Elasto-plastic Constitutive Models Of Rockfill Material And Soil-structure Interface And Their Applications On Concrete-faced Rockfiil Dam

The density and modulus of rockfill material become higher and higher with the application of thin layer vibration compaction technology. However, the deformation is still the main problem of concrete-faced rockfill dam (CFRD). The deformation of rockfill material is the root cause of slab stress. The slab stress also has closely related to the interaction between the concrete slabs and cushion layer. The slab ruptures and the separation voids between the concrete slabs and cushion layer observed during impounding and earthquake are strongly correlated with the behavior of rockfill materials and interfaces between face slabs and cushion layer.The constitutive laws of rockfill material and soil-structure interface are the important theory bases in the static and dynamic numerical analysis of CFRD. Static elasto-plastic finite element analysis of CFRD has been developed rapidly recently, but the development of dynamic elasto-plastic finite element analysis is relatively slow. The equivalent linear method widely used in the dynamic numerical analysis of CFRD can well capture the nonlinear behavior during low-medium magnitude earthquake. Nevertheless, the residual deformation induced by earthquake can't be reflected by the equivalent linear method, which limits the analysis of slab rupture and the seperation between face slabs and cushion layer during strong earthquake.In this study, the particle breakage, critical state and dilatancy of rockfill materials were first studied, and then a state-dependent elaso-plastic model of rockfill material considering particle breakage was proposed. Further, a three-dimensional state-dependent model of soil-structure interface was also proposed. The two models were implanted into nonlinear finite element software-GEODYNA, and then were applied to the static and dynamic numerical analysis of Zipinpu CFRD.The contents and main conclusions are as followed.(1) A series of large-scale consolidated drained triaxial tests were conducted to research particle breakage of Zipingpu rockfill/gravelly material under monotonic and cyclic loading for different initial void ratios. The results show that there exists a hyperbolic relation between the input plastic work and particle breakage, regardless of the initial void ratio or confining pressure under both monotonic and cyclic loading.(2) The translating critical state line due to particle breakage can captured by establishing the relation between particle breakage and plastic work. Tests showed there exists a critical state for Zipingpu and Milanhe gravel, and the paths in e-lnp'space approached to a nearly same critical void ratio with different initial void ratios.(3) The stress-dilatancy for Zipingpu gravel was analyzed and compared it to 2 rounded alluvial and 3 angular quarried gravelly/rockfill materials in the literature. The main characteristics of stress-dilatancy were:A nearly linear relationship between the dilatancy Dp and the stress ratio ? exists at medium-large stress ratio before peak under both constant mean pressure and confining pressure paths. The slope of the stress-dilatancy line before peak is slightly dependent of void ratio and confining pressure. The phase-transformation stress ratio decreases with the increasing of confining pressure except one subrounded gravel with little particle breakage. A nearly linear relationship exists between phase-transformation stress ratio Mf and state parameter ? for Zipingpu gravel regardless of void ratio and confining pressure.(4) Cyclic triaxial tests were performed on Zipingpu and AETS gravels, and they were followed by a discrete element study that was designed to investigate the stress-dilatancy relationship of gravelly soils under cyclic loading in triaxial stress states. Several conclusions emerged from the results. A nearly linear relationship was found between the stress ratio and the dilatancy ratio under conventional extension monotonic loading. The friction angle at the phase-transformation state was slightly different. The stress-dilatancy relationship was different during the virgin and cyclic loading. The dilatancy line under cyclic loading was located inside of the virgin/monotonic loading dilatancy lines, and the dilatancy relationship was related to the location of the most recent load reversal point. A nearly parallel linear relationship was found between ? and Dp in the d?>0 and d?<0 paths under cyclic loading. Numerical simulations were performed using the Particle Flow Code in 3-Dimension (PFC3D) software, and the results of these simulations showed that the stress-dilatancy characteristics under two-way cyclic loading are similar to the stress-dilatancy characteristics under one-way cyclic loading.(5) A constitutive model for gravelly soils was developed to reproduce their responses under repeated and dynamic loadings. The model is based on the theory of generalized plasticity and the framework of critical state soil mechanics, but it also borrow the concept of stress distances from bounding surface plasticity. The model has the following important features:the model parameters which are irrespective of initial void ratio, and proper modeling of paricle breakage, cyclic hysteresis, cyclic densification, and cyclic hardening of gravelly soils. The elasto-plastic model was incorporated into GEODYNA to study the effects of particle breakage due to vibration compaction on the deformation of rockfill dam during construction, impoundment and earthquake. It was found that the deformation would be underestimated without considering the effect of particle breakage due to vibration compaction, which was bad for the safety evaluation of high rockfill dam.(6) An elasto-plastic model based on critical state soil mechanics and generalized plasticity was modified to capture the three-dimensional (3D) behaviour of soil-structure interfaces. With a single set of parameters, the modified model simulated the monotonic and cyclic 3D behaviour of soil-structure interfaces over a wide range of soil densities, normal pressures and normal stiffnesses. A unified description of particle breakage under monotonic and cyclic loading was incorporated to reflect the effect of particle breakage. The effect of shear stress in one direction on the interface behaviour in another direction can be captured under various constrained boundary conditions. The elasto-plastic interface model was incorporated into GEODYNA to conduct a three-dimensional static and dynamic elasto-plastic finite element analysis on a CFRD. It was found that the stress path and shear deformation between slab and cushion layer calculated by the proposed model are different from hyperbolic and ideal elasto-plastic model during impoundment. The axial-direction stress induced by residual deformation is much larger for ideal elasto-plastic model. The proposed interface model can reflect the dilative and contractive responses, particle breakage and cyclic residual deformation, which is more close to reality.(7) A static and dynamic elasto-plastic finite element analysis was conducted to simulate the static construction process and the dynamic behahvior during Wenchuan earthquake for Zipingpu CFRD. The rockfill materials and interfaces between face slabs and curshion layer were modelled by the two elasto-plastic models proposed in this study. The model parameters of rockfill materials were obtained based on the deformation during construction and earthquake by feedback analysis. The feedback analysis showed that the elasto-plastic model of rockfill can well capture the static and dynamic deformation of Zipingpu CFRD. And on this basis, the phenomenon of the separation between face slabs and cushion layer was mainly discussed. The distribution of separation between concrete face slabs and cushion layer observed in Zipingpu CFRD after the Wenchuan earthquake had closely related to the slab dislocation and lower water level. The large separation at the top of stages II slabs may occur due to the lack of constraint at the construction joints after the slab dislocation. The large area of separation observed below the stage III slabs is due to the lower water level at the time of the Wenchuan earthquake, and the slab was more easily to open above the water level due to lack of normal stress constraint. The development of separation and re-contact between concrete face slabs and cushion layer during the Wenchuan earthquake can be closely predicted. The numerical results agree well with in-situ monitoring records, indicating that the two elasto-plastic model proposed can be used to evaluate the deformation of CFRDs.