oa Description of Subsidence Phenomena Due to Gas Extraction in Deep Layers with Advanced Three-Phase Constitutive Model

In coastal regions, the land subsidence due to industrial pumping of underground fluids such as<br>methane is documented on the basis of in situ surveys. Some laboratory characterization of the soils<br>hosting those fluids have also been published to complement the knowledge on compaction due to<br>changes of fluid pressures. The withdrawal of gas is simulated in the laboratory by injecting water<br>under a constant uniaxial or hydrostatic load, which results in the plastic compaction of the samples.<br>The paper proposes a new attempt to model the observed collapse of samples, as well as the changes<br>in compressibility and preconsolidation pressure during the process of wetting. The conceptual<br>framework essentially relies on unsaturated soil mechanics, as the subsidence phenomenon concerns a<br>three-phase material with solid grains, liquid water and gas. The developed constitutive model provides<br>a description of the water retention capability of the studied soils that is coupled with the mechanical<br>behaviour. Consequently, the elasto-plastic volumetric changes within the porous medium incorporate<br>the effects of saturation and suction, also called capillary effects. The formulation of the<br>preconsolidation stress is such that the shape of the yield limit depends on suction so that the apparent<br>added stiffness brought by low saturation is predicted. The modelling framework, based on the<br>generalization of the effective stress principle to three-phase media, also provides an elasto-plastic<br>comprehension of the well-known “wetting pore collapse” phenomenon. The ACMEG-s model shows<br>consistent understanding of changes of compressibility with the quantity of retained water. The<br>successive phases of isotropic compression and uniaxial mechanical compaction are used for the model<br>calibration. Interestingly, the phases of plastic compression during injection are captured with<br>accuracy, which evidence the applicability of this model to the boundary value problems that are the<br>large scale cases of subsidence.