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Introduction
A study of the behavior of embankments made with waste material like GGBS added to conventional fill materials on silty sandy subsoil. Staged construction of the embankment has been effectively modeled followed by the application of overburden pressure on the structure. The parameters required for modeling of GGBS fill embankment has been used from the work of various researchers determined in the laboratory using a prototype embankment.
3.3.2 Finite Element Model
A typical embankment of 8.0 metre crest width with 2:1 side slopes has been chosen for this study. The height of the embankment is 4 metre. The depth of subsoil below ground level is 5 m. The subsoil silty sand layer is assumed to be fully saturated. An overburden pressure of 100 kN/m2 is applied on the structure. The finite element model has been created and analyzed using PLAXIS 8.2 Professional geotechnical analysis software. Owing to the symmetry of the problem, only one half needs to be modelled. 15 noded plain strain elements have been used for discretizing both the embankment as well as the foundation material.
3.3.3 Material Models
The embankment consisting of different materials and foundation soil comprising of silty sand has been modelled using the Mohr-Coulomb soil model. The material properties of subsoil, embankment materials which are used in the current modeling analysis are listed in Table 3.3.3.
3.3.4 Mesh Generation
For the mesh generation, the global coarseness is set to ‘medium’ and the mesh is generated as shown in Figure 3.3.4
The deformation at the base of the silty sand layer is assumed to be zero. Hence the base is fixed in x and y directions. The two vertical boundaries are assumed to be fixed in x direction. The initial conditions include the existence of the phreatic level at the ground level. It is assumed that water can flow out from all boundaries and excess pore water pressures can dissipate in all directions.
Initial Conditions and Boundary Conditions
In the initial conditions, unit weight of water was set to 10 kN/m3. The water pressure is fully hydrostatic and is based on a general phreatic level. In addition to phreatic level, boundary condition for consolidation analysis can be additional input. The lines of consolidation need to be selected in vertical direction that means vertical boundaries must be closed to restrain the horizontal flow and no free outflow is allowed at that boundary. In the analysis, constant ground water level has been considered.
3.3.6 Initial Stresses
After the generation of water pressures, initial stresses were generated. When using Mohr Coulomb model, the analysis require the generation of the initial stresses by means of K0 procedure, which was then used to calculate the initial stresses. The suggested K0 procedure is based on Jacky’s formula (K0=1-sinϕ). After the generation of phreatic level and initial stresses, the input is complete and calculations can be generated. The calculation is involved in one phase only i.e. normal plastic calculation phase in which the embankment is constructed.
DYNAMIC ANALYSIS OF HIGHWAY EMBANKMENT
3.4.1 Introduction
In order to simulate the earthquake loading on the embankment, dynamic analysis was carried out. A real accelerogram of an earthquake recorded by U.S Geological Survey (USGS) in 1989 is used for the analysis. It is contained in the standard SMC format (Strong motion CD-ROM) which can be read by PLAXIS. In dynamic analysis, comparison of displacements and acceleration response of the embankment with normal fill, improved fill with GGBS under dynamic loading conditions was made.
3.4.2 Finite Element model
A typical embankment of 8.0 metre crest width with 2:1 side slopes has been chosen for this study. The height of the embankment is 4 metre. The depth of subsoil below ground level is 5 m. The subsoil silty sand layer is assumed to be fully saturated. The finite element model has been created and analyzed using PLAXIS 8.2 Professional geotechnical analysis software. Owing to the symmetry of the problem, only one half needs to be modelled. 15 noded plain strain elements have been used for discretizing both the embankment as well as the foundation material.
In dynamic analysis of embankment, the earthquake is modelled by imposing a prescribed displacement at the bottom boundary. In contrast to the standard unit of length used in PLAXIS [m], the displacement unit in the SMC format is [cm]. Therefore the input value of the horizontal prescribed displacements is set to 0.01m. The vertical component of the prescribed displacement is kept zero (ux=0.01m and uy=0.00m). At the far vertical boundaries, absorbent boundary conditions are applied to absorb outgoing waves. In this way the boundary conditions as described above are automatically generated.
3.4.3 Material Models
The embankment consisting of different materials and foundation soil comprising of silty sand has been modelled using the Mohr-Coulomb soil model.
Initial conditions and Boundary conditions
In the initial conditions, water unit weight was set to 10 kN/m3. The water pressure is fully hydrostatic and is based on a general phreatic level. After the generation of water pressures, initial stresses were generated. After the generation of the initial stresses the input is complete and the calculations can be defined.
3.4.6 Calculations
The calculation is involved two phases. The first one is normal plastic calculation in which the embankment is constructed. The second is a dynamic analysis in which the earthquake is simulated. To analyze the effects of the earthquake in detail the displacements are reset to zero at the beginning of each phase. Figure 3.4.6(a) presents a snapshot of different phases of the construction process as implemented in FEM program.