Biaxial Response of Polymeric Structural Membranes

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Polymeric membranes, commonly referred to as geosynthetics, are used for in a number of Civil Engineering projects to reinforce soils. They are commonly used in retaining walls, constructed slopes, roadways, and reinforced granular load transfer platforms. Advanced methods for the analysis of existing and especially new applications involving reinforcement geosynthetics require information from load tests that duplicate the type of loads seen in field applications. Field applications typically involve the simultaneous application of load in two directions of the material. To date, laboratory testing techniques have not been sufficiently developed to apply these types of loads. Research performed under this project will involve the use of a biaxial loading frame to evaulate the biaxial loading response of an array of geosynthetic reinforcement materials. These data will allow for the development of advanced material models describing the mechanical response of geosynthetics subject to load in two directions. These models can be incorporated into advanced analysis methods for the types of applications noted above. This work can also lead to optimization of manufacturing techniques that lead to superior mechanical properties.Polymeric membranes or sheets, commonly referred to as geosynthetics, are used for structural reinforcement in a number of applications including retaining walls, constructed slopes, roadways, and reinforced granular load transfer platforms. In these applications, geosynthetics experience load simultaneously in each principal material direction. These types of loads influence the load-strain properties typically in beneficial ways as compared to material response observed in uniaxial loading. Load-strain properties of geosynthetics are typically determined from uniaxial tension tests on wide-width samples (ASTM, 2011 and 2015), which simulate plane strain loading. The free sides of the sample make this simulation poor, meaning the loading condition is somewhere between uniaxial loading and plane strain loading. This test is typically regarded as an index test. A more advanced test is needed to describe material behavior for loads expected in field applications. Numerical models are commonly used as research tools for the examination of new applications involving reinforcement geosynthetics and for the development of robust design methods. These models have not been calibrated against realistic field loadings. Design methods should use load-strain material properties pertinent to the type of field loading expected. An advanced test device is needed to calibrate these models. Biaxial testing devices apply loads simultaneously in each principal direction of the material and can duplicate the types of loads seen in field applications. Biaxial tests have been developed for materials used for fabric buildings and roofing systems (Beccarelli, 2015), however only limited studies have been performed on geosynthetics. Montana State University has recently built a load frame for biaxial loading of geosynthetics. This research will use this device to examine the biaxial response of an array of geosynthetics. These data are needed to examine the suitability of various numerical models for describing biaxial load response and for providing linear elastic material properties pertinent to field loading conditions. The broader impacts of this project include generating information that will impact industry, engineering practice and education by: i) promoting the use of a sustainable construction material, ii) providing a means of assessing material response leading to optimization of geosynthetic manufacturing processes, and iii) supporting a graduate student who will take ownership of the testing program. Geosynthetics are a sustainable construction material in that they reduce the use of natural resources, may be manufactured from recycled content, lend themselves to simple construction techniques, are cost effective, and offer a higher degree of and resiliency due to their compatibility with the ductile behavior of surrounding soil materials. Geosynthetics reduce the carbon footprint of construction by the reduced use of natural resources and reduced energy associated with resource extraction and transport and by increasing the life of the constructed facility, thereby reducing maintenance and replacement operations.