A Back Analysis in Slide allows you to calculate the total support force required to achieve a specified factor of safety. For details about the Back Analysis option, see the Back Analysis of Support Force topic in the Slide Model section of this Help system.
If a Back Analysis has been carried out, then the slip surface determined by the Back Analysis and the required support force, can be displayed on the model in the Slide Interpret program.
Show Back Analysis Surface
By default, the surface determined by a Back Analysis, will automatically be displayed when you open the file in Interpret.
The display of this surface, can be turned ON or OFF, by selecting the Show Back Analysis Surface option in the toolbar or the Back Analysis sub-menu of the Data menu.
NOTE: this is the slip surface which requires the MAXIMUM amount of stabilizing force in order to achieve the specified factor of safety, OF ALL SLIP SURFACES ANALYZED.
Active Force / Passive Force
The required support force, can be assumed to act in either a PASSIVE or ACTIVE manner, in the limit equilibrium calculations. For a discussion of the significance of ACTIVE and PASSIVE support force, see the Active / Passive Force Application topic, in the Slide Model section of this Help system.
When the Back Analysis is carried out by the Slide COMPUTE engine, the required support force is always calculated using BOTH assumptions. The Back Analysis is actually performed independently for each assumption (ACTIVE and PASSIVE).
When the slip surface determined by the Back Analysis is displayed on the model, the required support force is also displayed. A horizontal force arrow, at the specified elevation, will be displayed, as shown in the figure below.
Back Analysis slip surface and force displayed on model
By default, the PASSIVE force is initially displayed, when a file is opened in Interpret. Either the ACTIVE or PASSIVE force can be displayed on the model, by selecting the desired option (Active or Passive) from the toolbar (use the drop-down shortcut menu beside the Show Back Analysis Surface toolbar button), or from the Back Analysis sub-menu of the Data menu. NOTE:
In general, the PASSIVE required support force will always be greater than the ACTIVE required support force. See the Active / Passive Force Application topic for more information.
In most cases, the Back Analysis slip surface which is determined (for a given limit equilibrium analysis method), is the SAME SURFACE, for both ACTIVE and PASSIVE force assumptions. However, in general, this may not always be the case. Since the Back Analysis is performed independently for the ACTIVE and PASSIVE cases, it is possible that two different surfaces may be determined.
Support Design Using the Back Analysis Results
So how can the Back Analysis results be used to design a support system? The results might be used as follows:
The force magnitude determined by the Back Analysis, can be used to estimate the spacing and capacity of support.
Depending on the type of support, either the ACTIVE or PASSIVE force can be used. For example, the ACTIVE force might be applicable for tensioned rockbolts. The PASSIVE force might be applicable for soil nails. In general, the PASSIVE (larger) force assumption, will lead to a more conservative support design.
The slip surface which is determined, can be used to estimate the required LENGTH of support (i.e. the user should ensure that the support has sufficient embedded length beyond the slip surface, which can supply the required support force).
Summary of Results in Info Viewer
A summary of all Back Analysis results can be found in the Slide Interpret Info Viewer. The Active and Passive force required, for each limit equilibrium analysis method performed, will be listed in the Info Viewer.