In this study, a realistic approach to modeling the three-dimensional honeycomb shape of the geocell is adopted by maintaining the actual curvature using Fast Lagrangian Analysis of Continua in 3D (FLAC3D) a finite difference software. An attempt has been made to numerically simulate laboratory-scale geocell-reinforced pavement systems under monotonic loading to study the strengthening mechanism of geocell reinforcement.
Preamble
The effect of geocell in the ultimate bearing capacity of the geocell reinforced sand bed was also investigated numerically. The definition sketch of the geocell-reinforced sand bed model used for the experimental study is shown in Figure 4.1. The numerical model of the unreinforced case used for the analysis is shown in Figure 4.3.
Comparison of the numerical simulation results of the unreinforced and geocell-reinforced case is presented in Figure 4.7. This will improve the performance of the geocell-reinforced sand bed there by increasing the ultimate bearing pressure. The numerical model of the unreinforced case used for the analysis is shown in Figure 5.3.
Comparison of the numerical simulation results of the unreinforced and geocell-reinforced case is presented in Figure 5.7. This study mainly focuses on the confinement mechanism of the geocell-reinforced sand bed over the weak sand layer under monotonic loading.
Mechanism of Geocell mattress
Mechanism of Geocell Reinforcement
In the case of geocell that has a three-dimensional honeycomb structure, there exists an additional lateral confinement on the infill material, which improves the performance of the reinforced sand bed to a greater extent. The present study focused on modeling the actual three-dimensional structure of the geocell reinforcement and performing numerical analysis on the geocell reinforced homogeneous and layered sand beds.
Objective and Scope of the Study
Organization of the Thesis
Introduction
Geocell and its Application in Pavements
In the case of vertical loading, hoop stresses are mobilized in the cell walls and soil resistance in the adjacent cells, so that the soil inside the cells is limited and the soil's strength and stiffness are increased. Al Qadi and Hughes (2000) reported that geocell inclusion increased the elastic modulus of the aggregate layer in a flexible pavement by approximately two times.
Experimental Studies on Geocell reinforcement under Static Loading
An additional layer of geogrid placed at the bottom of the geocell mattress further increased the bearing capacity and stiffness of the foundation bed. The performance of the geocell with a higher modulus of elasticity had a greater bearing capacity and stiffness of the reinforced section.
Numerical Studies on Geocell reinforcement under Static Loading
A three-dimensional mechanical response model was used to model the geocell-reinforced foundation due to the complex geometry of the geocell. They also found that the size of the geocell pocket inversely affects the performance of reinforced beds.
Summary
The properties of the geocell and fill material play an important role in improving the performance of the reinforced bed. In the following chapters, the reinforcement mechanism of the geocell in homogeneous and layered cases is studied using the FLAC 3D numerical model.
Introduction
Due to the complexity in modeling the actual geocell honeycomb shape, limited literature is available on numerical geocell simulations while maintaining the actual geocell shape. 2010) modeled the geocell honeycomb shape for a single geocell using FLAC 3D software and simulated static and cyclic tests. also adopted the same method for modeling the honeycomb shape of the geocell and simulated static tests considering sand and clay as substrates, respectively.
Finite Difference Approach
FLAC 3D Software
Introduction to FLAC 3D Software
Explicit Dynamic Solution Scheme
Using the Gaussian divergence theorem for the tetrahedron element, the derived velocities at each mass point are used to express the deformation rate of the tetrahedron element. Speeds and other variables are assumed to be frozen for box operation, meaning that the newly calculated voltages do not affect existing speeds.
Mechanical Time Step for Numerical Stability
The vertical stress contour in unreinforced and reinforced casing is shown in Figure 4.15. The vertical displacement contour in unreinforced models and models with geocells is shown in Figure 5.23.
Materials Models
Soil Model
The elastic-perfectly plastic Mohr-Coulomb model was used to model the sand layers in the numerical simulation. The Mohr-Coulomb criterion in FLAC 3D is expressed by the principal stresses 1, 2 and 3.
