A mixed finite element approach based on Key's variation principle is presented for elasto-plastic analysis of incompressible soils. Using this approach, undrained bearing capacity of rigid structures on thin Arctic silt layers is analyzed. The analysis indicates that the present approach can provide very rapid solution convergence for highly nonlinear elasto-plastic problems.
The Arctic sea bed is characterized by the presence of thin layers of weak silty soils resting on permafrost or dense soils. In order to recover hydrocarbons in this region, the use of gravity platforms may be necessary to cope with lateral ice loading. For saturated soils having low permeability such as the Arctic silt, the time required for pore pressure adjustment is usually very long compared to that within which the gravity loads are applied. The gravity platform may be built at a distant location and towed to the site to be placed. Even if the platform is constructed on site, the period of construction may be relatively short in comparison with that required for consolidation of the soil. Hence, virtually undrained conditions prevail at least during and for a short time after the application of the load. Previously, Baligh1 et al have investigated, as a research effort for the MIT Center for Scientific Excellence in Offshore Engineering, the undrained bearing capacity of rigid gravity structures on thin Arctic silt layers. The investigation consisted mainly of parametric studies using the displacement based finite element method to assess the influence of several variables on the bearing capacity. The variables included the soil compressibility, strain hardening rate (Et/E), and the platform diameter to the layer thickness ratio (B/H). The investigation showed that for given values of Et/E and B/H, the soil compressibility has a drastic effect on the bearing capacity predictions.