Theoretical Studies On Penetration And Lateral Bearing Mechanism Of Suction Caissons In Clay

Suction caissons are popular in offshore engineering to fix gas and oil platforms due to their competitive technical and economic advantages.With the development of renewable energy,such as utilization of wind power,suction caissons have been increasingly used to act as foundations for offshore wind turbines.Before providing the lateral bearing capacity,suction caissons must be successfully penetrated to the seabed.Thus,in this dissertation,the limit analysis theory and numerical simulation were adopted to obtain the suction range during caisson penetration and the lateral ultimate bearing capacity of the caisson in service.During caisson installation,the suction pressure inside the caisson must be predicted accurately:if the suction pressure is less than the required suction(pr),the caisson cannot overcome the total penetration resistance consisting of skin frictions along the inside(Fi)and outside(Fo)of the caisson wall and the bearing capacity at the caisson-tip(Fx);if the suction pressure is greater than the critical suction(pc),the reversed bearing capacity failure at the caisson-tip will be motivated,leading to a higher soil plug heave that prevents the caisson from penetrating to the designed depth.As the caisson penetrates seabed,bearing capacity failure of the soil at the caisson-tip occurs.The penetration resistance of caisson-tip may be equal to the bearing capacity of the soil.However,there are few studies on the soil failure mechanism at the caisson-tip,leading to an appreciable difference exists in selecting the value of bearing capacity factor Nc among researchers,especially,for the caisson-tip with internal or external bevels.Furthermore,the general bearing capacity failure at the bottom of the caisson was usually adopted by researchers to obtain the critical suction pressure(pc).But the experimental test has been affirmed that more volume of soil than that displaced by the caisson wall would enter the caisson before the general reverse bearing failure,resulting in higher soil plug heave.In addition,suction caissons for offshore wind turbines will encounter vertical loadings,great moment(M)and lateral(H)loadings induced by winds and waves.The traditional plasticity analysis adopting a conical-wedge and full-flow failure model was used to obtain the lateral ultimate bearing capacity of the caisson.For short suction caissons,however,traditional plasticity theory tends to over-predict the lateral bearing capacity,because the lateral and moment loads will induce rotation in the same direction on suction caissons for offshore wind turbines,resulting in rotation failure mechanism rather than full-flow failure mechanism.In Chapters 2 to 4,the required and critical suction for caisson penetration were researched by using the lower-bound method of limit analysis theory.In Chapters 5 and 6,the ultimate lateral bearing capacity of the suction caisson was studied by using the upper-bound and limit equilibrium methods of limit analysis theory.The main contents and conclusions are summarized as follows:(1)The asymmetric failure model considering the effects of different friction factors along the inside(αi)and at the base of the caisson wall(αb)on the failure model at the caisson-tip was proposed.The contribution to the penetration resistance of caisson-tip from the effective unit weight of soil within plastic zones was involved.The theoretical results of penetration resistance of caisson-tip were obtained in terms of the lower-bound method of limit analysis theory.It was indicated that the factor Nc increases with increasing friction factor along inside and at the base of caisson wall,which increases from 6.28 for smooth caisson wall(αi=αb=0)to 9.42 for rough caisson wall(αi=αb=1),involving two typical values of Nc=7.5 and 9.0 usually recommended in design.(2)The failure model at the caisson-tip with internal or external bevels considering the effects of flat base ratio(κ/t)and inclination angle of the internal bevel(δ),friction factors along inside(αi),outside(αo)and at the base of caisson wall(αb)was put forward for caisson penetration.The contribution to the penetration resistance of caisson-tip from the effective unit weight of soil within plastic zones was also included.The theoretical solution for the penetration resistance of the caisson-tip was deduced by using the lower-bound method of limit analysis theory.It was found that the factor Nc of caisson-tip with internal or external bevels are less than that of flat caisson-tip,which increases with increasing friction factors(αi,αo and ab),caisson-tip geometry ratio(κ/t)and decreasing inclination angle(δ),indicating that the internal or external bevels at the caisson-tip reduce the penetration resistance of the caisson-tip.(3)The local reverse bearing capacity failure model at the caisson-tip causing the soil plug heave was presented.The local reverse bearing capacity failure factor Ncr was deduced by using limit analysis where the effects of friction factors along the inside(αi)and outside(αo)of the caisson wall on the local reverse bearing capacity failure model was considered.It was shown that the factor Ncr ranges from 5.14 for smooth caisson wall(αi=αo=0)to 8.28 for rough caisson wall(αi=αo=1).Besides,with the increase of suction pressure,the local reverse bearing capacity failure extends inward and outward from the caisson wall.Ultimately,the general reverse bearing capacity failure will be formed at the bottom of the caisson,leading to the acceleration of the soil plug heave or termination of the caisson penetration.(4)The meniscus conical wedge-spherical failure model was proposed to analyze the ultimate bearing capacity of suction caissons subjected to eccentric lateral loadings in undrained clay.Comparing with the traditional ring-conical wedge and flow-around failure model,the meniscus-conical wedge having a meniscus shape at the soil surface reflects the deformation of the soil surface that close to a real condition.Meanwhile,the spherical failure accurately describes the rotation failure of the caisson for offshore wind turbines and keeps a kinematically admissible velocity field,ensuring the accuracy of theoretical results.Accordingly,the upper-bound solution for laterally loaded suction caissons was deduced based on the work-energy balance equation,following by two parameter optimizations of c and α’ to approach the optimal depth of rotation center that offers the minimum energy dissipation.(5)Based on the proposed failure model of meniscus conical wedge-spherical failure,an analytical solution to the horizontal bearing capacity of the suction caisson was obtained in terms of the limit equilibrium method.The parametric optimization of c was conducted to obtain the optimal depth of rotation center that satisfies both the force equilibrium in the loading direction and the moment equilibrium to the axis of rotation.In the upper-bound and limit equilibrium analyses,both one-sided and two-sided failure models were considered.Meanwhile,the effects of the caisson aspect ratio(L/D),lateral loading eccentricity(e)and the friction factor(αo)at the caisson wall on the lateral bearing capacity were studied.It was found that the rotation centers for one-sided and two-sided failure models were located at 3/4L and 2/3L,respectively.The lateral bearing capacities of suction caissons with two-sided failure model were about 15%～23%higher than that with one-sided failure model.The lateral bearing capacities of suction caissons with rough caisson wall were found to be about 14～22%higher than that with the smooth caisson wall.