Geocell Model
- Diamond Shape
- Primitive Honeycomb Shape
- Honeycomb Shape
- Comparison of Performance of Different Geocell
The curved part of the geocell was modeled using the cylindrical mesh and the two angular ends of the geocell were modeled using the degenerate wedge mesh. The width of geocell is kept as 0.3m in x direction and 0.2m in y direction as 0.2m respectively as shown in figure 3.5. Two cylindrical mesh areas are used to simulate the actual honeycomb shape of the geocell.
The curved shape of the geocell was modeled using a cylindrical mesh area with a radius of 0.075 m, and the corners were modeled using a cylindrical mesh area with a radius of 0.075 m in the x-direction and 0.15 m in the y-direction, respectively, as shown in Figure 3.6.
Validation of Geocell Model using Large Triaxial Test
Overview
Material Models and Parameters
- Aggregate
- Geotextile Geocell
The parameters used to model the geocell reinforcement and the dimensions of the geocell used are given in Table 3.2 and Table 3.3, respectively. The geocell was placed on the cylindrical surface of the large triaxial test model and the test was simulated.
Modeling of Triaxial Test using FLAC 3D
- Numerical mesh and Boundary Conditions
- Validation using Experimental Results
The vertical and lateral displacements were fixed at the bottom boundary of the cylindrical model, and the cylindrical surface of the model remained free, allowing horizontal and vertical deformations on the specimen under load. The model was solved in 75000 iteration steps until the vertical displacement at the top of the soil reached 10 percent of the model diameter (= 30 mm). In general, the numerical model simulated well the stress-strain curves of unreinforced sand and sand reinforced with geocells.
A comparison of the experimental and numerical results of the unreinforced model and the model with geocells is shown in Figure 3.10 and Figure 3.11, respectively.
Summary
Overview
Material Models and Parameters
Sand
Geocell
The variation of horizontal stresses in both x-direction and y-direction at the middle height of the geocell was analyzed as shown in Figure 4.8 and Figure 4.9.
Modeling of Static Load Test using FLAC 3D
Numerical Mesh and Boundary Conditions
The complete laboratory test box with dimensions of 1 m wide, 1 m wide and 0.9 m deep was modeled. The vertical movement was fixed at the bottom boundary of the model and the horizontal movement was fixed at the four side boundaries. The model was solved for 12,000 iteration steps until the top soil settlement ratio reached 10 percent.
In a geocell-reinforced bed casing, the geocell is positioned to provide a clearance of 0.02 m from the surface.
Experimental Validation
Results and Discussions
To study the confinement mechanism in the geocell reinforcement, two sections such as section A-A and section C-C were considered, and the variation of confining stresses developed in the backfill soil across these two sections was investigated. It can be observed that the confinement stress is maximum near the center of the load and it gradually reduces towards the edges. This will result in the development of confining stresses in the geocell wall as well as in the backfill soil.
In the reinforced case, the vertical displacement was limited by the geocell so that the vertical displacement was reduced.
Numerical Modeling of Single Geocell Reinforced Sand Bed
Loading Patterns
- Loading Plate diameter less than the Width of Geocell
- Loading Plate diameter more than the Width of Geocell
It can be seen that the vertical stress magnitude increased due to the geocell confinement under the same slab setting of 30 mm. It can also be observed that the geocell wall moves radially inwards which further increases the confinement in the backfill soil. Numerical simulations were performed in the previous model by increasing the diameter of the loading plate to 0.4m which is less than the diameter of the geocell reinforcement.
The displacement vectors are drawn considering two vertical sections along the geocell and one horizontal cross section at the average height of the geocell.
Summary
Overview
Material Models and Parameters
Sand
Using F2 and h1/a (h1 is the first layer thickness), the modulus of elasticity E1 of the top layer is found from Figure 5.2. Of the elastic modulus, the shear modulus and the bulk modulus values were determined by assuming the Poisson's ratio of 0.25. The parameters used for modeling 75% relative density sand and 30% relative density sand are shown in Table 4.1.
