UBC Theses and Dissertations
On the comparative behaviour of geogrids in tension and pullout Fuller, Anthony L.
Two principle mechanisms of resistance to imposed loads in geosynthetic reinforced soil walls are the tensile strength of the geosynthetics and soil - geosynthetic interface bond. Although both are treated separately in design, they are intimately related. Through soil - geosynthetic interaction lateral loads from soil are transferred to the reinforcement putting it into tension. A central theme to this study is the investigation of geosynthetic reinforcement in the small, in - service strain range of 0.50 % to 1.0 % typically found in field structures. This will also be referred to in the text as the "strain range of interest." Geotextiles and geogrids are two types of geosynthetics used for soil reinforcement; this study investigates the behaviour of geogrids exclusively. Three separate apparatuses were utilised to investigate the tensile and pullout characteristics of three commercially available geogrids. The geosynthetics represent two different types of geogrid: those of low and high junction strength. Unconfined tensile tests were carried out with an industrial (Instron) testing machine testing machine. Long-term creep pullout tests were performed with a modified pullout box, designed and built at the B.C. Ministry of Transportation and Highways' laboratory in Victoria, B.C.. Constant rate of displacement pullout tests were carried out with a large scale pullout apparatus designed and built at the University of British Columbia in Vancouver, B.C.. Unconfined tensile tests investigated the load - strain characteristics of all three geogrids and the load - time (relaxation) and strain - time (creep) characteristics of the high junction strength geogrid. Creep pullout tests, carried out at comparable loads to unconfined creep tests, allowed comparison of confined and unconfined creep behaviour. Pullout tests were carried out to investigate the soil - geogrid interaction behaviour of the two geogrid types. The effects of increases in normal stress were also investigated. Results of unconfined tensile tests reveal that, in the strain range of interest, little significant difference exists in the load - strain behaviour of the different geogrid types, despite the dramatically different ultimate short term tensile strengths reported for the materials in specification guides. Unconfined creep tests reveal a limit of instability for a high density polyethylene geogrid of approximately 15 to 20 % strain. This finding is corroborated by other literature. Unconfined creep test data compare reasonably well with isochronous load - strain curves supplied by the manufacturer for the same material. Creep pullout tests carried out at comparable load levels to the in - isolation creep tests demonstrate that creep strain appears to be mitigated by confinement. Pullout tests reveal significant differences in the mobilisation of normalised pullout resistance. There are two chief components to interaction between soil and geogrids: friction between the geogrid and soil and passive bearing of transverse members against the soil. Results of pullout tests point to significant differences in the mobilisation of interaction between the geogrids tested. These differences include the magnitude of normalised pullout and displacement necessary to mobilise it. Although under low confining stress these differences are pronounced, under relatively high normal stress they are small.
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