CONSOLIDATION OF A DOUBLE-POROSITY MEDIUM

The classical one-dimensional column consolidation boundary value prob lem is studied for a double-porosity material. Uniaxial strain and con stant vertical stress conditions are applied to a column that is drain ed at the top and undrained at the bottom. An initial fluid pressure d ifferential develops between the matrix and fracture phases in respons e to surface loading when variations in the mechanical and flow parame ters of the matrix and fracture exist. The mean stress is shown to be a linear combination of the fluid pressure in the matrix and the fluid pressure in the fracture. The time dependent general analytical solut ion is given for the matrix and fracture pressure histories and surfac e displacements using fracture and matrix storage coefficients defined for constant stress (constant confining pressure) and the assumption that the cross-storage coefficient at constant stress is negligible. P ressure and displacement histories are controlled by the mechanical an d flow properties of both the matrix and fracture and on the magnitude of the differences between the two : phases. The double-porosity solu tion approaches the equivalent single porosity solution for closely sp aced fractures, a small permeability contrast and a large cross-flow t erm. Pressures and displacements are also compared to previous results in the literature based on storage coefficients defined at constant v olumetric strain and the assumption that the constant strain cross-sto rage coefficient is negligible. The previous results included small ma trix fluid pressures and fracture fluid pressures in excess of the app lied stress. The constant stress formulation of the double-porosity co lumn consolidation problem produces physically intuitive results. (C) 1998 Elsevier Science Ltd. All rights reserved.