tutorial 06 intersecting tunnels

Intersecting Tunnels Tutorial 6-1

Intersecting Tunnels Tutorial

Unwedge is generally used to examine the safety of a single tunnel. However it is possible to simulate two or more intersecting tunnels with a simple trick. This tutorial demonstrates how to model a drawpoint and adjacent drift in a hard rock mine. This junction is assumed to be a high traffic area so support is applied to achieve a design factor of safety of 2.0.

Topics Covered

• Intersecting tunnels

• Importing dxf

• Scaling wedge size

• Bolt support

Unwedge v.3.0 Tutorial Manual


Model

Select Project Settings from the toolbar or the Analysis menu.

Select: Analysis → Project Settings

In the Project Settings dialog, make sure that the units are Metric, stress as tonnes/m2. Also ensure that the box is checked for Compute End Wedges. Select OK.

For this tutorial we will start by reading in an Autocad DXF file which contains the geometry of the intersecting tunnels.

Select: File → Import → Import DXF

Or click the Import DXF button on the sidebar. Navigate to the Examples > Tutorials folder in your Unwedge installation folder and open the Tutorial 06 Drawplan.dxf file.

The model should appear as follows:



Tunnel Orientation Switch to the 3D Wedge View.

Select: View → Select View → 3D Wedge View

As you can see, by default, Unwedge assumes that the tunnel is horizontal. Since we have imported a plan view, we need to rotate the tunnel 90°.

Under Tunnel Axis Orientation on the sidebar, set the Plunge to 90°. You will now see the two tunnels oriented such that the main drift is heading North. If your actual tunnel is heading in a different direction you must change the Trend. For example, to set the main drift heading East, set the trend to 90°.

For this tutorial, we will assume that the main tunnel is heading North so leave the Trend at 0°. You will now see the correct 3D representation of the tunnels. The geometry will be clearer if you rotate the Perspective view approximately 90° as shown.

TIP: To look directly down the tunnel in the Persepctive view, click in the Perspective view and then click the button for Reset Tunnel Rotation just to the right of the Wedge Translation slider in the sidebar.

The height of the tunnel appears significantly too large. This will be addressed later by scaling wedges.

Note that this analysis assumes that the roof and floor of the tunnel are flat. You CANNOT define the intersection of tunnels with a curved roof, because then this becomes a full 3-dimensional problem. For the analysis in this tutorial, the problem is simplified to assume that the roofs of the tunnels are flat.



Input data Open the input data dialog.

Select: Analysis → Input Data

Now enter the data by following these steps:

1. Ensure the General tab is selected. Since miners are constantly at work in this area of the mine, we want a high factor of safety. Set the Design Factor of Safety to 2.0.

2. Select the Joint Orientations tab in the Input Data dialog. By default, 3 joint orientations are already defined. Enter the Dip and Dip Directions for the joint sets as shown.

3. Select the Joint Properties tab in the Input Data dialog. For Joint Properties 1, enter Phi = 30 and Cohesion = 0. Note that all three joint sets are assigned these properties by default.



4. Click OK to close the dialog.

Your screen should now look like this:



Scaling Wedges

In order to account for the actual height of the tunnel, you must use the Scale Wedges dialog, and enter the actual tunnel height. This will calculate the actual maximum wedge size for the tunnel height.

To scale down the size of wedges, select the Scale Wedges option from the Analysis menu.

Select: Analysis → Scale Wedges

You will see the Scale Wedges dialog. Let’s assume that the height of the tunnel is 5 m. Therefore check the box next to Tunnel Height and set the scaling value to 5 m.

Your screen should now look like this:



Now you can see that the wedges have a maximum height of 5 m. However, the graphical display of the tunnel "height" will still appear too large (i.e. larger than the tunnel height you have defined). This is a display issue only – just remember that the wedge size corresponds to your true tunnel height, even though the graphical tunnel height will appear larger.

Perimeter Wedge Support You will see in the sidebar that all of the wedges have a factor of safety below 1. Recall that we want a factor of safety of at least 2. This can be easily accomplished by adding a few spot bolts.

Switch to the Perimeter Support Design view.

Select: View → Select View → Perimeter Support

We will support the perimeter wedges using cable bolts so click on the sidebar button for bolt properties

Select: Support → Bolt Properties

Change type to Cable Bolt as shown.



Click OK. Now you can simply add one or two bolts through each wedge to stabilize them.

Select: Support → Add Spot Bolt

You will see the Add Spot Bolt dialog, which allows you to choose the bolt property type and length. The maximum apex height for the perimeter wedges is less than 1.5 meters so the default 2 meter bolts should be sufficient.

Click OK to use the default Bolt properties.

Now click on the perimeter such that the bolt goes through one of the wedges. Repeat these steps until wedges 5 and 7 are supported by one bolt and wedges 2 and 4 are supported by 2 bolts. The screen should look like this:



You can see in the sidebar that the factor of safety for all perimeter wedges is now greater than 2, thus achieving the design factor of safety for these wedges.



Roof Wedges

To see the wedges that form at the top and bottom of the tunnel intersection, we need to look at the end wedges.

Select: View → Select View→ End Wedges

You can rotate the views to get a realistic view of the roof and floor wedges like this:

The maximum wedge found in the roof of the excavation is very large with an apex height of approximately 13 meters and a weight of almost 3000 tonnes. However, the largest trace length actually observed underground was 10 meters in length so we can scale the wedge to reflect this.

Select: Analysis → Scale Wedges

You will see the Scale Wedges dialog. At the bottom, change the Wedge to Scale to Roof. At the top under Trace Lengths, select all joints and enter 10 m for the Scaling Values as shown.



Click OK to close the dialog.

You will now see that the wedge is now ~ 150 tonnes. This can be supported by shotcrete or cable bolts. Using the default shotcrete, the layer of shotcrete would have to be 40 cm thick to support the wedge with a factor of safety of 2, which is required because the drawpoint and drift are commonly travelled working areas and must be safe. Alternatively, the wedge could be supported with a pattern of 8 meter long, end-plated cable bolts installed at 1.3 meter by 1.3 meter centres.

We will choose the cable bolt support option. To do this, first switch to the End Support Designer.

Select: View → Select View → End Support

Ensure that the End Wedge Visibility in the sidebar is Roof. We will support the roof wedge using the same cable bolts as used for the perimeter wedges so there is no need to change the bolt properties. Now add an array of bolts by selecting the Add Pattern button.

Select: Support → Add Bolt Pattern

Set the length to 8 m and the vertical and horizontal spacing to 1.3 m as shown.



Note that the maximum apex height of end wedges may be greater than 8 m. However, this maximum is for the floor wedge, which we do not care about. Therefore 8 m bolts are fine. Click OK. You will now see an array of cable bolt locations. Click anywhere on the screen to place the array. Your screen should now look like this:

You can see in the sidebar that the factor of safety for the roof wedge is now greater than 2.



3D Wedge Views Return to the 3D Wedge View. We don’t care about the floor wedge so we can turn it off by going to Wedge Visibility and selecting User Defined. Click on the three dots to the right of the pull-down menu.

Uncheck 10 Floor and click OK. The screen should now look like this:

Check that all wedges are within the desired factor of safety by going to Wedge Visibility and selecting FOS < Design. You should now see no wedges.

That concludes this tutorial on how to model intersecting tunnels.


tutorial 06 intersecting tunnels

Documents

main tunnel

intersection of tunnels

tunnel rotation

actual tunnel

single tunnel

d wedge view

perspective view

analysis input data