Thesis: S.D. Thomas

Doctor of Philosophy, Jesus College, University of Oxford, Trinity Term 1987

The Consolidation Behaviour of Gassy Soil

Summary

The consolidation behaviour of gassy soil has been studied in a programme of experimental and theoretical research.

This research is of particular importance to the offshore geotechnical industry as the presence of gas in the seabed can have a dramatic effect on the material properties of a marine sediment. Initial numerical modelling based on existing unsaturated soil theory combining the gas and the water phase into a compressible fluid in the pores of compressible soil skeleton failed to simulate the soil behaviour previously observed experimentally at Oxford. Therefore, there was scope for further study in this field.

Chapters 2 to 4 describe the experimental preparation, consolidation technique and experimental results of the two series of tests on artificially prepared gassy soil samples. The results of these tests indicated that the gas appeared to be affected by the total stress rather than the pore water pressure, with the saturated soil matrix outside the gas voids being controlled by the consolidation stress.

Chapter 5 presents the one-dimensional numerical modelling of the experimental results. Poor simulations were again made using compressible fluid theory. Treating the gas as compressible solid inclusion embedded in a saturated soil matrix, however, resulted in excellent simulations of the observed pore water pressures and settlements. Chapter 6 attempts to explain the results of the experimental and numerical modelling in terms of elastic and plastic soils behaviour. This includes the introduction to the double compressibility model in which the deformation behaviour of the saturated matrix is governed by changes in consolidation stress, whereas that of the gas is governed by changes in total stress.

Chapter 7 presents the development of the governing gassy soil consolidation equations under both plane strain and axisymmetric conditions. Chapter 8 describes the approximation of the governing consolidation equations using the Galerkin finite element method in terms of nodal displacements and pore water pressures. The resulting finite element approximation is subsequently formulated for rectangular elements under plane train and axisymmetric conditions in Chapter 9.

The remainder of the thesis describes the structure of the finite element model DCFEM2 and the constitutive relationships that are required for such a model. The code is verified with existing analytical solutions and then is used to simulate the observed gassy soil behaviour under laboratory and field conditions.

This thesis can also be downloaded from the ORA website