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Open Pit Stability Analysis

1. Introduction

This tutorial will demonstrate how an open pit mine can be modelled by importing surface and sub-surface geometry from DXF and STL files.

Want to watch the video version of this tutorial? Check out our Youtube channel here.

Select File > Recent > Tutorials and read in the file Open Pit Stability Analysis – starting file from the installation folder.

This file contains all of the required material properties but does not contain the model geometry.

2. Import DXF

Select Geometry > Import/Export > Import Geometry

The ground surface and pit geometry have been stored in an *.STL file. Open the file 'Open Pit Geometry.stl' (3D Object).

Select the layers in the Filter Entities dialog by clicking the Open Pit Geometry.stl dropdown option.

Make sure the layers are selected with checkmarks on the box as shown in the dialog. You should see the following:

Import Geometry dialog
Import Geometry dialog

Select Post-Processing to select the Find and Repair Defects.

Import Geometry dialog
Import Geometry dialog
Import Geometry dialog
Import Geometry dialog

Select Done to import the file Open Pit Geometry.stl.

Select the Geometry > Import/Export > Import Geometry Import option again and import the file Open Pit Layer1.dxf and Open Pit Layer2.dxf.

You should see two more surfaces representing the boundaries between material regions.

Import Geometry dialog - Open Pit Layers 1 and 2
Import Geometry dialog - Open Pit Layers 1 and 2

Press Post-processing and Select Find and Repair Defects.

Import Geometry dialog - Find and Repair Defects
Import Geometry dialog - Find and Repair Defects

Select Done once the find and repair defect has finished.

3. Create External Volume

In the sidebar Visibility pane, select the first entity at the top of the list (Open Pit Geometry). This is the pit surface imported from the STL file.

Visibility Tree
Visibility Tree

From the Geometry menu, select the Create External from Surface option. Select OK without changing the default values in Create External from Surface dialog. You may then have to wait a few seconds.

Create External From Surface dialog - Elevation tab
Create External From Surface dialog: Elevation tab

The External Volume should be created by extruding downwards from the outer perimeter of the pit surface geometry

3.1 DIVIDE ALL

Select Geometry > 3D Boolean > Divide All Geometry

This will intersect the two material surfaces with the External Volume. The model should appear as follows:

Model (2x2 view)
Model (2x2 view)

4. Assign Materials

In the sidebar Visibility pane you should see the following entities:

Visibility Tree
Visibility Tree

The Divide All operation has created an External Volume which consists of four different volume regions. We need to assign the correct material properties to each external volume region.

The model layer order may differ in visibility pane depending on the order of the layer import or extrusion. Click on the first/last entity in the visibility tree list (this should be one of the very top layer). In the Properties pane, notice that the default material assignment is Sandstone. We need to change this to Limestone. In the Properties pane, change the Applied Property to Limestone. Make sure this is applied to the top layer of the model.

Properties pane
Properties pane

Click on the third entity (top layer) in the list and also assign the Limestone material property. We have now assigned Limestone to the upper material layer of the model.

Click on the second volume entity (very bottom layer). This is already assigned the default Sandstone assignment, which is correct, so you do not have to change the material assignment.

Click on the top volume entity (middle layer) and assign the Ore material.

Visibility tree
Visibility tree

The material assignments are complete. Save the file with a new file name. The model should appear as follows:

Model View after Assigning Materials
Model View after Assigning Materials

4.1 ASSIGN MATERIALS USING A CUTTING PLANE

We will now mention an alternative method of assigning materials which can be very useful for assigning material properties to complex 3D models.

Select Materials > Assign Materials Using Cutting Plane

You will see the following dialog, and a moveable cutting plane will be displayed in the model views.

Assign Materials using Cutting Plane dialog
Assign Materials using Cutting Plane dialog

For complete flexibility the cutting plane can be moved or rotated to any angle using the Freehand Manipulation tool, to view any section of the model as shown below.

This is left as an optional exercise after completing this tutorial. Close the Assign Materials Using Cutting Plane dialog, and do not change the material assignments.

5. Slip Surface Options

Select Surfaces > Slip Surface Options

Slip Surface Options dialog
Slip Surface Options dialog

We will use the default settings (Ellipsoid, Cuckoo Search, Surface Altering). Select Cancel.

6. Compute

Save the model.

Select Analysis > Compute

Both Janbu and GLE analysis methods have been selected in Project Settings. The analysis should take a few minutes.

7. Results

Select the Results workflow tab

Select Show Contours. You should see the following:

Analysis Results (Number of Nests = 20)
Analysis Results (Number of Nests = 20)

The safety factor of the critical slip surface using the GLE analysis method is shown above.

8. Search Options

When using the default Slip Surface and Search settings, you should not assume that you have located the true Global Minimum surface on the first analysis.

Select Surfaces > Slip Surface Options

In the Slip Surface Options dialog, increase the Number of Nests for the Cuckoo Search to 50. In the Cuckoo search setting, increasing the number of nests will increase the chance of finding a surface with a lower factor of safety but it will take a longer time to compute. Select OK.

Cuckoo Search for Number of Nests = 50
Cuckoo Search for Number of Nests = 50

Select Compute and re-run the analysis.

In this case, the Global Minimum slip surface and the safety factor are similar to the previous analysis with 20 nests. However, in general, you should try different search options and parameters to verify the global minimum.

Since the nests of the Cuckoo search is set to 50 after adding the search volume, it'll take more computation time than keeping the number of nests to a smaller value.
Analysis Results (Number of Nests = 50)
Analysis Results (Number of Nests = 50)

9. Creating a Safety Map

After re-running the analysis and obtaining the results, create a safety map.

Select Interpret > Add Surface Safety Map

By selecting this feature, you can see the safety map of the computed model.

Surface Safety Map in 2x2 View
Surface Safety Map in 2x2 View
Surface Safety Map in 3D Perspective
Surface Safety Map in 3D Perspective

10. Additional Exercises: Search Limits

As an additional exercise, use the search limits option to define a volumetric box which limits the search area to one side of the open pit and re-run the analysis, and compare the result.

Select: Surfaces > Add Search Limit > Add Search Limit by Box

You will then be prompted to define the search area of interest. Select the area in RED, having the minimum safety factor, OR enter the following XYZ values:

Defining the Search Limits
Defining the Search Limits
Add Search Limit dialog
Add Search Limit dialog

Select OK twice to close both dialogs.

Select Compute and re-run the analysis.

Define Search Limits - Analysis Results
Define Search Limits by Volumetric Box - Analysis Results
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