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EOSC433: Geotechnical Engineering Practice – Finite Element Lab Exercise

Lab Practical - Finite Element Stress & Deformation Analysis
Part A – The Basics
In this example, some of the basic features of a finite element analysis will be demonstrated through the modelling of a simple excavation using the Rocscience program PHASE2. The first step is to define the geometry and parameters requiredfor the analysis. If you have not already done so, run the PHASE2 MODEL program by selecting the PHASE2 icon from the Start menu. Note that when the PHASE2 MODEL program is started, a new blank document is already opened, allowing you to begin creating a model immediately. The following steps are laid out in the general order usually followed to perform a finite-element analysis. 1) SET PROBLEMLIMITS Let’s first set the problem limits, so that we can see the model being created as we enter the geometry. Select: View → Limits Enter the minimum and maximum x-y coordinates as shown below. These limits will approximately centre the model in the drawing region.

2) DEFINE GEOMETRY First create the excavation as follows: Select: Boundaries → Add Excavation Enter the following coordinates inthe prompt line at the bottom right of the screen. Note – press Enter at the end of each line, to enter each coordinate pair. Enter vertex [a=arc,esc=quit]: -5 10 Enter vertex [a=arc,u=undo,esc=quit]: -5 0 Enter vertex [a=arc,u=undo,esc=quit]: 5 0 Enter vertex [a=arc,c=close,u=undo,esc=quit]: 5 10 Enter vertex [a=arc,c=close,u=undo,esc=quit]: a Number of segments in arc : press Enter Enter secondarc point [u=undo,esc=quit]: 0 15 Enter third arc point [u=undo,esc=quit]: c

EOSC433: Geotechnical Engineering Practice – Finite Element Lab Exercise

Note the series of prompts used for creating the arched roof. First the “a” command is entered, to begin entering the arc. Then we accepted the default number of arc segments, in this case 20, by pressing Enter at the next prompt (although wecould have entered a different number). Then an intermediate point on the arc is entered, (0,15), and by entering “c” at the last prompt, the arc closes on the first point of the excavation. In this case, we formed a semi-circle, although a flatter arch could have been formed by lowering the intermediate (second) arc point. 3) Now we will create the external boundary. In PHASE2, the externalboundary may be automatically generated, or user-defined. We will use one of the ‘automatic’ options. Select: Boundaries → Add External You will see the Create External Boundary dialog. We will use the default settings of Boundary Type = Box and Expansion Factor = 3, so just select OK, and the external boundary will be automatically created. The boundaries for this example have now been entered. 4)DISCRETIZATION The next step is to generate the finite element mesh. In PHASE2, meshing is a simple two-step process. First you must DISCRETIZE the boundaries, and then the MESH can be generated. You can also configure various Mesh Setup parameters before generating the mesh. We will do this first, although default parameters are in effect if you do not use the Mesh Setup option. Select: Mesh → SetupEnter the # of Excavation Nodes = 60, and select OK. Now discretize the boundaries. Select: Mesh → Discretize The discretization of the boundaries, indicated by red crosses, will form the framework for the finite element mesh. Notice the summary of discretization shown in the status bar, indicating the actual number of discretizations for each boundary type. Discretizations: Excavation=59External=49

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EOSC433: Geotechnical Engineering Practice – Finite Element Lab Exercise

Note that the number of excavation discretizations is 59, but we entered 60 in the Mesh Setup dialog. Don’t worry, this is normal. Due to the nature of the discretization process, the actual number will not always be the same as the number you entered. If you are not happy with a given discretization, it...