Most popular FAQs
- The Swedish Method can be used on circular slips, gives a more conservative FoS and is not suitable for submerged slopes or those with horizontal loads.
- Bishop’s Method is the most widely used and is suitable for all circular slips.
- Janbu’s Method is most widely used for non-circular slips
An explanation of the theory behind the Methods of Analysis used in Slope can be found in Section 2 of the Slope Manual.
With the default option of unchecked “anchor force is applied as surface load” box (BS8006 calculation method)
- For a given slip circle, Slope looks for the minimum anchor capacity based on pullout, tensile failure or stripping;
- Resolves it parallel to the slip surface at the point at which the anchor crosses the slip surface in order to derive a simple restoring moment;
- Adds this moment to the other restoring moment derived from slope stability theory to give a total restoring moment;
With the “anchor force is applied as surface load” box checked
- slope again calculates the minimum anchor capacity but applies it as a surface load;
- This means that it is worked through the method of slices calc, ultimately resulting in an increased normal force on the failure surface, and hence a greater restoring moment by virtue of increased friction;
- There is no explicit calculation of moment due to the reinforcement.
This means that the surface load option will tend to obtain a higher factor of safety and the BS8006 calculation is acknowledged to be conservative. In essence, the two solutions give the following results:
Default option: MSu/F + Mn/F = Mw
Surface load option: MSu/F + Mn = Mw
where Msu is soil restoring moment, Mw is soil disturbing moment, Mn is moment due to nail forces.
Points can be deleted by placing the cursor over the required point and clicking the left mouse button at the same time as holding down the Shift key on the keyboard.
Whole strata can be deleted by selecting the stratum and then the delete button on the graphics toolbar.
The Spencer method is the same as Bishop with constant inclination interslice forces.
The Slope methods with inclined interslice forces satisfy overall horizontal, vertical and moment equilibrium, and those with varying inclinations of interslice forces also satisfy horizontal and vertical equilibrium of each slice.
The Morgenstern-Price method is simply a way of deciding where the interslice forces act, and it can be difficult to decide this adequately. We feel that as the same equilibrium conditions are satisfied by the variably inclined interslice force methods in Slope there is little advantage in trying to add these methods. In practice they are found to give answers within about 5% of each other.
You can model an artesian groundwater basin by double clicking ‘Groundwater’ in the Gateway and entering the top of the artesian groundwater table and any other groundwater tables.
Then double-click on ‘Strata’ in the Gateway and only define perched water table under ‘GW surface’ for the strata that it appears in.
In the ‘Analysis Method’ dialog you can select how you would like the calculated factor of safety to be applied.
- ‘Shear Strength’ will apply to the soil strength
- ‘Disturbing surface loads’ will multiply the loads by the FoS to cause failure, representing bearing capacity problems
- ‘Restoring surface loads’ can be used to determine the required reinforcement.
For more details, please see Section 3.3.3 of the Slope Manual.
It is possible to analyse several different groundwater cases in one model.
Double click ‘Ground water’ in the Gateway and add the coordinates for your original groundwater profile. Then select the ‘Add GW profile’ tab and enter the coordinates of your draw down profile.
You can choose which groundwater case you want to analyse by double clicking on ‘Strata’ in the Gateway and selecting the appropriate ‘GW Surface’ that you have previously entered in.
For more information have a look at Sections 3.3.6-3.3.8 in the Slope Manual.
At the moment partial factors from British standards and Eurocode 7 are built-in to the program. These are:
- SLS – Serviceability Limit State
- BS8006 – Reinforced Soil
- BS8006Sec6 ULS – Design of walls and abutments
- BS8006Sec7ULS – Design of Reinforced Slopes
- BS8006Sec8ULS – Design of embankments with reinforced soil foundations on poor ground.
- BS6031 – Code of practice for earthworks
- BS8081 – Code of practice for ground anchors
- EC7DA1-1 – Eurocode for Geotechnical Design Approach 1 Combination 1.
- EC7DA1-2 – Eurocode for Geotechnical Design Approach 1 Combination 2.
However if you want to enter your own country codes double-click on ‘Method Partial Factors’ in the Gateway. Enter your partial factors, give them another name and select ‘Add’. This will save your sets of partial factors for future use.
It is very important to remember that Slope only analyses overall stability. Representation of the shape of a wall and providing parameters (e.g. high cohesion) just attempts to keep the slip surfaces from passing through it. The behaviour of the retaining wall and other possible failure mechanisms must be considered separately.
So although, retaining walls themselves can’t be put into Slope, you can represent the material that the retaining wall is made from. To do this you would select ‘Materials’ in the Gateway and enter a material with the same properties such as unit weight, cohesion and friction, as your retaining wall.
You will then need to double-click on ‘Strata’ in the Gateway and enter the coordinates in the shape of the retaining wall. Note that strata can not cross each other, vertical surfaces need to be represented slightly off vertical and you need to define the strata the full length of the section. To ensure that the slip circle doesn’t pass through the wall you can double click on ‘Slip Surfaces’ in the Gateway and enter a ‘Common point’ that you expect the slip will go through, ie. somewhere near the toe of the retaining wall.
1. The reinforcement does not intersect the slip circle. In this case it will be drawn in grey.
2. If both sides of an embankment are included, the program may not position the reinforcement correctly because it is only specified by level and offset from the ground surface.
3. The reinforcement has been set as Inactive in the Analysis Options dialog. This can be useful if you want to compare FoS with and without reinforcement.
Reinforcement is not considered in non-circular slips because the method is not covered in the British Standard. We may be adding it in the future.
The effect of reinforcement can be input into non-circular slips as a surface load.
Yes. Double-click on “Slip surfaces” in the Gateway.
Here you can enter the coordinates of your first slip. At the bottom of the worksheet select the ‘Add slip’ tab which will allow you to add any further slip coordinates that you wish to analyse. The number of non-circular slips is not limited by the program.
NB to view non-circular slips other than the most critical (which will be shown by default) click the button which will open a table of available slip surfaces – select the one you want to plot and the graphical output view will be updated.
You can enter the slips graphically by going to the graphical input view and selecting the ‘Non-circ’ button in the top toolbar
You can then click to create points on the non-circular slip that you’d like to analyse.
To enter additional non-circular slips graphically, go to the drop down box in the graphical input view and select ‘<New…>’.