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05 - Successive Failure

1.0 Introduction

Successive failure of a rock slope refers to a phenomenon where multiple failures occur in a rock slope in quick succession, leading to a large-scale slope failure. This type of failure can occur when initial failures trigger additional failures as blocks are further destabilized. This analysis is important for many reasons including:

  • Identifying key blocks. A key block is a block whose removal is required for the subsequent destabilization of larger masses of blocks. If key blocks are stabilized, then the slope is stabilized.
  • Identifying the total failure depth behind key blocks. Bolt supports must have adequate length to anchor blocks into competent rock mass.
  • Identifying extent of failure volumes due to successive failure of the slope.

This tutorial covers how to utilize the Successive Failure analysis options in RocSlope, and view the failure iteration and failure depth of blocks.

Finished Product

The finished product of this tutorial can be found in the Successive Failure.rocslope_model file. All tutorial files installed with RocSlope can be accessed by selecting File > Recent Folders > Tutorials Folder from the RocSlope main menu.

2.0 Opening the Starting File

  1. Select File > Recent > Tutorials Folder.
  2. Go to the Successive Failure folder and open the file Successive Failure.rocslope_model.

This model already has the following defined and provides a good starting point to start computing blocks:

  • Project Settings
  • Material Properties
  • External Geometry
  • Joint Properties
  • Measured Joints
3d view of starting geometry
3D View of Successive Failure file with Measured Joints

In the starting model, Measured Joints are defined over the west extents of open pit. The size of the joints are relatively large (Radius = 500 m) so blocks have the potential of forming deep behind the free surface of the slope, some completely behind daylighting blocks.

2.1 Project Settings

Review the Project Settings.

  1. Select Analysis > Project Settings project settings icon
  2. Select the Units tab. Ensure Units are Metric, stress as MPa.
    project settings dialog units
    Units tab in Project Settings dialog
  3. Select the Analysis tab.
    1. Ensure Design Factor of Safety = 1.5.​
    2. Ensure Successive Failure is ON. We will be analyzing successive failures by iterating through each stage of failed blocks until no more blocks can be removed.
    analysis tab in project settings
    Analysis tab in Project Settings dialog
  4. Click Cancel to exit the dialog.

2.2 Material Properties

Review the Material Properties.

  1. Select Materials > Define Materials define materials icon
  2. One (1) material property is defined in the Define Materials dialog. The Schist material property has:
    1. Unit Weight = 0.026 MN/m3.
    2. No Water Surface applied.
    define materials dialog schist property
    Schist Material Property in Define Materials dialog
  3. Click Cancel to exit the dialog.

2.3 External Geometry

The External is of a pit shell and composed of one volume assigned with the Schist material property.

2.4 Joint Properties

Review the Joint Properties.

  1. Select Joints > Define Joint Properties define joint properties icon. Two (2) joint properties are defined.
  2. The Smooth joint property has:
    • Strength Type = Mohr-Coulomb
    • Cohesion = 0 MPa
    • Phi = 10 degrees
    • Waviness = 0 degrees
    • smooth joint property
      Smooth Joint Property Strength tab in Define Joint Properties dialog
    • Water Pressure Method = Dry
    • smooth joint property water parameters
      Smooth Joint Property Water Parameters tab in Define Joint Properties dialog
  3. The Rough joint property has:
    • Strength Type = Mohr-Coulomb
    • Cohesion = 0 MPa
    • Phi = 30 degrees
    • Waviness = 0 degrees.
    • rough joint property strength tab
      Rough Joint Property Strength tab in Define Joint Properties dialog
    • Water Pressure Method = Dry
    • rough joint property water parameters
      Rough Joint Property Water Parameters tab in Define Joint Properties dialog
  4. Click Cancel to exit the dialog.

2.5 Measured Joints

Review the Measured Joints.

