Bearing Capacity Study Of Spudcan In Two-Layer Clay By Numerical Computation Method

Mobile jack-up rigs are widely used in offshore oil and gas exploration and increasinglyin temporary production and maintenance work. A typical modern jack-up has threeindependent legs, each equipped with a footing known as "spudcan". In the past 20 yearsbearing capacity study of spudcan at home and abroad is widely concerned. However, itsmechanism and theoretical study is still backward in practice. In Bohai Bay and other watersit is encountered that stratified soil profiles with a relatively thin layer of sand or stiff clayoverlying a weaker layer. Currently study of bearing capacity for spudcan in this two-layer(strong soil overlying soft soil) soil is few.In this thesis, the author takes a review on the study of the bearing capacity of spudcan.Then the finite element analysis software-AFENA is employed to analyze the ultimatebearing capacity of spudcan in two-layer clay with two-dimensional FE model. Theconstitutive model employed is Mohr-Coulomb plasticity with linear elasticity assumedwithin the yield strength surface. By varying the depth of the strong layer,the soil strengthratio of soil strength and the unit weight of the soil, a parametric study of the bearing capacityofa spudcan in two-layer clay is investigated. The main conclusions are remarked as follows:(1) During large penetration of a spudcan, the bearing capacity of spudcan responds tothe top soil layer first, thus the bearing capacity increases with spudcan penetration. When thespudcan is close to the bottom layer clay, a reduction in the bearing capacity occurs, showingthe influence of the bottom soft layer. Punch-through failure occurs when a peak in thepenetration resistance is reached.(2) The penetration depth with potential"punch through"failure is mainly affected by thetop layer soil thickness (H/D), the top layer soil strength and the soil strength ratio (S_ut/S_ub).The strength ratio of soil layers has little effect on the potential failure depth when thestrength and thickness of the top soil layer are constant. By FEM analysis, an empiricalformula with depth of punch-through is given. It is also found that depth of punch-through is0.8-1.5 times the diameter of the spudcan, it is wrong that it is deemed that punch-throughusually happens at the soil interface.(3) It is found that there are three distinct penetration mechanisms during spudcaninstallion: during initial penetration, an open cavity is formed with vertical walls; with further penetration, soil flows partially around the spudcan into the cavity; during deep penetration,the spudcan is fully embedded and the soil flow mechanism is entirely localized.(4) The depth of cavity depends not only on the top layer soil strength and soilself-weight but also on the soil strength ratio. During initial penetration, the depth of cavity isvery small. When penetration becomes deeper and soil back-flow continues to occur onto thespudcan, the depth of cavity is highest. During deep penetration, the spudcan is fullyembedded and the soil flow mechanism is entirely localized, the depth of cavity is unchanged.