Anchor Design Through Back-Analysis

A feature of SVOffice™ 2009

December 5, 2011

SoilVision Systems Ltd. is pleased to announce the addition of a new back-analysis feature in our SVSlope® 2D/3D software. The back-analysis feature allows end-users to design anchor patterns in order to stabilize slopes. The back-analysis feature has been implemented in both the 2D and 3D versions of SVSlope® and therefore allows the full 3D back analysis of arbitrary patterns of anchors which vary spatially in a 3D fashion.

Figure 1: The Back-Analysis 2D Interface

All of the existing supports available in the SVSlope® software may be incorporated into an anchor design back-analysis. Support objects such as grouted tiebacks, soil nails, end anchored support, etc. may be used to match the equivalent horizontal force required by the slope back-analysis. The equivalent horizontal force method also allows the engineer to experiment with combinations of support which will result in the target factor of safety being reached. For example, the user may experiment with a combination of grouted tiebacks and soil nails of differing installation lengths.

This new feature is easy to use and may be incorporated in any existing analysis. It is also being provided freely to our existing customers at no charge. Simply download the latest version of our SVOffice™ 2009 software and the feature will be enabled for all licensed users.

The design of anchor patterns becomes more complex in a 3D analysis. Therefore, an additional functionality enabling the user to design patterns of anchors in a 3D slope has been enabled. The user merely enters the toe and the crest of the slope along with appropriate spacing information and a pattern of anchors will be generated in 3D. After the pattern is generated, the user is free to modify each individual anchor as needed or add additional anchors.

How It Works

The new back-analysis feature works by allowing the user to specify a target factor of safety and a horizontal support force elevation.

Figure 2: The Back-Analysis Dialog

The analysis is then run using either the Bishop's or Janbu Simplified methods. The results of this analysis will be a recommendation for how much horizontal force was required to raise the factor of safety to the target level. Once this horizontal force is known, the user can then experiment with different anchor designs in the SVSlope® front-end which replicate this horizontal force. There must be recognition given that there is not a unique solution. In other words, there are any number of anchor combinations which may yield a given factor of safety. The user is encouraged to use their own professional judgment in each case. An example design which solves the model shown earlier in this article is shown in Figure 3, below.

Figure 3: Back-Analysis Results

The results of a 3D analysis are of particular use in that the intersection of elliptical or fully specified slip surfaces may be taken into account. It should be noted that a 2D analysis assumes a cylindrical slip surface in the third dimension. In reality, most slip surfaces are elliptical. This means that the point at which they intersect various anchors may change at different places on the slip surface. This detail is significant in the design of grouted tiebacks. An illustration of this concept may be seen in Figure 4, below, which shows the results of a three-dimensional back-analysis.

Figure 4: Back-Analysis 3D Results

Additional Information

Additional help on the specifics of this implementation is available in the User's Manual. There are also training videos available on the use of the back analysis feature for both 2D and 3D. Feel free to contact us directly with any additional questions you have on this type of analysis!