CISM has developed PLASCON3D, a geomechanical simulation code primarily interested in the study of soil erosion, compaction and subsidence phenomena.

The F.E. code PLASCON3D is based on the theory described below and comes directly as an extension of the 2D version PLASCON.

The code develops coupled thermo-hygro-mechanical compaction/subsidence analyses within a saturated (locally partially saturated) porous medium subjected to external loads and water/gas withdrawals from deep layers (aquifers/reservoirs). The code is being updated to take into account material non-linearities (critical state plasticity model), necessary once rebound phenomena are to be considered. It is planned to further extend the elasto-plastic constitutive law to partially saturated states by means of the BSZ model.

In addition of the assumption of small strains and the absence of inertial effects, the following simplifications have been introduced:

- creep is not accounted for;
- body forces and hydraulic heads are assumed to remain constant and act in the direction of one of the global coordinates;
- the permeability matrix is assumed to have zero off-diagonal terms and is assumed constant throughout each element.

The problem of subsidence above gas reservoirs

Surface subsidence due to extraction of underground fluids (water, hydrocarbons) plays an important role in reservoir engineering. In recent years a great deal of attention has been directed towards this phenomenon, also because it affects historical cities, like Venice and Ravenna in Italy. Subsidence analyses are computationally intensive because they involve problems of regional scale and very long time spans: e.g. in the case of the Groningen gas field, subsidence predictions for the year 2050 have been made from the year 1973 on.

References

• B.A. Schrefler, X. Wang, V. Salomoni, G. Zuccolo, "An efficient parallel algorithm for three-dimensional analysis of subsidence above gas reservoirs", Int. J. Num. Meth. Fluids, 31(1), 247-60, 1999.
• R.W. Lewis, B.A. Schrefler, "The finite element method in the deformation and consolidation of porous media", Wiley, Chichester, 1987.
• G. Bolzon, B.A. Schrefler, O.C. Zienkiewicz, "Elastoplastic soil constitutive laws generalized to semisaturated states", Géotechnique, 46(2), 279-289, 1996.
• M. Zambon, "Abbassamenti anormali del suolo nel Ravennate", La Bonifica, 1-2, 58-64, 1968 (in italian).
• L. Carbognin, P. Gatto, G. Mozzi and G. Gambolati, "Land subsidence of Ravenna and its similarities with the Venice case", Evaluation and Prediction of Subsidence, S.K. Saxena Ed., ASCE, New York, 254-66, 1978.
• Ministero Dell' Agricoltura e Delle Foreste, "L'abbassamento del suolo della zona litorale Ravennate", Concessione Studio D. M. 9-11-71, No. 481, 1976 (in italian).
• T. Ippolito, "La subsidenza di Ravenna", Le Scienze, 99, 104-11, 1976 (in italian).
• B.A. Schrefler, R.W. Lewis and C.E. Majorana, "Subsidence above volumetric and waterdrive gas reservoirs", Int. J. Num. Meth. Fluids, 1(2), 101-15, 1981.
• L. Simoni, V. Salomoni, B.A. Schrefler, "Elastoplastic subsidence models with and without capillary effects", Comp. Meth. Appl. Mech. Engng., 171, 491-502, 1999.