biaxial testing of geogrids: recent developments
At different recent German and international conferences some publications discussed a so-called
biaxial effect of geogrid reinforcement. Generally it is reported that synthetic materials (e.g. membranes used for roofing or in aeronautic applications) show somewhat different mechanical behaviour when loaded under biaxial stress conditions compared to uniaxial stress conditions. In the case of geogrid reinforcement it is the production technology, especially the formation of the crosspoints between the reinforcement elements of the longitudinal direction (MD) and the cross direction (CMD), which was declared to be the basic reason for this phenomenon. When loaded biaxially, geogrid reinforcement products made from punched and stretched polymer sheets and those made from welded or glued strips would perform significantly better (stiffer) than woven or knitted materials. The positive effect of biaxial loading would be apparent especially when analysing the creep-strain-behaviour of the materials.
The interpretations and conclusions basically with regard to the derivation of reduced reduction factors for creeprupture are critically reviewed in this paper. Based on an extensive literature study, FEM simulations as well as additional biaxial testing it is intended to further contribute to the ongoing discussion and research.
This paper will present that the effect of boundary conditions for biaxial testing especially the clamping and loading arrangement needs to be carefully considered to avoid misinterpretation of test results.
Detailed knowledge about biaxial testing of geogrids is limited to date, elaborate testing has been reported only by Kupec (2004) and McGown et al. (2004). Analysis of the available information suggests that boundary conditions of the testing equipment especially the clamping and loading arrangement may have caused misinterpretation of the mechanical behaviour under biaxial stress conditions.
Additional testing at University of Essen was carried out in order to provide a basis for comparison and
verification of the actual information. Significantly improved loading arrangement and specially designed single strip clamping was available there. It was found that the substantial differences in the material behaviour reported by others cannot be verified for the tested geogrid from welded PET strips neither in short term (CRS-testing) nor in sustained loading tests. The production technology, especially the formation of the crosspoints between the reinforcement elements of the longitudinal direction (MD) and the cross direction (CMD) of a geogrid, which was declared to be the basic reason for a potential stiffening effect under biaxial loading seems to be without any importance in that regard.
The derivation of reduced partial reduction factors for creep rupture and biaxial loading conditions, as was published by Heerten et al. (2005), needs careful revision.
Significant variation of the initial modulus observed for the testing carried out at the University of Strathclyde so far gives reason to assume that the same conclusion can be drawn for other geogrid types also, regardless of the raw material and the formation of the cross points. Additional testing is required to verify this assumption.
Important conclusions about the load transfer mechanism of geogrids in reinforced soil structures that have been based on findings by Kupec et al. may need careful revision.