Geogrid Reinforcing Effects Considering Creep And Geogrid-Soil Interaction

Geogrid has been widely used in various reinforced soil structures in practice due to its good engineering properties and cost advantages.The geogrid creep behavior and geogrid-soil interaction have great influences on the design and long-term service performance of reinforced soil structures.Based on laboratory tests,theoretical analyses and discrete element modeling,this dissertation investigated the geogrid reinforcing effects and mechanisms considering the creep and geogrid-soil interaction using high-density polyethylene?HDPE?uniaxial geogrids.The main research contents and results are summarized as follows:?1?Analysis of the creep behavior and creep damage constitutive model of geogrid.The creep characteristics of geogrid were studied under different temperature and load levels by laboratory creep tests.Then long-term creep strength and creep strength reduction factors were obtained within the design service life of reinforced structures.Based on the characteristics of polymer molecular motion and Rabotnov damage theory,the viscoelastic-plastic-creep damage constitutive model of geogrid was set up.The creep parameters of geogrids at different temperature and load levels were determined.Comparison of experimental results with the theoretical results proved that the rationality and reliability of the constitutive model.Finally,a sensitivity analysis of the creep model parameters was carried out,and results showed that the accelerated creep fracture stage of geogrids was mainly affected by viscoplastic material parameters.The effects of viscoelastic materials parameters was minor.?2?Interaction mechanism of geogrid-soil interface with the creep damage effects.The longitudinal rib length increased and tensile strength decreased due to geogrid creep under the long-term loads.To analyze the effects of creep on the interaction mechanism between reinforcement and soil,this dissertation simulated different geogrid elongation after creep with different spacing of transverse ribs.Based on strain hardening and strain softening of the HDPE geogrid with different cross rib spacing during pullout procedure,broken linear shear stress-displacement hardening model and trilinear shear stress-displacement damage softening model were proposed.Based on the basic control equations of the interface,analytical solutions of interface tension,shear stress and displacement at different stages were derived considering the strain hardening of the geogrid under creep load and strain softening based on creep damage,and the development of shear stress and progressive failure mode of the geogrid-soil interface at different pullout stages were revealied.?3?Investigation of geogrid reinforcing effects under static load by pullout test.Based on the pullout test results,geogrid-soil interaction mechanism,the friction coefficient,interface shear stress and the relative displacement relationship between geogrid and soil have been investigated.The contribution rate of resistance force of transverse ribs to the interface between geogrid and soil was analyzed and the modified calculation model of the ultimate geogrid pullout force was proposed.?4?Analysis of geogrid reinforcing effects under the cyclic load and flexible boundary by pullout test.The interaction mechanism of geogrid-soil was dramatically influenced by frequency and amplitude.The interface cohesion and friction coefficient under dynamic loads were smaller than those under static loads.The maximum pullout force under top flexible and front flexible boundaries was smaller than that under rigid boundaries.The cohesion and friction coefficient under the top flexible and front flexible boundaries were smaller than those under the rigid boundaries.Results showed that dynamic load and flexible boundary would weaken the geogrid reinforcing effects.Therefore,the strength of geogrid should be reduced when quasi-static analysis was used for reinforced soil structures.In pullout test of geogrids,it is recommended to apply normal load using flexible top boundary and flexible boundary condition at the front.If it cannot be realized due to testing conditions,a reduction factor is suggested.?5?Research of geogrid reinforcing effects under the cyclic load and flexible boundary by discrete element simulation.The formation and evolution mechanism of shear band at the geogrid-soil interface were analyzed.The macro and meso relations of reinforced soil interface parameters in pullout test were clarified.Based on the discrete element method,geogrid pullout test in sands was simulated using Particle Flow Codes(PFC^2D)and simulated results were compared with the pullout test results.The influences of static loading,the coordination number in the shear zone under rigid/flexible boundary,porosity,particle rotation,contact force chain and other factors,as well as the load transfer at the interface of the reinforced soil were studied.From a mesoscopic perspective,it was revealed that particle movement and rotation were important features of shear band deformation evolution.The distribution of contact force determined the macroscopic mechanical properties of the sample and the basic shape of shear band.The mesoparameters,such as coordination number and porosity,the rotation of the soil particles within the sample and the distribution of contact force between the sand particles intuitively showed the effects of static and dynamic loads and rigid/flexible boundary conditions on the pullout test results.