  1. Select Joints > Define Measured Joints define measured joints icon. 40 Measured Joints are defined.
  2. Listed in order of Dip, Dip Direction, X, Y, Z, Radius, and Joint Property:

    DipDip DirectionXYZRadiusJoint Property
    59 48 -390 -115 1018 500 Rough
    77 131 -319 -337 1004 500 Rough
    78 123 -313 -157 1003 500 Rough
    78 313 -486 128 1037 500 Rough
    61 13 -793 -447 1099 500 Rough
    89 255 -272 -65 994 500 Smooth
    51 341 -375 -594 1015 500 Smooth
    67 107 -548 47 1050 500 Smooth
    57 86 -281 91 996 500 Smooth
    82 105 -758 33 1092 500 Smooth
    53 62 -546 -100 1049 500 Smooth
    46 84 -665 -224 1073 500 Smooth
    55 79 -772 -386 1094 500 Smooth
    29 223 -798 -400 1100 500 Smooth
    50 82 -305 193 1001 500 Smooth
    34 33 -699 -504 1080 500 Smooth
    39 57 -348 -272 1010 500 Smooth
    81 36 -258 -51 992 500 Smooth
    58 314 -544 -193 1049 500 Smooth
    39 211 -744 -599 1089 500 Smooth
    81 268 -797 108 1099 500 Smooth
    65 336 -455 104 1031 500 Smooth
    78 317 -575 -54 1055 500 Smooth
    62 133 -656 -479 1071 500 Smooth
    27 283 -769 -258 1094 500 Smooth
    30 167 -458 -465 1032 500 Smooth
    33 132 -253 -539 991 500 Smooth
    60 336 -580 80 1056 500 Smooth
    30 39 -583 168 1057 500 Smooth
    28 76 -362 -30 1012 500 Smooth
    17 101 -362 -432 1012 500 Smooth
    10 249 -511 -162 1042 500 Smooth
    19 343 -631 112 1066 500 Smooth
    45 58 -789 -329 1098 500 Smooth
    26 213 -628 60 1066 500 Smooth
    84 351 -516 -421 1043 500 Smooth
    51 162 -339 -320 1008 500 Smooth
    49 178 -253 -309 991 500 Smooth
    77 291 -403 114 1021 500 Smooth
    54 51 -417 -421 1023 500 Smooth
    define measured joints dialog
    40 Joints in Define Measured Joints dialog
  3. Click Cancel to exit the dialog.

3.0 Compute

RocSlope has a two-part compute process.

3.1 Compute Blocks

The first step is to compute the blocks which may potentially be formed by the intersection of joints with other joints and the intersection of joints with the free surface.

To compute the blocks:

  1. Navigate to the Compute workflow tab compute workflow tab
  2. Select Analysis > Compute Blocks compute blocks icon

As compute is run, the progress bar reports the compute status. Once compute is finished, the Results node is added to the Visibility Tree and All Valid Blocks are blocks are shown in the viewport. The Results node consists of the collection of valid blocks and the socketed slope. The original External and Measured Joints visibility is turned off.

compute blocks 3d view
3D View of all Valid Blocks

Once compute is finished, the blocks are coloured according to the Block Color option (Random Colors) set in the Results node's Property pane.

Compute Blocks only determines the geometry of the blocks. In order to obtain other information such as the factor of safety, Compute Kinematics needs to be run.

3.2 Compute Kinematics

The second and final compute step is to compute the removability, forces, and factor of safety for each of the valid blocks.

To compute the block kinematics:

  1. Ensure that the Compute workflow tab compute workflow tab is the active workflow.
  2. Select Analysis > Compute Kinematics compute kinematics icon

As compute is run, the progress bar reports the compute status. By default, after Compute Kinematics is run, only Removable Blocks are shown.

compute kinematics 3d view
3D View of Removable Blocks only

In a Successive Failure mode of analysis, several Failure Iterations may be performed. In the first Failure Iteration, only immediately removable daylighting blocks are analyzed. Any blocks which have a factor of safety less than the Design Factor of Safety (set in Project Settings) is considered "failed". These blocks are also known as "key blocks". The stability of key blocks control the global stability of the slope. As key blocks fail and are detached from the slope, potential constraints in joint faces (in regards to removability and valid sliding direction) are removed and become free faces which no longer support the block or provide shear resistance to sliding. As such, the failure proceeds in a successive manner. See the Analysis topic for more information on Successive Failure.

