Flac Intro

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    Associate Professor Nagaratnam Sivakugan School of Engineering James Cook University Townsville, Qld 4811 Australia Tel.: 61 7 47814431 Email: Siva.Sivakugan@jcu.edu.au

  • A FLAC Primer

    Dr. Nagaratnam Sivakugan (2006) 2

    A FLAC Primer

    Table of Content Page

    1. FLAC versus Others 2 2. Purchasing FLAC 2 3. Units and Notations 3 4. Running FLAC 4 5. Mesh Generation 5 6. Boundary Conditions 9 7. Solving the Problem 9 8. Outputs to Screen, Printer and Documents 10 9. Some Constitutive Models 13 10. Calculation Modes 15 11. Large/Small Strain Modes 15 12. FISH 15 13. Seepage 16 14. Consolidation

    Appendices:

    A A Note on Permeability 19 B - Geotechnical Applications

  • A FLAC Primer

    Dr. Nagaratnam Sivakugan (2006) 3

    Preface

    I confess I only know about 10% of FLAC, but I know that 10% quite well. For those who try to learn FLAC from scratch, it is a horrendous effort to master this wonderful software. FLAC comes with a 10-volume user manual which is not that friendly for a beginner. It took me three full weeks (and a ruined holiday) to understand the little I know of FLAC. Since then I had been helping the several postgraduates and honours students (or they help me in fine tuning my understanding) who had been applying FLAC and FLAC3D to geotechnical and mining problems. With my limited knowledge of FLAC, I was able to teach FLAC to academics and postgraduates at Indian Institute of Technology, Kanpur (India), Sultan Qaboos University (Oman) and State University of Rio de Janeiro (Brazil). And they have become ardent fans of FLAC. The purpose of this manual is to explain the concepts and commands in the simplest possible way for the beginners. Dr. Michael Coulthard has contributed significantly to our FLAC modeling through his prompt and precise answers to all our queries, reviewing sections of this manual and visit to JCU. Consolidation, being a coupled problem, involving mechanical deformation and fluid flow, requires a bit more had work to master. The section on consolidation is written by Ms. Briony Rankine, a postgraduate who has worked on consolidation extensively, using FLAC and FLAC3D. Her contribution is gratefully acknowledged. Using this document, one can understand the key features of FLAC to get started. However, the modeling strategies given in Itascas FLAC manual (available on-line too) are far superior. There are several programs covering a wide range of applications in these manuals and they are certainly worth trying. Siva June 2006

  • A FLAC Primer

    Dr. Nagaratnam Sivakugan (2006) 4

    A FLAC Primer 1. FLAC versus Others FLAC (Fast Lagrangian Analysis of Continua) is one of the most popular general purpose geotechnical modeling softwares used worldwide. It is a finite difference software, and the solution scheme is explicit. The solutions are reached through a process known as time-marching or time-stepping, which is simply adjusting the values of each node in the mesh through a series of cycles or steps. These adjustments take place on the basis of the selected constitutive model and equation of motion (similar to equilibrium equations). The adjustment continues until the error (e.g., unbalanced force in the system) becomes very small. The other numerical modeling software that are used widely are PLAXIS, ABAQUS, ANSYS, CRISP etc. PLAXIS is a geotechnical software that is popular among geotechnical engineering practitioners, mainly due to its user friendliness. It operates on portable computers and is widely used in design offices. ABAQUS is quite powerful and operates from mainframe computers. It is used in civil and mechanical engineering applications, but not specifically developed with geotechnical engineering in mind. It also requires some good learning for one to master the features and use them comfortably. Like ABAQUS, ANSYS can be used for geotechnical and structural modeling. Recent addition of CIVIL FEM module makes ANSYS attractive for geotechnical modeling. CRISP is a critical state soil mechanics program, developed in UK. FLAC is developed specifically for geotechnical and mining applications and includes most constitutive models that we can think of. It runs on portable computers, and is quite versatile in features. FLAC is a very useful research tool, in spite of some difficulty in getting started. The purpose of this document is to provide a plain English guide, with very little jargon, in helping the beginners. Most of the time, I could teach a 4th year honours student or a postgraduate and get him or her started within an hour. Many of our honours students and postgraduates use FLAC and FLAC3D comfortably in their research projects. 2. Purchasing FLAC FLAC is marketed and distributed worldwide by Itasca Consulting Group of Minneapolis, USA (http://www.itascacg.com) through its agents located in many countries. The other softwares distributed by ITASCA are in Table 1. The 2-dimensional versions were the ones developed first and were later extended to 3-d.

