Block Search

A Block Search is one of the Search Methods which can be used in Slide for locating the Global Minimum safety factor for NON-CIRCULAR slip surfaces. In order to perform a Block Search, the Surface Options dialog must be configured as follows:

NOTE that the term Block Search is used, because the sliding mass can be considered as consisting of active, passive and central "blocks" of material, as shown in the figure below.

Active, central and passive blocks

image\blocksearch2.gif

Block Search Objects

A Block Search requires one or more Block Search objects to be defined by the user. The Block Search objects are used to generate the locations of slip surface vertices.

A Block Search object can be:

For details about creating Block Search Objects, see the Block Search Objects topic and the Block Search Polyline topic.

How Does the Block Search Work?

Slide performs the Block Search using:

as described below.

Block Search Windows, Lines, Points

If your search uses Block Search Windows, Lines or Points:

    1. For EACH Block Search object, a single point is randomly generated (i.e. within a window object, or along a line object. If a point search object is used, then the exact point is used).

    2. The points generated by the search objects, are then joined by straight line segments.

NOTE: if more than two Block Search objects have been used, the points are first sorted by X-coordinate, to ensure that the slip surface is a kinematically admissible surface i.e. it does not reverse direction at any point.

    1. The Left and Right Projection Angles (entered in the Surface Options dialog, see below for details) are then used, to project the slip surface up to the ground surface, from the leftmost and rightmost points generated by the Block Search objects.

    2. Steps 1 to 3 are repeated, for the Number of Surfaces specified in the Surface Options dialog.

Block Search Polylines

A Block Search Polyline works differently than other Block Search objects. It allows you to easily perform a slip surface search which can follow a weak layer of material. For a Block Search Polyline:

    1. TWO points are randomly generated along the polyline.

    2. The slip surface is constrained to follow the polyline between the two points.

    3. If NO other search objects are defined (windows, lines or points):

      • then the Left and Right Projection Angles are used to project the slip surface up to the ground surface, from the Left and Right points generated on the polyline.

    4. If you are using a Block Search Polyline with other search objects (window, line or point):

      • the portion of the slip surface which is generated by the Block Search Polyline, will be incorporated into the overall surface, including the vertices generated by the other search objects.

      • The Left and Right Projection Angles are used to project the slip surface up to the ground surface, from the leftmost and rightmost points generated by all Block Search objects.

For more information, see the Block Search Polyline topic.

NOTE: each slip surface thus generated, must intersect the slope surface within the Slope Limits, or the slip surface will not be analyzed. See the Slope Limits topic for details.

Multiple Groups

The Multiple Groups option allows you to simultaneously search different regions of a model (e.g. multiple weak layers) using a single Block Search. This works as follows:

  1. Select the Multiple Groups checkbox in the Surface Options dialog.

  2. When you define a Block Search object (window, line, point or polyline) you will be able to assign a Group ID number to the object.

  3. When the analysis is run, the Block Search will be carried out independently for EACH GROUP of Block Search objects, according to the algorithm described above.

  4. For example, if you wanted to simultaneously search two (or more) weak layers, you could define a Block Search Polyline in EACH weak layer, and assign a different Group ID to each polyline.

NOTE:

Left and Right Projection Angles

The definition of the Left and Right Projection Angles used for the Block Search, is shown in the following figure.

image\proj_ang2a.gif

NOTE:

 

Left Projection Angle

Right Projection Angle

Minimum Start Angle

95

5

Maximum End Angle

175

85

Allowable Projection Angle Ranges for a typical Block Search

image\blocksearch5.gif

However, sometimes it is necessary to search for slip surfaces which daylight into the slope face, and have a downward dip angle (for example, to follow a weak layer which dips downwards into the slope face). For such cases, the allowable Start and End angles are as follows:

 

 

Left Projection Angle

Right Projection Angle

Minimum Start Angle

95

– 85

Maximum End Angle

265

85

 

Maximum Allowable Projection Angle for downward dipping surfaces

image\blocksearch8.gif

Convex Surfaces Only

image\surfopt_dlg9a.gif

During the Block Search surface generation process, it is possible (depending on the number and relative placement of your Block Search objects) that NON-CONVEX slip surfaces may be generated – i.e. the EXTERNAL angle between any two adjacent line segments of a slip surface, is LESS THAN 180 degrees, as illustrated in the above diagram from the Surface Options dialog.

If you DO NOT want Slide to analyze such surfaces, then select the Convex Surfaces Only checkbox in the Surface Options dialog. In this case, during the Block Search surface generation process, only CONVEX surfaces will be accepted – i.e. the EXTERNAL angle between any two adjacent line segments of a slip surface is GREATER THAN (or equal to) 180 degrees. If a NON-CONVEX surface is generated, it will be discarded. Therefore only CONVEX surfaces will be analyzed.

image\surfopt_dlg9b.gif

Optimize Surfaces

A supplementary optimization can be applied to slip surfaces, after the initial analysis, in order to locate slip surfaces with an even lower factor of safety. See the Optimize Surfaces topic for details.

Surface Filter

See the Surface Options topic for details about the Minimum Elevation, Minimum Depth, Minimum Area, Minimum Weight slip surface filter options.

Random Number Generation

The generation of slip surfaces by the Block Search is based on the generation of random numbers. For the Block Search, random numbers are used to generate:

The method of generating random numbers is determined by the Random Numbers option in the Project Settings dialog. There are two options available, Random or Pseudo-Random. Pseudo Random generation uses a constant seed value to generate random numbers. For a given model, this will generate exactly the same slip surfaces each time the analysis is run, and therefore the results will be reproducible. Random generation uses a variable seed value. This will generate DIFFERENT surfaces each time the analysis is run, and therefore results will be different each time Compute is selected. For details about the Random and Pseudo Random options, see the Random Numbers topic.