Multi-Directional 3D Stability Analysis

A feature of SVSlope® 3D Elite*

June 27, 2013

One of the early feature requests received by SoilVision Systems Ltd. pertaining to the 3D SVSlope® limit equilibrium slope stability package was the ability to analyze a slope at any particular sliding direction. Limit equilibrium 3D solutions in the past have been restricted to only solving problems at a single sliding direction, which is parallel to the x-axis.

The reason for this restriction in software is that all theoretical formulations of the 3D limit equilibrium method presented in literature presume the slip direction is parallel to the x-axis of the coordinate system. This is done for the sake of convenience as the theoretical formulations in 3D for any arbitrary direction become complicated. However, more and more consulting firms are building models in 3D coordinate systems and subsequently are interested in analyzing slips at a variety of directions. Therefore SoilVision Systems performed a comprehensive research and development project aimed at extending the existing abilities of the limit equilibrium method such that any slip direction could be analyzed in a 3D problem. The R & D Program was a success and has resulted in an updated version of SVSlope® being released which accommodates the analysis of slips in any direction.

The new implementation of analysis of any sliding direction is executed in the following manner. Once the user builds the 3D model they must first select a primary sliding direction when looking at the 3D model in plan view. This is done by selecting plan view and drawing a line on a problem as illustrated in the following figure.

Figure 1

Once the primary slip direction is identified all other analysis parameters remain the same. In addition to the primary sliding direction the user may specify a range of sliding directions relative to the primary sliding direction to analyze, therefore the user can easily ascertain if the selected sliding direction produces a critical slip surface. The analysis of any arbitrary sliding direction works with all existing methods of specifying the critical slip surface which are currently available in the software. Such methods include the grid and tangent method, the entry and exit method, as well as wedge slides and arbitrary surfaces. Ellipsoidal, combined wedge and surface failures and arbitrary slip surface failures are supported with the new sliding direction specification. There is also an optimization algorithm to determine the most critical sliding direction.

Figure 2

Figure 3

Once the sliding surface direction is assessed the grid of columns is set to be parallel and perpendicular to the specified sliding direction. The user does not need to perform any modifications to their existing 3D model. This is useful as the existing model can then be made to be easily compatible with the GIS coordinates of the existing site. This makes the representation of the results as plotted over a GIS location very amenable.

Once the user has specified the primary sliding direction and the number of different sliding directions to analyze, the software places the grids over the new geometry and determines the locations of all the intersections of each column in the analysis. The analysis then proceeds as forward with very little change from an existing analysis and the results may be visualized in the AcuMesh™ backend. An example of a typical analysis maybe as follows: It is often necessary to analyze the slope stability of a bluff. This may be a bluff at the edge of a city in which structures need to be placed at the top of the bluff. Therefore a load may be placed at the top of the bluff and the sliding direction which is prominent may not be readily obvious. This is the case in the example described above. In this case a distributed load should be placed at the top of the slope representing the placement of the purposed building. Subsequently the direction of a number of potential failure directions can be specified, the directions are all analyzed in the SVSlope® software as shown and the most critical failure surface can be determined. This is particularly useful where the geology at the site is not regular and may influence the direction of sliding. Therefore the analysis of setback distances and potential volumes of slides which may happen on a bluff overlooking a city is greatly simplified. It should also be noted that such an analysis is approximate at best using a 2D analysis and not recommended in a 2D model.

In order to provide continuity to existing 2D analysis techniques a 3D model may be sliced at any angle in the software and the results of the slice exported to a 2D model that can be run for comparison purposes. This allows the geotechnical consultant to understand the potential differences between a 2D approximate analysis and a detailed 3D analysis. The tool is designed to be simple to use and easy for a geotechnical consultant to provide results for their client.

*Example models illustrating the multi-directional feature in SVSlope™ may be found under the Slopes_3D project. To perform multi-directional 3D stability analysis, SVSlope® 3D Elite level licensing authorization is required.

If you are interested in applying this tool in your consulting firm and developing additional business please contact us.