    Table 1. Itasca Software 2-D 3-D Description FLAC FLAC3D Fast Lagrangian Analysis of Continua tor treating soil as a

    continuum UDEC 3DEC Universal Distinct Element Code for discrete blocks such as

    jointed rocks PFC2D PFC3D Particle Flow Code for assemblage of circular or spherical

    particles Universities and educational institutions can purchase Itasca products at discounted prices. They come with hardware locks (dongles Itasca calls them keys). When you need more than one key, the 2nd, 3rd and 4th keys of the same software can be bought at 50%, 33% and 25% of the price of

  • A FLAC Primer

    Dr. Nagaratnam Sivakugan (2006) 5

    the first one. At JCU, we have 3 FLAC 5.0 keys and 2 FLAC3D 3.0 keys, but under the same site license with one contact person. Itasca distributor will not entertain queries from more than one person per site license. Most geotechnical problems can be modeled as plane strain (e.g., strip footings, embankments, retaining walls, braced cuts..) or axisymmetric (pile foundation, circular footing, triaxial test, etc.) problems. Therefore, FLAC is adequate for one to get started, and it is much cheaper than FLAC3D which is required for analyzing truly 3-d problems. When you purchase FLAC, you will get the student version free (only for universities). The student version can solve a mesh of up to about 600 elements, and that is adequate for learning. All the examples provided in the extensive FLAC manuals can run on this student version. The main advantage of student version is that it does not require a dongle and can be installed in any personal computer. That makes it suitable for teaching in classrooms. The student version is adequate for viewing any large mesh and also to manipulate any *.sav files saved using save command. The dongle is only required to run (using step or solve commands) FLAC. FLAC comes with a very comprehensive on-line manual. Hardcopies are available in the form of 10 separate volumes, which can be bought at nominal price. At IIT Kanpur I had postgraduates doing term papers on FLAC applications just using this student version. The student version has default RAM of 0.15 MB, which can solve up to about 500 elements. 3. Units and Notations SI or imperial units can be used in FLAC. Stick to consistent sets of units to avoid any confusion. My suggestion is to use the following: Length in m; Mass in kg; Time in s; Force in N; Stress or Pressure in Pa; Density in kg/m3; Gravity in m/s2. Obviously, all FLAC outputs will be in matching units (e.g., pore water pressures in Pascals, deformations in meters, etc). In FLAC, 6.23 x 105 is written as 6.23e5. Compressive stresses are negative in FLAC. Pressure is positive when applied towards a surface, and negative when it is directed away from the surface. These are two different parameters stress and pressure in FLAC. A FLAC program will consist of a series of instructions. Each line starts with a command, which is followed by appropriate key words and necessary values. Some examples are: MODEL elastic PROPERTY den = 1600 bulk = 1e8 PLOT hold grid bou Here, MODEL, PROPERTY, and PLOT are three of the 50 or so commands available in FLAC. Whatever that cannot be done using FLAC commands can be done using FISH (for FLACish) language compiler built into FLAC. In the x-y Cartesian coordinate system, the normal stresses in x and y direction are x and y respectively (Fig. 1). The shear stress is xy (often denoted by ). These are written in FLAC as sxx, syy, and sxy respectively. The normal stress in z direction is szz.The displacements of a node in x and y directions are denoted by xdisp and ydisp respectively. Pore water pressures at an element or node are denoted by pp and gpp respectively. Xvel and yvel are the velocity

  • A FLAC Primer

    Dr. Nagaratnam Sivakugan (2006) 6

    components in x and y directions, which are simply the displacements per time step. Large velocity vectors indicate instability in a geotechnical system.

    Figure 1. Notation for Stresses 4. Running FLAC By default FLAC allocates 8 MB RAM for FLAC, which is sufficient for up to 30,000 elements. If your mesh is larger, you need to allocate more memory. This can be done by adding the required number of RAM (say 16 for 16 MB) at the end of the target string. Right-click the mouse on the FLAC short-cut icon, select properties and add 16 at the end of the target string. Next time you start FLAC, the new RAM allocated will appear on the screen. Until Version 4.0, FLAC was available only as command-driven. Now, it can be used command-driven or menu-driven. Although the menu-driven FLAC appea