Investigation Of Bearing Capacity Of Suction Caisson Foundations In Sand

With the rapid growth of economy,China’s demand for energy is increasing day by day.China’s oceans are rich in natural resources,including oil,natural gas and wind energy.In the process of obtaining these marine energy,it is necessary to establish an offshore platform foundation to support the superstructure.Due to the different construction environment,the traditional foundations(such as the single pile foundation and gravity foundation)will become impractical and uneconomical.Suction caisson foundation is a new type of offshore platform foundation being developed in recent years.It has the characteristics of strong wind and wave resistance,high bearing capacity,simple construction and short construction period.Therefore,it is widely used in sand in marine engineering.Accurately predicting the bearing capacity of suction caisson foundations in sand plays an important role in the safe operation of offshore engineering superstructure.Existing studies mainly use the traditional Mohr-Coulomb(MC)model to evaluate the bearing capacity of suction caisson foundations in siliceous sand,while ignoring the influence of strain softening characteristics of medium dense and dense sand on its bearing capacity,which leads to inaccurate calculation of bearing capacity.In addition to siliceous sand,marine foundations also contain a large number of calcareous sand,which has the characteristics of high compressibility and easy crushing.These characteristics make the mechanical properties of calcareous sand significantly different from those of siliceous sand.This makes the calculation method for estimating the bearing capacity of suction caisson foundations in siliceous sand no longer suitable for calcareous sand.In this paper,the bearing capacity of caisson foundations in marine sandy soil(including siliceous sand and calcareous sand)is studied by means of numerical simulation,laboratory test and theoretical analysis.The main research work and conclusions are as follows:(1)The modified Mohr Coulomb(MMC)model of siliceous sand is improved and embedded into ABAQUS finite element software for calculation and analysis through secondary development.Through numerical analysis,the horizontal and moment bearing capacity of shallow caisson foundations for offshore wind turbines in siliceous sand are studied.It is found that the horizontal and moment bearing capacity increase significantly with the increase of the vertical load,caisson aspect ratio and relative density of sand.In addition,with the increase of load eccentricity,the horizontal bearing capacity of shallow caisson foundations decreases gradually,while its moment bearing capacity increases gradually.Based on the numerical results,a method for calculating the horizontal and moment bearing capacity and caisson size of shallow caisson foundations in siliceous sand with different relative densities is proposed.(2)Based on the improved MMC model,the uplift bearing capacity of suction caisson anchors in siliceous sand is explored through numerical simulation,and it is revealed that the change of padeye position changes the failure mode of soil around the suction caisson anchor,which affects the uplift bearing capacity of caisson anchors.It is found that when the caisson anchors have the maximum uplift bearing capacity,the corresponding optimal padeye depth is related to the caisson aspect ratio and the relative density of sand,and the depth is generally in the range of 0.6L ～ 0.7L from the top of caisson lid.Based on the numerical results,the calculation method for the V-H failure envelope of suction caisson anchors in siliceous sand is proposed,and the feasibility of this calculation method is verified by comparing with the existing numerical and experimental results.(3)A series of consolidated drained triaxial tests of calcareous sand with different densities and axial strains are carried out.It is found that there is an obvious strain softening phenomenon in the shear process of medium dense and dense calcareous sand,and the peak strain increases with the increase of relative density and effective confining pressure.In addition,based on the particle sieving test before and after the test,it is found that the particle breakage increases with the increase of relative density and effective confining pressure,and the particle breakage speed first increases and then decreases with the increase of axial strain.Based on the modified particle breakage index,the relationship between the axial strain,relative density,stress level and particle breakage value is established,which provides a basis for the study of constitutive model of calcareous sand.(4)Based on the improved MMC model,the existing theoretical framework of sand particle breakage and the triaxial test data of calcareous sand,a MMC-PB(Modified Mohr Coulomb-Particle Breakage)constitutive model considering strain softening and particle breakage of calcareous sand is proposed.Through the secondary development of user subroutine,the MMC-PB constitutive model is embedded into ABAQUS finite element software for calculation and analysis.Compared with the current and existing triaxial test data,it is found that most particle breakage under triaxial conditions occurs around the shear zone,and the shear leads to a gradual increase in the non-uniformity coefficient,resulting in more non-uniform samples.It is verified that the model can be used to capture the particle breakage phenomenon and strain softening characteristics of medium and dense calcareous sand.It shows that the MMC-PB constitutive model of calcareous sand can be applied to the calculation of numerical model.(5)A group of centrifuge tests and numerical analysis are carried out to explore the uplift bearing capacity of suction caisson anchors in calcareous sand.It is verified that MMC-PB model can accurately simulate the interaction between calcareous sand and caisson anchors.It is revealed that when the suction caisson anchor is pulled out in calcareous sand,the particle breakage mainly occurs in the passive earth pressure area and the bottom of the skirt.When the caisson anchor foundation is translational under loadings and does not rotate,the particle breakage of the soil in the passive area is more significant.Based on the numerical results,the calculation method for V-H failure envelopes of suction caisson anchors in calcareous sand is proposed,which can provide a theoretical basis for the design of suction caisson anchors in calcareous sand.