PISA, the Program for Incremental Stress Analysis, is a computer program for geotechnical deformation analysis. PISA has advanced modeling features which can simulate many geotechnical structures such as dams, embankments, tunnels, excavations,retaining structures, etc. Complex construction procedures can be modeled using a step approach. It has a wide variety of material models which include linear and nonlinear elastic, anisotropic elastic plastic, frictional and non-frictional plastic, critical state and creep models. Undrained and drained deformation with user specified phreatic lines can also be modeled effectively using PISA. A graphical user interface allows the engineer to draw the finite element mesh using a mouse. Modeling parameters are keyed in through dialog boxes. A variety of outputs, such as deformed meshes, contours of stresses and strains, displacmemnt arrows, stress trajectories and x-y plots of many quantities such as stress paths, settlement profiles, horizontal deflection with depth, etc. allow the engineer to examine many important aspects of the structure. PISA is a multi-document interface (MDI) program which allows many windows to be simultaneously displayed. As well, PISA has a comprehensive on-line help which provides instant explanations of PISA commands and modeling procedures.

The main features of the program include:

• Graphic input using the mouse, combined with key entry for accurate specification of coordinates;
• Efficient generation of finite element mesh using element blocks;
• Graphic output showing finite element mesh, deformation of soil structures, contour of stresses, strains and displacements, displacement vectors, stress vectors and x-y plots of various quantities such as stress paths, settlement profiles, deformation history etc;
• Execution of finite element code within the program and continuous display of progress of analysis and convergence characteristics for nonlinear problems;
• Step modeling with complete flexibility of adding and deleting elements;
• A variety of soil and solid models based on linear and non-linear, elastic, plastic and critical state theories;
• Effective stress analysis for drained and undrained materials with user defined phreatic lines;
• Interactive help facilities providing explanation of program commands and operations.

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