Study On Uplift Bearing Characteristics Of Suction Caisson Foundation Under Vertical Load

Suction caisson foundation is an important anchor foundation of deep-water offshore floating platform.It is a thin-walled circular foundation with sealing at the top and opening at the bottom.During the installation of suction caisson foundation,the caisson is sunk into a certain depth by its own weight and then pushed to full depth with differential pressure produced by pumping water out of the interior.Due to their easy installation,reusability and low construction costs,suction caissons have been increasingly used in offshore engineering.The suction caisson foundation in tension leg platform(TLP)is mainly affected by uplift load.For this reason,a lot of studies about this aspect have been done by many scholars.Generally,there are two steps from the load application to the caisson start working.The first step is the process of load application,which has been studied by many scholars.but there are still no wide spread engineering specifications on design and calculation of uplift bearing capacity for suction caisson foundation.So the failure mode,uplift mechanism and calculation method of suction caisson foundation still need to be further studied.The second step is the load holding stage.For this stage,the soil at the bottom of the suction caisson foundation is in unloading state.However,there is little research on the long-term uplift bearing characteristics of suction caisson foundation.In the actual marine engineering,the suction caisson foundation not only resist the working load from the superstructure but also suffer the cyclic load.The strength of soft clay will be weakened and the deformation will be cumulative by cyclic loading.However,very little research is focused on the case of axial stress unloading.At the same time,the failure surface at the bottom of suction caisson foundation is still uncertainly.A large number of studies have shown that cyclic load and long-term load will reduce the uplift bearing capacity of suction caisson foundation and the uplift bearing capacity of suction caisson foundation is lower under drainage condition.But there is no report on how to improve the uplift bearing capacity of suction caisson foundation.In this paper,the gravitational reinforced composite suction caisson foundation is proposed to solve this problem.At the same time,the uplift bearing characteristics and improvement technology of suction caisson foundation are studied by the finite element,upper bound solution and theoretical analysis in this paper.The main research contents are as follows:Suction caisson foundations are often subjected to vertical uplift loads,but there are still no wide spread engineering specifications on design and calculation method for uplift bearing capacity of suction caisson foundation.So,it is vital important to establish the uplift failure criterion for analyzing the uplift bearing capacity of suction caisson foundations.In order to study the uplift bearing mechanism and failure mode of suction caisson foundation,a series of model tests were carried out to study the uplift bearing characteristics of suction caisson foundation with the effects of aspect ratio,soil permeability and loading mode.Combined the uplift bearing characteristics of caisson in soft clay,the bearing capacity composition and the calculation method are proposed for analyzing the uplift bearing capacity of suction caisson foundations.It can provide a reference for the engineering design of suction caisson foundation under vertical load.Secondly,the cyclic bearing characteristics of suction caisson foundation under different cyclic load ratio and different aspect ratio are studied by the model test of suction caisson foundation under vertical cyclic load.Last,the long-term uplift bearing capacity of suction caisson foundation under different static load ratios is studied through the test of suction caisson foundation model.Firstly,the conventional triaxial compression and tensile tests of saturated soft clay under different confining pressures were carried out to obtain the basic mechanical characteristic parameters of soft clay.Then,axial unloading triaxial creep test of saturated soft clay were carried out to obtain the variation law of axial strain with time and deformation mechanism of soft clay under different confining pressures.Based on the tests,different creep models are introduced to describe the test results of soft clay under different stress levels.The results show that the fitting degree of the four creep models is good under the condition of low stress level.With the increase of stress level,the fitting degree of the fractional model can be better than other models in the inflection point of fitting curve.For the later development,the five parameter Burgers model is more close to the test value.Another result shows that the soft clay has typical nonlinear creep characteristics.With the increase of deviator stress,the nonlinear creep characteristics become more obvious.Based on the tests,a method of determining Merchant model parameters related to confining stress and stress level was established.On this basis,a three-dimensional non-linear viscoelasticity constitutive model which reflects time effect was built to analysis the unloading creep characteristics of soil and the long term uplift bearing capacity of suction caisson foundation.As the anchoring foundation of the tension leg platform(TLP),suction caisson foundation is not only subjected to the vertical pullout load but also subjected to the cyclic load.However there are few studies on the mechanism of the cyclic characteristics of soft clay under unloading condition.So,the cyclic cumulative deformation characteristics of soft clay under axial unloading are studied by GDS test system.Based on the tests,strain softening model related to stress level is introduced to describe the test results of soft clay under different stress levels.An improved creep empirical model is established by creep empirical model.combining with creep element model,the improved creep empirical model is proposed by considering softening index,static deviator stress and dynamic deviator stress.By introducing the improved creep model and considering the influence of cyclic stress and softening index,an equivalent cyclic creep model was established to describe the cumulative plastic strain of saturated soft clay.On this basis,a three-dimensional nonlinear equivalent cyclic creep model which reflects time effect was built to analysis the cyclic creep characteristics of soil and the cyclic uplift characteristics of suction caisson foundation.In order to study the upper-bound theorem of bearing capacity of suction caisson foundations under vertical uplift load,introducing the viewpoint of reverse bearing capacity and three different failure modes for study.Select the appropriate foundation failure mode and displacement velocity field,it means that the active area of failure mode become the passive area under the vertical pullout loads and the logarithmic spiral direction is opposite.At the same time,reasonable upper bound theorem is derived by consider the factors of soil weight,cohesion and earth pressure.Then,drawing the relationship curve between the upper bound solution and the cohesion,the internal friction angle,soil pressure and friction coefficient for analysis.The upper bound solution calculated by using the Matlab calculation program and compared with the previous experimental data and other upper bound solution,it is proved that the failure mode of the soil and this method are reasonable.At the same time,analyzed the influencing factors of three reverse failure modes.The uplift bearing capacity of caisson is only composed of the internal and external frictions and caisson weight under the drainage condition,so the uplift bearing capacity is very small.Therefore,the gravitational reinforced composite suction caisson foundation is proposed to solve this problem.The uplift bearing capacity of the new suction caisson foundation and the shearing characteristics of caisson-cement soil interface are studied through the model tests,large-scale shear tests and push out tests.At the same time,combined with the finite element,upper bound solution and theoretical analysis,obtained the shear strength of the caisson model and the calculation formula of the overall interface shear strength of the caisson model.Based on the uplift bearing characteristics of new caisson by model tests and push out tests,the interface failure mode,the overall shear strength,the bearing capacity composition and the relevant method are proposed for analyzing the uplift bearing capacity of suction caisson foundations.It can provide a reference for the engineering design of the new suction caisson foundation under vertical loads.