Development of a test protocol for cyclic pullout of geosynthetics in roadway base reinforcement
Holley, Timothy Michael.
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Geosynthetics, or manmade materials used in soils engineering, have successfully been used as base reinforcement of pavements for over 40 years. Use of geosynthetics can result in cost savings by allowing the aggregate base layer to be reduced in thickness and/or the service life of the pavement to be extended. Design methods for this type of reinforcement have typically been developed by individual manufacturers for specific products. These methods are not widely used by state transportation agencies because 1) they are proprietary, 2) they are empirically based, and 3) they lack compatibility with the current national trend towards mechanistic-empirical pavement design procedures. This project was initiated to develop testing methods to determine one of the critical material properties needed for mechanistic-empirical base-reinforced pavement design, namely, the resilient interface shear stiffness. This property describes the interaction, in particular the shear stiffness, between the geosynthetic and the surrounding aggregate. This new test protocol closely mimics vehicular load patterns, resulting in design parameters pertinent to the use of the geosynthetics to reinforce the base course. A study was conducted to evaluate the repeatability of these tests and to develop a standardized test method. Specific parameters under investigation include load pulse and rest period duration, embedment length of the geosynthetic, and differences in results using different soils and types of geosynthetics. Some parameters seemed to have little effect on values of resilient interface shear stiffness, while others vastly impacted the results. Load pulse and rest period durations did not affect output results significantly. Maintaining a constant confinement or shear stress during the test duration produced higher repeatability and correlated well to the adapted resilient modulus equation. Three-aperture length tests on polyester geogrid also correlated well with this equation, however repeatability was moderately low. Polypropylene geogrid and a woven geotextile confined in Ottawa sand displayed low correlation to this equation. During testing, very small displacements occur, and therefore, every effort should be made to ensure that these measurements are accurate and not skewed by electrical noise and interference.