# General Modeling

This page contains the answer to commonly asked general modeling-related tech support questions.

**How are loads applied in an axisymmetric analysis?**

As with plane strain, the loads in axisymmetric mode are per unit of distance out of plane (arc length). If you add a point load with a magnitude of 1.0 then the total load for 360 degrees revolution of the model is 2*pi*r, where r is the distance the load is from the x=0 axis.

**How do I define the stress field?**

In the Field Stress dialog, the following parameters are used to define a constant field stress. Sigma 1 is the major in-plane principal field stress, and Sigma 3 is the minor in-plane principal field stress. Sigma Z is the out-of-plane field stress. The Angle is measured between the positive x-axis and the direction of Sigma 1. The Angle is measured in degrees, and the counter-clockwise direction is defined as positive.

Note that **RS2** is plane strain. Inherent to the plane strain formulation, the out of plane stress is a principal stress and therefore normal to the plane of analysis. When you transform the 3D stress field to be in line with your plane of analysis, you'll probably find that there is shear stress out of plane. This shear stress is generally ignored, and the out of plane stress is used as if it was a principal stress. Whatever you do, you end up making a simplification assumption to adhere to the plane strain formulation.

**RS2** has a Stress Transform tool to assist with stress transformation.

**Is it possible to model truck loads as distributed load in RS2?**

Yes, you can define truck loads as either line loads or distributed loads. Just be careful - all loads are per meter depth in *RS2* and *Slide*. Below is a comment on tire pressure, which may be useful:

To constrain a distributed load to a certain location, you need to define vertices on the boundaries that represent the extents of the load on the boundary. Note, be careful with the distributed load. Remember that **RS2** is 2D, so all distributed loads are per meter out of plane. You’ll want to look at two cases, the case of a distributed load equal to the pressure under the tire, which will be the worst case scenario, and another case where you account for the out of plane spacing of the tires and divide the pressure by the wheel base length (best case).

You could of course also take the entire weight of the truck, divide by the wheel base length and add this as a point load. If the size of the truck is small in comparison to the slope or structure being modeled, this approximation may be close enough.

**How can I carry out a back analysis?**

The sensitivity or probability options can be used to perform a back analysis. In the case of a sensitivity analysis, you can set the possible magnitude range for the parameter of interest and run the analysis, then pick off the FS=1 value of the parameter.

For cases where there are multiple parameters, a probabilistic analysis can be used. Set the parameters to have a uniform distribution with the possible range of magnitude for the parameters. Then look at the scatter plot of FS values and look at all the possible options for values that yield a FS=1.

**What strain limit can RS2 handle?**

Small strain theory implies that there is no distinction to be made between the undeformed and deformed configuration. Since **RS2** uses Cauchy strain (or engineering strain) definition, the results will be accurate up to a strain of 1%. The accuracy of the results decreases as the values increase beyond 1%. However, the accuracy depends on many other variables as well.

**How do I specify in situ stress?**

This is a question with no exact answer, since the lateral stress ratio is rarely known.

*Chapter 10: In situ and induced stresses* discusses this topic in detail. Hoek's Corner provides links to the individual chapters of Practical Rock Engineering.

Other papers on determining in situ stress, include the following:

- Sheory, P.R. (1994). A Theory for In Situ Stresses in Isotropic and Transversely Isotropic Rock. Int. J. Rock Mech. Min. Sci. & Geomech. 31(1)23-34
- Stacey, T.R. and Wesseloo, J. (1998). In situ stresses in mining areas in South Africa. Journal of the South African Institute of Mining and Metallurgy. November/December: 365-368
- Stacey, T.R., Xianbin, Y., Armstrong, R., and Keyter, G.J. (2003). New slope stability considerations for deep open pit mines. Journal of the South African Institute of Mining and Metallurgy. July/August: 373-390
- Grov, E. (2006). The importance of in-situ rock stress in design and construction of sub-surface opening. International Symposium on Utilization of underground space in urban areas. 6-7 November, Sharm El-Sheikh, Egypt.

The above mostly deal with rocks. For soils, Ko=(1-sin(phi)) could be a value to start with if the soil is loose and normally consolidated. Another option is to start with Ko=1 and perform a sensitivity analysis by varying it to see the effect. Starting with Ko as a function of the Poisson ratio may not be the best method, as we have found that this causes weak materials to be initially close to failure, causing a number of modeling issues.