Slope in a Closely Jointed Rock Mass (FLAC2D)

Example Application

This example reproduces example 1.4.6 from the FLAC/Slope 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

The effect of joint orientation on the stability of a rock slope is illustrated in this example. The rock mass contains a single closely spaced joint set. Three orientations of the jointing are investigated: joint angles of 10°, 45° and 135°, measured counterclockwise from the horizontal plane.

Model Generation

The model geometry is created using Sketch. After creating a new Sketch set, open the Slope Wizard and enter the following parameters:

Select Auto-size from the mesh drop-down menu and set the zone size to 1. Now select Mesh All Polygons . Finally, Click the button to Create Zones and save the project and the model state. Create a plot of zones to see the model (Figure 2). Optionally, you can also select the State Record tab above the console pane, right click, and select Save to file as Data File.

Figure 2: The zoned model

Properties and boundary conditions are specified with commands in data files (data files) The behavior of the jointed material is simulated with FLAC2D’s ubiquitous-joint model. The rock mass has the following intact material properties.

• density 2500 kg/m3

• cohesion (c) 0.1 MPa

• friction angle (φ) 45°

• tensile strength (\(σ_t\) ) 0.0

The joint properties are

• cohesion (\(c_j\) ) 0.01 MPa

• friction angle (\(φ_j\)) 40°

• joint tensile strength (\(σ_{jt}\) ) 0.0

The stability condition for a 20 m high 1:2 (H:V) slope is evaluated for the three jointing orientations.

Results

For comparison, a simulation is first made neglecting the presence of the jointing (i.e., by using the Mohr-Coulomb material model and the intact rock properties). The calculated factor-of-safety for this case is 2.19, and the failure surface results are shown in Figure 3.

When the jointing is included with a joint orientation of 10°, the effect of the jointing is negligible. The factor of safety for this case is 2.16, and the failure surface results are very similar to the Mohr-Coulomb material case (compare Figure 4 to Figure 3).

As the joint orientation angle is increased, the effect of the jointing becomes more evident. For the case of the 45° joint orientation, the factor of safety is 1.75, and the failure surface is more aligned with the angle of the slope, indicating a toppling mode of failure. See Figure 5. The failure mode now includes both shear and tensile failure along the joints. At a joint orientation of 135°, slip along the jointing is the predominant mode of failure. This is evident from the failure plot in Figure 6. The factor of safety in this case is 1.09.

Figure 3: Factor-of-safety plot for rock slope neglecting the presence of jointing – velocity vectors and shear strain contours.

Figure 4: Factor-of-safety plot for rock slope with jointing oriented at 10° - velocity vectors and shear strain contours.

Figure 5: Factor-of-safety plot for rock slope with jointing oriented at 45° – velocity vectors and shear strain contours.

Figure 6: Factor-of-safety plot for rock slope with jointing oriented at 135° – velocity vectors and shear strain contours.

Data Files

mohr-coulomb.dat

model new
program call 'mesh.dat' suppress

model gravity 9.81
model large-strain off

zone cmodel assign mohr-coulomb 

zone property density 2500.0 bulk 1E8 shear 3E7 ... 
              cohesion 100000.0 friction 45.0 dilation 0.0 tension 0.0
              
zone face skin
zone face apply velocity-x 0 range group 'East1' or 'West'
zone face apply velocity (0,0) range group 'Bottom'
model factor-of-safety filename 'mc'

ubi10.dat

model new
program call 'mesh.dat' suppress
model gravity 9.81
model large-strain off

zone cmodel assign ubiquitous-joint

zone property density 2500.0 bulk 1E8 shear 3E7 ... 
              cohesion 100000.0 friction 45.0 dilation 0.0 tension 0.0 ...
              angle=10.0 joint-cohesion=10000.0 joint-friction=40.0

zone face skin
zone face apply velocity-x 0 range group 'East1' or 'West'
zone face apply velocity (0,0) range group 'Bottom'

model factor-of-safety filename 'ubi10'

ubi45.dat

model new
program call 'mesh.dat' suppress
model gravity 9.81
model large-strain off

zone cmodel assign ubiquitous-joint

zone property density 2500.0 bulk 1E8 shear 3E7 ... 
              cohesion 100000.0 friction 45.0 dilation 0.0 tension 0.0 ...
              angle=45.0 joint-cohesion=10000.0 joint-friction=40.0 
              
zone face skin
zone face apply velocity-x 0 range group 'East1' or 'West'
zone face apply velocity (0,0) range group 'Bottom'
model factor-of-safety filename 'ubi45'

ubi135.dat

model new
program call 'mesh.dat' suppress
model gravity 9.81
model large-strain off

zone cmodel assign ubiquitous-joint

zone property density 2500.0 bulk 1E8 shear 3E7 ... 
              cohesion 100000.0 friction 45.0 dilation 0.0 tension 0.0 ...
              angle=135.0 joint-cohesion=10000.0 joint-friction=40.0

zone face skin
zone face apply velocity-x 0 range group 'East1' or 'West'
zone face apply velocity (0,0) range group 'Bottom'

model factor-of-safety filename 'ubi135'

Endnote

[1]

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