Slope with Geogrid Reinforcement (FLAC2D)

Example Application

This example reproduces example 1.4.4 from the FLACSlope 8.1 manual. The project file for this example may be viewed/run in FLAC2D.[1] The main data file used is shown at the end of this example.

How To Read and Use Examples

Problem Statement

In this example, two layers of geogrid are used to stabilize a slope. The slope conditions and material properties for this model are shown in Figure 1.

Figure 1: Slope with geogrid reinforcement.

Model Geometry

The model is created using the Slope Wizard in i Sketch. The following parameters were used to build the slope.

A zone size of 0.25 is specified and the zones are generated.

It is possible to specify group names in Sketch, but for this model the groups were specified in the i Model workspace by first clicking on the top block to select it, and then choosing “Assign a Group to the Selection” from the right-most icon in the tool bar. The group for the top block is set to “sandy clay”. Then the bottom two blocks are selected and the group is set to “silty clay”.

Create a new plot, add zones and change the group slot to “Default”. The model will appear as in Figure 3.

This model is saved. Data files are then used to restore this saved state, assign properties and boundary conditions, add reinforcement and run the factor of safety analyses (see data files). Note that in 2D, geogrid reinforcement can be simulated with cables.

Figure 3: The zoned model with assigned groups.

Results

The slope is unstable without the geogrid reinforcement. The results for the unsupported case are shown in Figure 4. The factor of safety is calculated to be 0.9.

The properties selected for the geogrid reinforcement are assigned in the following command.

structure cable property cross-sectional-area=5.0e-3 young=1.04e11 ...
    yield-tension=1.0e7 yield-compression 0 grout-friction 10 ...
    grout-stiffness=3.46666675E9 grout-cohesion=1000 grout-perimeter 2.01

Note that in FLAC2D, the structural element strength properties are not reduced during a factor-of-safety analysis. The factor-of-safety calculation results are shown in Figure 5. The safety factor is now increased to 1.07.

The effect of the bonding resistance provided at the geogrid/soil interface can be seen when we increase the bond cohesion from 1000 N/m to 10,000 N/m. For this case, the calculated factor-of-safety is now 1.16, as shown in Figure 6.

Figure 4: Factor-of-safety results for unsupported slope.

Figure 5: Factor-of-safety results for geogrid support with bond cohesion = 1000 N/m.

Figure 6: Factor-of-safety results for geogrid support with bond cohesion = 10000 N/m.

Data Files

nogeogrid.dat

model restore 'mesh'

model gravity 9.81
model large-strain off

zone cmodel assign mohr-coulomb

zone property density 1800.0 bulk=1E8 shear=3E7 ... 
              cohesion 10000.0 friction 20.0 dilation 0.0 tension 0.0 ...
              range group 'sandy clay'
zone property density 1800.0 bulk=1E8 shear=3E7 ... 
              cohesion 10000.0 friction 25.0 dilation 0.0 tension 0.0 ...
              range group 'silty clay'
              
zone face skin
zone face apply velocity-x 0 range group 'East' or 'West'
zone face apply velocity (0,0) range group 'Bottom'

model factor-of-safety filename 'nogeogrid'

geogrid1.dat

model restore 'mesh'
model gravity 9.81
model large-strain off

zone cmodel assign mohr-coulomb


zone property density 1800.0 bulk=1E8 shear=3E7 ... 
              cohesion 10000.0 friction 20.0 dilation 0.0 tension 0.0 ...
              range group 'sandy clay'
zone property density 1800.0 bulk=1E8 shear=3E7 ... 
              cohesion 10000.0 friction 25.0 dilation 0.0 tension 0.0 ...
              range group 'silty clay'
              
structure cable create by-line 2.0,10.0 15.0,10.0 segments 10 group 'geogrid1'
structure cable create by-line 6.0,6.0 19.0,6.0 segments 10 group 'geogrid2'
structure cable property cross-sectional-area=5.0e-3 young=1.04e11 ...
    yield-tension=1.0e7 yield-compression 0 grout-friction 10 ...
    grout-stiffness=3.46666675E9 grout-cohesion=1000 grout-perimeter 2.01

zone face skin
zone face apply velocity-x 0 range group 'East' or 'West'
zone face apply velocity (0,0) range group 'Bottom'

model factor-of-safety filename 'geogrid1'
    

geogrid2.dat

model restore 'mesh'

model gravity 9.81
model large-strain off

zone cmodel assign mohr-coulomb

zone property density 1800.0 bulk=1E8 shear=3E7 ... 
              cohesion 10000.0 friction 20.0 dilation 0.0 tension 0.0 ...
              range group 'sandy clay'
zone property density 1800.0 bulk=1E8 shear=3E7 ... 
              cohesion 10000.0 friction 25.0 dilation 0.0 tension 0.0 ...
              range group 'silty clay'
              
structure cable create by-line 2.0,10.0 15.0,10.0 segments 10 group 'geogrid1'
structure cable create by-line 6.0,6.0 19.0,6.0 segments 10 group 'geogrid2'
structure cable property cross-sectional-area=5.0e-3 young=1.04e11 ...
    yield-tension=1.0e7 yield-compression 0 grout-friction 10 ...
    grout-stiffness=3.46666675E9 grout-cohesion=10000 grout-perimeter 2.01

zone face skin
zone face apply velocity-x 0 range group 'East' or 'West'
zone face apply velocity (0,0) range group 'Bottom'

model factor-of-safety filename 'geogrid2'

    

Endnote

[1]

This example is listed under the present document title in the i Examples dialog (use the menu command Help -> Examples… to access).