Phase2 FAQs > Interpret
Below are answers to Interpret FAQs for Phase2. This section answers questions related to data interpretation and analysis results. Click on the question to read the answer.
I notice that I am not getting the exact stresses I anticipate on the boundary. Why?
For three noded constant strain/stress triangular finite elements the stresses are calculated at the center of the element. We then use nodal averaging to get the stresses at a node. The nodal averaging takes the stresses from all attached elements and averages it. As a result, stresses at boundary nodes are calculated from stresses computed just inside the rock mass. In areas of high stress gradient you will not see the exact stress on the boundary as a result. Going to quadrilateral elements or elements with midside nodes should improve your results.
There are two issues with this. First, make sure you are looking at the normal stress across the boundary. The normal stress is continuous but the tangential stress might not be due to different modulus. Often people look at principal stresses that are not normal to the boundary and have different directions on both sides of the interface. These will not be continuous. Second, Phase2 calculates stresses for 3 noded triangles and 4 noded quadrilaterals in the center of the element. Quite a bit of nodal averaging and interpolation goes into calculating the stress contours. This can also be a source of error.
See the Ubiquitous Joints help topic.
The Ubiquitous joints dialog (in Interpret, select Data->Ubiquitous Joints) allows inclinations to be entered from -360 degrees to +360 degrees. There is a bug in version 4.x of Phase2, so that inclinations greater than 180 degrees or less than 0 degrees may display incorrect results.
Work-around in version 4.x: Use inclination values in the range 0 to +180 degrees.
This problem has been fixed since version 5 of Phase2.
In Interpret, turn on the Line Loads checkbox in the Display Options dialog. This will display the spring force.
Bolts require a certain amount of strain from one end to the other in order to create load in the bolt. The amount of strain to create a certain load depends on the modulus and stiffness that you enter for the bolt. Whether the load in the bolt produces any effect on the rock mass depends on the current state of stress and deformation in the rock mass. In the case of an elastic analysis, the strains are generally (but not always) too small to cause enough strain in the bolt to have a significant effect on the overall analysis. If there is a significant amount of overstress in the rock mass, a plastic analysis should be done to account for the failure and stress redistribution. Generally, but not always, higher strains will result in the rock mass and result in significantly higher loads in the support system. If the rock mass is not overstressed, obviously the elastic and plastic analyses will be identical and there will be no difference in the loads in the support system.
You can do this a number of ways, but one method is to construct a simple User Data equation using the Data > Define User Data option in the Phase2 Interpreter.
Assuming that the pipe is modeled as a circular excavation boundary, the equation for radial closure at a point is simply:
ux*((xc-x)/r) + uy*((yc-y)/r)
where ux,uy is the displacement vector at a point, xc,yc is the center coordinate of the pipe, x,y are the coordinates of a point, r is the radius of the pipe.
For example, if the center of the pipe is at (0.0, 0.0) and the pipe radius = 1.0, then you could construct the following User Data equation in Phase2 Interpret:
dx*((0.0-x)/1.0) + dy*((0.0-y)/1.0)
If you plot the above radial closure equation (i.e. view the contours of the user data variable you have defined), you can then query the boundary (right-click on the pipe), or use View > Data Tips > Query Data Tips, or graph the boundary. This gives you a distribution of radial closure. Depending on the stress field and the complexity of the model, this will vary along the surface of the pipe. The change in diameter is the radial closure of two opposite sides of the pipe.
For information on how to define User Data variables, see the User Data help topic.
Note: this equation assumes that the pipe center coordinates do not change (e.g. the pipe is subjected to a constant field stress). If the entire excavation boundary moves in a particular direction, then the above equation will not give exact results. In this case, you would have to compute the relative displacement of points on opposite sides of the pipe diameter.
The bolt results (e.g. axial force) reported in the Phase2 Interpret program, are the results for an individual bolt.
During the Phase2 analysis (compute), the out-of-plane bolt spacing is used to normalize the bolt properties per unit out-of-plane depth. This allows the bolts to be incorporated in the 2-dimensional analysis.
However, when the final bolt results are reported in the Phase2 Interpret program, the bolt results (e.g. axial force) apply to an individual bolt (i.e. the out-of-plane bolt spacing is again used to determine the force and stress per individual bolt).
How can I construct a flow net after a groundwater seepage analysis?
See Tutorial 13 and Tutorial 21.