Numerical modeling of centrifuge test procedure for different embankment cases

Physical modelling through full-scale and small-scale models is widely implemented in order to define specific aspects of the prototype behaviour. On the other hand, numerical modeling is essentially required to cope with the complex geotechnical problems due to the ability of considering and analyzing all aspects of the model and can afford more perception about the behavior of structures such as geosynthetic-reinforced embankments. In this study, four different cases of unreinforced and reinforced embankment models constructed on soft and stiff grounds were studied. Small-scale physical modelling by means of centrifuge tests and numerical modelling by means of finite element simulations were performed. As the small-scale model was rotated in different acceleration fields during the centrifuge test, the dimensions of the centrifugal model were different from the original state of the prototype in different stages of the test. This paper focused on developing a finite element simulation based on the dimensions of a centrifugal model in different incremental acceleration fields applied during the stages of the test. Comparing the results of finite element simulations with the measurements of the centrifuge tests showed a good agreement between the two methods, which verified the reasonableness of the finite element models in analysis of embankments based on small-scale centrifugal dimensions. Moreover, the results showed the different deformation behaviour for embankments on soft and stiff grounds and indicated the significant effect of the geosyntheic reinforcement on increasing the stability of the embankment on soft ground

NUMERICAL MODELING OF CENTRIFUGE TEST PROCEDURE FOR DIFFERENT EMBANKMENT CASES

ABSTRACT: Physical modelling through full-scale and small-scale models is widely implemented in order to define specific aspects of the prototype behaviour. On the other hand, numerical modeling is essentially required to cope with the complex geotechnical problems due to the ability of considering and analyzing all aspects of the model and can afford more perception about the behavior of structures such as geosynthetic-reinforced embankments. In this study, four different cases of unreinforced and reinforced embankment models constructed on soft and stiff grounds were studied. Small-scale physical modelling by means of centrifuge tests and numerical modelling by means of finite element simulations were performed. As the small-scale model was rotated in different acceleration fields during the centrifuge test, the dimensions of the centrifugal model were different from the original state of the prototype in different stages of the test. This paper focused on developing a finite element simulation based on the dimensions of a centrifugal model in different incremental acceleration fields applied during the stages of the test. Comparing the results of finite element simulations with the measurements of the centrifuge tests showed a good agreement between the two methods, which verified the reasonableness of the finite element models in analysis of embankments based on small-scale centrifugal dimensions. Moreover, the results showed the different deformation behaviour for embankments on soft and stiff grounds and indicated the significant effect of the geosyntheic reinforcement on increasing the stability of the embankment on soft ground

Geometrical effect on the behaviour of embankment on soft ground

Many embankments constructed on soft ground are susceptible to failure and large settlements due to its low strength soil condition. Geosynthetics are used effectively as a reinforced material to increase the shear strength, and stiffness of the reinforced embankment and consequently, to reduce the total and differential settlements. In the first part of the study, four different cases of embankments with and without reinforcement, constructed on soft and stiff grounds were studied through small-scale physical modeling using centrifuge test and numerical modeling using finite element simulation. Comparison between the results using both finite element models and centrifuge tests was carried out to validate and identifies the reliability of the finite element method. In centrifuge test, a model scale with various sizes was simulated to a constant full-scale dimension using different acceleration fields. The results show the different deformation behavior for these different embankment cases and indicate the significant effect of the geosynthetics reinforcement on increasing the stability of embankment. The comparison analysis presents a good agreement between results of these two methods. It validated the finite element technique in analysis of different embankment cases. The second part of the study focus on the geometrical effects on the behavior and failure mechanism of embankments. Two full-scale case history embankments in Malaysia and Canada, the Muar trial embankment and Vernon highway embankment were verified. Three dimensional effects on Muar trial embankment were evaluated by comparing the results of two and three-dimensional analysis, in terms of predicted displacements, lateral movements, excess pore pressure, factor of safety, and failure height of the embankment fill. Moreover, this study attempt to evaluate the boundary limits for the applicability of two and three-dimensional analyses by determining the suitable geometry configuration of embankment in utilizing the geotechnical analysis. The ratio of the calculated failure height of three to two dimensional Finite Element analyses (Hf,3D/Hf,2D) has been determine for embankment cases with different base aspect ratio of the length to width (L/B). Two shape-factor equations related to the bearing capacity of spread footings and safety factor of embankments also utilized to account for the geometrical behavior of the embankment regards to its geometrical configuration. Results of three-dimensional analyses have better agreement with the actual field measurements. It is concluded that neglecting the three dimensional effects could mislead the design of the embankment in some condition. In conclusion, it is recommended that for “long embankment” with the length to width ratio more than two (L/B > 2), it may appropriate to use two-dimensional analysis as the three-dimensional safety factor converges to two dimensional safety factor. For “short embankment” with the length to width ratio less than two (L/B < 2), three dimensional effects on the embankment behavior becomes considerably great and should be considered as important factor in design and analysis of embankments

3D numerical analysis of centrifuge tests on embankments on soft and stiff ground

The behavior of reinforced and unreinforced embankment on soft and stiff grounds has been investigated using the centrifuge tests and verified using numerical simulations. Four different cases have been investigated in this study based on various types of foundation materials and reinforcement condition. Two-dimensional (2D) and three-dimensional (3D) finite element programs, Plaxis 2D and Plaxis 3D Foundation respectively used to simulate and analyze the prototypes behavior provided by centrifuge tests. Deformation behavior, settlements and effect of reinforcement have been studied in this study. Comparison of the results of the numerical analysis with the measurements obtained from the centrifuge tests shows good agreement in terms of settlement and the reduction of settlement due to geosynthetics reinforcement

Effect of weathered surface crust layer on stability of Muar trial embankment

This paper attempts to evaluate the effect of surface crust layer on the stability and deformation behavior of embankment. A full-scale case history trial embankment constructed on Muar flat in the valley of the Muar River in Malaysia was modeled and analyzed. The Muar trial embankment was simulated in two- and three-dimension (2-D and 3-D) utilizing finite element programs PLAXIS 2-D AND PLAXIS 3-D FOUNDATION, using staged-construction procedure. Sensitivity analysis was performed by varying the thickness of weathered crust layer beneath the embankment fill, i.e., three models of embankment with no surface crust, 1 m surface crust and 2 m surface crust layer. Predictions were made for the vertical and the horizontal displacements of the embankment. Factor of safety for each meter increase in the embankment height was defined until the failure is reached. It is concluded that the bearing capacity of the ground and the deformation behavior of the embankment were sensitive to the thickness of the weathered crust layer. The surface crust layer has a positive effect on the stability of the embankment and consequently reduces the settlement and increases the failure height of the embankment fill up to 37%

Three-dimensional face stability and deformation analysis of tunneling with TBMs

This paper presents three-dimensional (3D) face stability and deformation analysis of the Earth Pressure Balance (EPB) shield tunneling utilized in the construction of Shiraz subway twin-tunnels in Iran by analytical and finite element (FEM) simulation approaches. A FEM program, PLAXIS 3-D Tunnel is used to simulate step-by-step excavation and tunnel construction processes by tunnel boring machine (TBM). The result of FEM in terms of required face stability pressure is compared with analytical calculation using 3-D wedge stability model which is set up based on the limit equilibrium analysis. Heterogeneity of soil layers and 3-D soil arching effect are considered in this model. The results of wedge stability model and FEM simulations are in good agreement with Shiraz subway actual measurements. The numerical finite element procedure is found to be an effective approach for predicting the face stability pressure as well as the deformations and settlements due to EPB shield tunneling