Reliability Analysis for the Geogrid-Reinforced Unpaved Roadway System

Abstract eng:
When soft subgrades are encountered as foundation soil in an unpaved roadway, excessive rut depth always occurs such that the design expectation cannot be met. One engineering means to deal with the situation is to employ geogrid synthetic material in the unpaved roadway system. It was also known that the undrained shear strength of the subgrade clay material, along with the design thickness of the aggregate material, played crucial roles in the overall performance of the unpaved roadway system. In terms of design, however, with the variation of undrained shear strength of the subgrade clay material, using single value of undrained shear strength for design to represent the roadway performance could overlook variability of the soil properties and thus failure might occur in sections of high variability. In this study the design approach proposed by Giroud and Han (2004) was employed to evaluate the system rut depth. Reliability analyses of the system rut depths through Monte Carlo Simulation were conducted using the suggested design approach and discussed. In this study, the reliability of the unpaved roadway is assessed considering the variability of the subgrade undrained shear strength and five values of loading cycles. Based on the results of the analyzed cases, it is observed the probability of failure decreases significantly with increasing aggregate thickness under different reinforcing scenarios, as expected. Results also indicated that the influence of loading cycles is not so significant when good geogrid reinforcement (for example, geogrid type 2) is used. When comparing the results of different coefficient of variation (C.O.V.) under the same reinforcing scenario, it is found that lower C.O.V. indicates a faster reduction of the probability of failure for both unreinforced and geogrid 1 reinforced cases. In addition, when large aggregate thickness is employed in design of unpaved roadways, the influence of C.O.V. of the subgrade undrained shear strenght on the probability of failure is not also very significant in the geogrid 1 reinforced case.

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