Truncated Cone Testing

Hullings and Koerner prepared a research paper for the 1991 Geosynthetics Conference in Atlanta titled “ Puncture Resistance of Geomembranes Using a Truncated Cone Test” (Hullings & Koerner, 1991). The truncated cone test method attempts to simulate the puncture mechanism in a performance-oriented test. This type of test was originally developed by the U.S. Bureau of Reclamation.

The author’s concluded that the results of many standardized index puncture tests (such as ASTM D4833), which are commonly reported as puncture strength, do not completely describe the phenomena and may be misleading. In this research the low modulus, high elongation (more flexible materials) required taller cones to induce failure. Stiffer materials such as the semi crystalline HDPE, and the fabric reinforced CSPE sample, were easier to puncture in this field performance based test.

Based on this research it can be concluded that flexible membrane liners (FML’s) will provide more actual puncture resistance under realistic service conditions, when compared to stronger but stiffer materials such as HDPE 60 mil. The many other advantages of FML materials, such as prefabrication into large panels, conformability to uneven terrain, and lower installation costs also make them attractive choices for a geomembrane containment system.

The ability to resist puncture is a critical performance property for a geomembrane containment system. However accurately measuring a geomembrane’s ability to resist puncture in service has proven to be a difficult proposition. Index puncture tests such as ASTM D4833 measure puncture resistance by pressing a rounded metal probe through the material and recording the force required to make a hole. The results of this test are almost directly proportional to the materials tensile strength. It is widely agreed that a test such as ASTM D4833 fails to model the actual behavior of geomembranes and can give misleading results.

The truncated cone testing procedure involved using a pressure vessel to force a geomembrane sample down over three ceramic cones. The cones were tapered to a point, and then the tip was truncated at a 45-degree angle to blunt it. The cones were spaced 25 cm apart, to form an equilateral triangle. The effective height of each cone was varied by varying the depth of a layer of sand surrounding the cones’ base. The results of the study showed that a critical cone height (CCH) existed for each material tested. For cones shorter than the CCH the geomembranes would not puncture, even at very high pressures. For cone heights taller than the CCH the Geomembrane samples punctured at relatively low pressures (usually less than 14.5 psi, or 100 kPa).

The results of Hullings and Koerner’s research found that puncture resistance cannot be measured only in terms of the tensile strength of the material. Strong but stiff materials such as HDPE 60 mil recorded critical cone heights much shorter than more flexible materials such as PVC 20 mil. Table 1 shows the CCH’s found for the four materials tested:

 

Geomembrane Tensile Strength
(MPa)
Elongation
(%)
Critical Cone Height
(CCH)
HDPE (60 mil) 19 (Yield) 16 1.0 cm
Reinforced CSPE
(37 mil)
30 25 1.8 cm
PVC (20 mil) 16 310 7.0 cm
VLDPE (40 mil) 10 400 8.9 cm

If you would like more information on this truncated cone testing, or on Layfield’s extensive line of flexible membrane liners, please contact your Layfield Representative.

References: Hullings, D., Koerner, R., “Puncture Resistance of Geomembranes Using a Truncated Cone Test” Geosynthetics ’91, Atlanta, USA, 1991, pp. 273 –285.

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