure of various geosynthetic materials to chemical or biological incubation.

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