A Study on Uplift Bearing Characteristics of Suction Caisson Foundation in Soft Ground

Abstract: As the exploitation depth of marine resources is progressively deepening from shallow sea toward the deep sea (depth of water is lager than 500 meter), the mooring problem of large floating constructions in deep-sea submarine presents some new challenges. In engineering practice, a lot of thick soft and weak ground is inevitable encountered with. Complex load combination and feasibility and economy of execution and so on must be considered. The traditional pile foundation is no longer applicable in design, so the newly type of foundation for deep-sea platform is required. To compare with platforms of shallow sea, vertical loading of deep-sea platform foundations is no longer the pressure downward, but the drawing force upward. Moreover, there exit horizontal load component and moment component. As a novel type of offshore foundation, suction caisson foundation is being researched and popularized in recent years. Suction caisson foundation suits for soft ground and formidable ocean environment and has advantages of low cost, short construction period and repeatable application. It can supply omnidirectional anchoring ability and play stable tensile force. However, because the work mechanism of suction caisson foundation is different with that of traditional gravity foundation and pile foundation, there lacks sophisticated design code and unified analysis method for this type of foundation at present. Therefore it will be theoretically important and practically significant to examine the working bearing capacity of suction caisson foundation in soft ground under complex conditions. In this dissertation, the studies are emphasized on numerical methods of evaluating the bearing capacity and failure mechanism of suction caisson foundation in soft ground under wave loading. The main investigations consist of the following parts.1. Based on general- purpose finite element software ABAQUS, the finite element computational model for suction caisson foundation in soft ground under monotonic loading is established. According to finite element analysis, the failure mechanism of stability for suction caisson foundation in soft ground is different with that of traditional gravity foundation and pile foundation. When the component of load is applied slowly, the vertical uplift failure mechanism of suction caisson foundation may be local shear failure mode. The caisson is only pulled out of the subsoil and the soil plug remains at the same place. The horizontal failure mechanism of suction caisson foundation may be a single-sided failure mode. The potential crack between the posterior wall of caisson and neighboring soil in the active zone is constructed. At the same time, only an individual wedge near the mudline exists in neighboring soil of the passive zone, and a truncate spherical slip surface exists in the base of caisson obviously. The moment failure mechanism of suction caisson foundation may be also a single-sided failure mode, but the rotation center of the caisson is higher than that under horizon and the weight soil in the passive zone is smaller. When the component of load is applied rapidly, the vertical uplift failure mechanism of suction caisson foundation may be the overall instability. A soil plug exists in the caisson and the subsoil under the bottom of the caisson is extracted accompanying the uplift of the caisson. The horizontal failure mechanism of suction caisson foundation may be a double-sided failure mode. No crack between the posterior wall of caisson and neighboring soil in the driving side is constructed, and two wedges in both active and passive zones near the mudline exist. At the mean time, the moment failure mechanism of suction caisson foundation may be also a double-sided failure mode, but the rotation center of the caisson is lifting. Moreover, by parametric computation and comparative analysis the ultimate bearing capacity of suction caisson foundation under monotonic load is evaluated for different aspect radio of caisson, loading speed, shear strength of subsoil, deformation modulus of subsoil and strength reduction factor. When the action point of horizontal load is about z/L=0.6, the horizontal bearing capacity is maximal. Moreover, the different damage mechanisms occur for different action point of horizontal loading.2. According to the uplift damage mechanism and ultimate bearing capacity feature of suction caisson foundation in soft ground, a three-dimensional upper bound limit analysis method for uplift ultimate bearing capacity is proposed based on the viewpoint of “reverse bearing capacity”. The failure mechanism is assumed to be overall instability, a soil plug exists in the caisson and a little soil around caisson is extracted accompanying the uplift of caisson. The reverse bearing capacity calculation uses typical Prandtl failure mechanism. For the aspect ratio L/D =2.0, the uplift ultimate bearing capacity by upper bound limit analysis method is validated by finite element numerical computations and limit equilibrium method. Three results are consistent with each other and maximum error is less than 10%. For the same soil parameters and different aspect ratio of caisson, upper bound limit analysis results are consistent with finite element numerical results, but little more than limit equilibrium results.3. Based on the concept of cyclic strength proposed by Andersen et.al. to consider cyclic softening behavior of soft clay seabed induced possible by wave loading, a three-dimensional finite element model is developed to evaluating the cyclic bearing capacity of suction caisson foundation by combining cyclic strength and Mohr-Coulomb yield criterion. The quasi-static numerical computation determines the cyclic bearing capacity of suction caisson foundation under different load component, and these results are compared with the ultimate bearing capacity of suction caisson foundation under monotonically loading pattern. The computational results indicate that non-uniform distribution of static stress ratio and cyclic strength in subsoil induces the bearing capacity of suction caisson foundation reduced greatly, but the aspect ratio of caisson has no significant effects on the decreased degree of bearing capability induced by cyclic softening behavior of soft clay. when considering variate characteristic of load and cyclic softening behavior of soft clay, the cyclic bearing capacity of suction caisson foundation is decreasing with increasing of cyclic loading number to failure, and the decreased degree is drive to flat.4. The failure envelopes of suction caisson foundation are searched by Swipe test procedure, fixed displacement ratio procedure and load-displacement controlling procedure, furthermore the bearing capacity behavior of suction caisson foundation under combined loading mode is developed. According to finite element analysis, for the different aspect ratio of caisson and strength reduction factor of subsoil the failure envelope shape of ultimate capacity of suction caisson foundation is similar, but the dimension may change. The V-M-H three-dimensional failure envelope of suction caisson foundation is a closed asymmetrically approximate 1/4 ellipsoid forms. When moment component is zero, the failure envelope is symmetric to V axis. With the increasing of moment component, the failure envelope is obviously asymmetrical to V axis. At the meantime, this asymmetry is gradual enhanced with increasing moment component. Then comparative numerical computations indicate the failure envelope of cyclically combined loading pattern locates in that of monotonically combined loading pattern all along. The former is significantly reduced compared with the latter, about 30%…
Key words: Suction Caisson Foundation; Uplift Ultimate Bearing Capacity; Finite Element Method; Cyclic Strength; Upper Bound Limit Analysis Method; Combined Loading Mode

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