Punch-through Failure Of Spudcan Foundation In Sand Overlaying Soft Clay Soils

With the rapid development of economy, energy demand is growing quickly, especially of oil and gas. The available oil and gas onshore are becoming less and less after one hundred years'hardly extracted. Oil and gas exploration have been developed to sea. Seabed are often layered soil profile, which make it possible to occur punch-through failure during the installation and operation of foundation, especially in a layer hard soil overlaying soft soil. Based on the literature review, punch-through failure is the most important factor of failure of the jack-up units. In the literatures, the study is mainly focusing on clay overlaying clay seabed, while the study on punch-through failure of spudcan in sand overlaying soft clay is relative rare. To enhance the understanding of the mechanism of punch-through failure of spudcan foundation in sand overlaying clay soils, this thesis present the characteristics of the bearing capacity of spudcan in sand overlaying clay by small strain and large displacement finite element method. The main work could be summarized as follows:(1) Based on Mohr-Coulomb failure criteria, small strain FE analysis is applied to compute the bearing capacity of spudcan foundation in sand overlaying soft clay soils with different frictional angle, clay cohesion, sand layer thickness. These results are compared with that from empirical formula, such as Hanna-Meyerhof equation, projected area method.(2) In large displacement finite element analysis, RITSS is employed to simulate the penetration process of foundation, which has been successfully used to simulate deep penetration of spudcan into double layered clay soil. The effects of the undrained shear strength of the lower soft clay, frictional angle of sand, the relative thickness of the upper layer sand and the density of the upper layer sand on the bearing capacity are analyzed. Based on the bearing capacity curves punch-through depth are obtained, which is useful to direct the installation of spudcan foundation in layered soils to avoid foundation failure.(3) A set of apparatus is developed to measure bearing capacity of strip foundation deep penetration into sand soil. Particle Image Velocimetry (PIV) technique is used to capture sand flow mechanism. Using the developed apparatus, nine testes with three foundation width and three sand relative density have been done. The effects of relative density of sand and foundation width are discussed.