Successive Failure analysis in RocSlope does not model the interaction between blocks; the unravelling mechanism is much more complex. Instead, RocSlope looks at each block in isolation during each Failure Iteration.

4.0 Interpreting Results

Once both blocks and kinematics are computed, all block results can be viewed in a table format.

    4.1 Block Information

    To view all block results:

    1. Navigate to the Results workflow tab results workflow tab
    2. Select Interpret > Block Information block info icon

    Visualizing blocks can be difficult when the slope extents are large compared to the block extents.

    To zoom into all blocks:

    • Select Interpret > Zoom To All Blocks zoom all blocks icon

    The Block Information pane shows the collection of blocks according to the Results Set settings. The Results Set shown can be selected in the Results tab of the Display Options, or the Properties pane for the Results Node. By default, only Removable Blocks are coloured. In the case of Successive Failure, it may be more beneficial to only colour the failed blocks. Failed blocks are blocks which have a factor of safety less than the Design Factor of Safety.

    To colour Failed Blocks Only:

    1. Select View > Display Options display options icon
    2. Navigate to the Results tab in the Display Options dialog.
    3. Set Results Set = Failed Blocks (FS < Design FS)
    4. Click OK to close the dialog.

    OR

    1. Select the Results node from the Visibility Tree.
    2. In the Results node Properties pane, set Results Set = Failed Blocks (FS < Design FS).

    Failed Blocks Only are now coloured according to Random Colors.

    failed block information
    Block Information pane showing only Failed Blocks
    A failed block is inherently geometrically removable, but a removable block may not fail. The removability is geometry-dependent, while factor of safety is force-dependent.

    Note that the Failure Iteration column is visible for a Successive Failure analysis. The Failure Iteration indicates the the step in which the block fails. It is not a true representation of the complex unraveling mechanism, but rather for notation and filtering.

    4.2 Edit Filters

    To see the progressive unravelling of the slope, we can toggle the Failure Iteration quick filter under the Filter Options > Edit Filters section.

    1. At the top of the Block Information pane, click Filter Options Filter Options icon
    2. Expand the Edit Filters section.
    3. For the Failure Iteration quick filter, increase the minimum value from 1 to 2 and hit ENTER. All blocks which are immediately removable and have failed (i.e., key blocks) are filtered out and hidden in the viewport.
    4. block information filtered
      3D View and Block Information pane showing Failed Blocks with key blocks filtered out
    5. Step 2 can be repeated to show the successive failure of the slope.
    6. Select the Clear Filter button to remove the filter.

    Alternatively:

    1. Hover the mouse over the Failure Iteration header in the Block Information listing and click the small filter symbol filter icon that appears.
    2. In the Filter Rules tab, set the operator to Is greater than and the value to 1; OR
    3. In the Filter Values tab, select all checkboxes except leave 1 unchecked. The key blocks are hidden.
    4. Select the Clear Filter button in the pop-out dialog to remove the filter.

    4.3 Contour Blocks

    In RocSlope, blocks can be contoured by several metrics. In a Successive Failure analysis, the Failure Iteration and Failure Depth are especially relevant.

    To show block contours:

    1. Select Interpret > Contour Blocks contour blocks icon
    2. From the Contour pane on the right of the screen, select Failure Iteration. Blocks are coloured by the failure iteration that it belongs to. This is also an easy way to quickly identify key blocks and visually see the effects of block failures on adjacent blocks.
      blocks contoured by failure iteration
      3D View of Failed Blocks contoured by Failure Iteration
    3. From the Contour pane on the right of the screen, select Failure Depth. The daylighting blocks are contoured by the maximum Failure Depth, measured from the free surface to the deepest failed block, in a direction normal to the free surface. This metric is a good indicator of the minimum bolt length required to anchor into competent rock if bolts are installed normal to the slope surface.
      blocks contoured by failure depth
      3D View of Failed Blocks contoured by Failure Depth
    When visualizing All Valid Blocks or Removable Blocks results sets, some blocks may have a Failure Depth of 0. Blocks with a Failure Depth of 0 are either non-daylighting blocks or daylighting blocks which are safe (i.e., Factor of Safety >= Design Factor of Safety)

    This concludes Tutorial 05.

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