Clay
Geocell
Modeling of Static Load Test using FLAC 3D
Numerical mesh and Boundary Conditions
Experimental Validation
Results and Discussions
Figure 5.11 and Figure 5.12 show the change of horizontal stresses in the x and y directions on the section at the mid-height of the geocell. In Figure 5.16 and Figure 5.17, the changes in confining stresses developed in the fill soils in these two sections A-A and C-C were studied. To better understand the mechanism of geocells, three geocell-reinforced sand subgrade sections, such as A-A, B-B, and C-C, are considered as shown in Figure 5.19, and the displacement vectors of sand and geocells are plotted.
From Figure 5.20 it can be seen that the upper part of the geocell bends inwards and the lower part of the geocell moves outwards.
Parametric Studies
- Effect of Modulus of Geocell
- Effect of Geocell-Soil Interface Shear Modulus
- Effect of Apperture on Geocell Walls
- Geocell eith Geogrid at the base
It can be observed that the shear modulus of the geocell soil interface has significant influence on the bearing capacity of the foundation bed. To study the effect of opening on the geocell walls, the geocell was modeled with openings with an area of 5% of the total area of the geocell as shown in Figure 5.29. It can be observed that the presence of the opening reduces the ultimate bearing pressure of the sand bed.
It can be observed that the geogrid improves the performance of the geocell-reinforced sand subgrade by 7% at 3%, 5% and 10% settlement ratios, which is clearly seen in Figure 5.32.
Geocell Reinforced Sand Bed over Clay Subgrade under Monotonic Loading
Summary
Numerical Modeling of Geocell Reinforced Sand Bed Over Weak Sand Layer Under Monotonic Loading.
Overview
Material Models and Parameters
Patterns of Geocell Reinforcement
- Type 1- Weld over Weld
- Type 2- Weld over Cell
- Type 1 with Geogrid
- Type 1 with Geogrid
The central weld of the upper geocell layer was placed above the central weld of the lower geocell layer as shown in Figure 6.2. In this case, the central weld of the upper geocell layer was placed above the cell of the lower geocell layer as shown in Figure 6.3. A geonet layer was placed between the two layers of geocell in the first case as shown in Figure 6.4 and the behavior of the sand bed is studied.
A layer of geogrid was placed between two layers of geocells in the second case, as shown in Figure 6.5, and the behavior of the sand base was studied.
Modeling of Static Load Test using FLAC 3D
Numerical Mesh and Boundary Conditions
Results and Discussions
It can be observed that the developed stresses were maximum near the load center and gradually decrease towards the boundaries of the model. The maximum stresses observed in geocell 1 and 2 in all cases expect the lower geocell of the two cases of Type 2. To study the displacement mechanism in the geocell reinforcement, a vertical cross-section was considered and the displacement in the geocell reinforcement as and in the soil analyzed separately, as shown in Figure 6.11 and Figure 6.12.
When a geogrid is placed between two layers of geocell, the geogrid is providing additional tension membrane support there improving the performance of the reinforced sand bed.
Summary
General
Conclusions
Numerical Modeling of Geocell Reinforced Sand Bed under Monotonic
Numerical Modeling of Geocell Reinforced Sand Bed over Weak Subgrade
Numerical Modeling of Stacked Geocell Reinforced Sand Bed Over Weak
Scope of Future work
Three-dimensional numerical analysis of Geocell-reinforced soft clay beds by considering the actual geometry of Geocell Pockets. 20] Moghaddas and Dawson, Laboratory model tests for a strip foundation supported on geocell reinforced sand beds. Special Geotechnical Publications No. 207, 2010. Behavior of foundations on reinforced sand subjected to repeated loading, comparing the use of 3D and flat geotextiles.
