5.3 Modeling of Static Load Test using FLAC 3D
5.3.3 Results and Discussions
For studying the confining mechanism of geocell reinforced sand bed, the stresses developed in the geocell walls and in the infill sand are analysed. The confining stresses developed in the geocell wall under monotonic loading is showed in the Figure 5.8. It can be observed that, the stress developed is maximum at the centre of the loading and it gradually reduces towards the other sides. But in numerical analysis, the confining stress developed in the geocell is assumed to be constant throughout the geocell. From this study, it is clear that the confining stress developed in the geocell varies with distance from loading and so the minimum value of the confining stress must be considered for the design applications.
The stress developed in the geocell under monotonic loading is plotted by selecting four locations (1, 2, 3 and 4) as shown in the Figure 5.9 and Figure 5.10. It can be observed that the stress developed in the geocell increases with increase in applied stress and the increase in developed stress is higher at the location 1 which is directly below the centre of the loading and the rate of increase in developed stress is less as the distance of location from the loading centre increases.
Figure 5.8: Confining stress developed in the geocell wall
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Figure 5.9: Locations in the geocell reinforcement
Figure 5.10: Stress developed in the geocell wall under monotonic loading
The Figure 5.11 and Figure 5.12 shows the variation of horizontal stresses in both x- direction and y-directions at a section at the mid height of the geocell. As similar to the
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homogeneous sand case, the maximum stress is developed in the cells 1 and 2, which are directly below the loading plate and with the loading area more than that of the other cells.
Figure 5.11: Horizontal stress in x-direction at the mid height of geocell
Figure 5.12: Horizontal stress in y-direction at the mid height of geocell
Figure 5.13 and Figure 5.14 shows the variation of horizontal stresses along section A-A and section C-C. It can be observed that the stress developed is maximum at an h/D ratio of 0.4 with a magnitude of 193.89 kPa.
2 1 1
2
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Figure 5.13: Horizontal stress in x-direction at vertical cross-section
Figure 5.14: Horizontal stress in y-direction at vertical cross-section
The variation of confining stresses developed in the infill soil across these two sections A-A and C-C were studied in the Figure 5.16 and Figure 5.17. Confining stress variation is also plotted at four different h/D ratios as shown in Figure 5.15, where h is the height of geocell and D is the diameter of the loading plate, to see the variation of confining stress with depth.
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-0.3 -0.2 -0.1 0 0.1 0.2 0.3 Distance across A-A (m )
-0 -40 -80 -120 -160 -200
Confining Stress in soil (kPa)
h/D = 0 h/D = 0.4 h/D = 0.67 h/D = 1.33
In Figure 5.16, the variation of confining stress along section A-A is shown. It can be observed that the confining stress is maximum near the centre of the loading and it gradually reduces towards the edges. The maximum confining stress of magnitude 170 kPa is developed at h/D ratio of 0.4 which is at the surface of the geocell reinforcement. The negative sign indicates the developed stress is compressive in nature.
The confining stress variation in infill soil along section C-C is plotted in the Figure 5.17.
The same trend along section A-A can be observed along section C-C. The maximum confining stress developed is 158 kPa and is nearer to the centre of the loading where h/B ratio of 0.4.
Figure 5.15: Locations along the depth of geocell reinforcement
Figure 5.16: Variation of confining stress in the infill soil across A-A at various h/D ratios
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-0.3 -0.2 -0.1 0 0.1 0.2 0.3
Distance across C-C (m ) -0
-40 -80 -120 -160
Confining Stress in soil (kPa)
h/D = 0 h/D = 0.4 h/D = 0.67 h/D = 1.33
Figure 5.17: Variation of confining stress in the infill soil across C-C at various h/D ratios
The displacement of sand in unreinforced case is shown in the Figure 5.18. As there is no restriction for movement, the direction of sand movement will be both downwards and radially outwards. In geocell reinforced case the geocell walls restrains the movement of sand thereby developing confining effect on the infill material. This is shown in the Figure 5.20. The displacement vectors of soil is represented in green and geocell displacement vectors are represented in red.
For better understanding of the geocell mechanism, three sections across geocell reinforced sand bed is considered such as A-A, B-B and C-C as shown in the Figure 5.19 and the displacement vectors of sand and geocell are plotted. From the Figure 5.20, it can be observed that the top portion of geocell is bending inward and the bottom portion of the geocell is moving outward. This same trend can be observed in Figure 5.21 and Figure 5.22.
Thus it can be concluded that the geocell bed is undergoing bending under monotonic static when it is placed over a weak subgrade.
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Figure 5.18: Displacement vector plot of unreinforced case
Figure 5.19: Sections considered for studying the displacement mechanism of geocell
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Figure 5.20: Displacement vector plot at section A-A
Figure 5.21: Displacement vector plot at section C-C
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Figure 5.22: Displacement vector plot at section B-B
The vertical displacement contour in the unreinforced and geocell reinforced models are shown in the Figure 5.23. It can be observed that the vertical movement is more in unreinforced case than in geocell reinforced case. In the reinforced case the vertical displacement was restrained by geocell and so the vertical displacement magnitude got reduced.
Figure 5.23: Variation of vertical displacement
Figure 5.24 shows the horizontal stress contour in the unreinforced and geocell reinforced layered models. It can be observed that the magnitude of horizontal stress is more in geocell reinforced case than in unreinforced case because of the lateral confinement offered by the geocell walls on the infill sand. The maximum horizontal stress developed in the unreinforced sand was 61.44 kPa. Whereas the maximum horizontal stress developed at the
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geocell reinforced case was 149.5 kPa. A sudden change of horizontal stress was observed at the location of the geocell in the reinforced model, which means that the horizontal stress inside the geocell was taken mostly by the geocell rather than the same outside the geocell.
Figure 5.24: Variation of horizontal stress
The displacement in the geocell is plotted in the Figure 5.25. The maximum displacement can be observed at the region directly under the loading and has a magnitude of 1.4*10-2. The Figure 5.26 shows the coupling stress developed in the geocell under loading. The maximum stress is 50.15 kPa and is observed at an h/D ratio of 0.4.
Figure 5.25: Geocell displacement
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Figure 5.26: Geocell coupling stress