Numerical Study On Time-dependent Maro-and Micro-mechanical Behavior Of Rockfill

The construction of airport high embankments and dams is under rapid development in China.And the long term deformation of rockfills is one of the major threats faced by these geotechnical structures.To deal with this problem,the time-dependent mechanical behavior of rockfills should be well understood.And in this study,discrete element method(DEM)is used to study the micro-mechanism behind the short term and long term mechanical behavior of rockfills.And based on the micro-mechanism,a time-dependent constitutive model is proposed to simulated the simulate the nonisotach behavior of rockfills.The main conclusions are as follows:(1)The conventional large traixial tests and others stress paths tests were simulated with the proposed DEM model.Rockfill particles are modeled as breakable agglomerates,and reasonable consistency is found between the predicted and experimental results.The simulation highlights the influence of agglomerate breakage on both the mechanical behavior of the assembly.The micro parameters during the tests are analyzed.The evolution of both the number and mode of agglomerate breakage during shearing is significantly influenced by the confining pressure,deviator stress ratio,and loading direction.A consistent relationship,which is independent of the stress paths,is identified between the macro-scale and the micro-scale.(2)In order to simulate time in DEM,the bond strength within an agglomerate decreases with the elapsed time as a proposed function.The macro-and micro-behavior of rockfill,during creep and stress relaxation are investigated with the DEM model.The deviator stress level is found to have significant influence on the evolution of the agglomerate breakage number and mechanical coordination number during creep,whereas these two numbers increase slightly during stress relaxation in all situations.The relationship between the macro-and micro-responses is explored.The micro-mechanism of the observed nonisotcah phenomenon is also investigated.(3)Based on the proposed time-dependent DEM model,the breakage mode of rockfill during the tests is studied.For specimens composed of large number of particles,the breakage mode is dominated by “asperity breakage”.A series of single particle creep tests are also simulated with different numbers of loading points.And the results show that for more loading points,the breakage mode tends to be “asperity breakage”;while for fewer loading points,the breakage mode tends to be “splitting breakage”.(4)Based on the obtained micro-mechanism with DEM,a time-dependent constitutive model is proposed to simulated the simulate the nonisotach behavior of rockfills.The model is validated against the results of triaxial creep tests on various granular materials,including three rockfills and a sand.A comparison is further made between the proposed nonisotach model and an existing isotach model.The time scale effect is then proposed to explain the discrepancy between the numerical results and measured results of high rockfill dams.