Study On Buoyancy And Adhesion Force Of Mat Foundation On Soft Clay Seabed

Compared with the traditional spudcan platform,the new marine operation platform has many advantages.During the development of the new offshore platform,the interaction between mat foundation and seabed is not clear.Through the analysis of the whole operation process of the mat foundation,it is considered that the buoyancy and the adhesion force of the foundation should be paid more attention to.Based on these two points,field model tests and relevant numerical analysis were carried out in this paper to explore the variation of buoyancy during the sitting process and the factors affecting the adhesion force of mat foundation.The main conclusions are as follows:1.A test device that can simultaneously measure the buoyancy and adhesion force of the mat foundation model is designed,and field tests are carried out with this device.The results show that the mat foundation is still affected by buoyancy after its lower part is sunk into the seabed.At the beginning of the sitting process,due to the significant excess pore pressure in the seabed soil,the buoyancy acting on the foundation could reach twice the theoretical value(calculated according to Archimedes’ law).With the increase of consolidation time,the buoyancy gradually decreased and finally stabilized near the theoretical value.The buoyancy of the foundation fluctuates slightly due to tides.The results of adhesion force test show that the suction caused by negative pore pressure on the bottom surface is the most important factor of the adhesion force,accounting for more than 85%of the total adhesion force,and it is preliminarily believed that the adhesion force increases with the increase of consolidation time.2.The correctness of the subroutine is verified by the numerical calculation of the element to the three-dimensional model,and the buoyancy of the mat foundation is numerically studied by the subroutine.The results show that the buoyancy of mat foundation is directly related to the excess pore pressure in the surface soil beneath the foundation.For the soil directly below the foundation,the excess pore pressure near the edge is more significant under the influence of shear action at the beginning of the sitting process,and the excess pore pressure in the middle is more significant under the influence of drainage path at the end of the process.Finally,the formula for calculating the buoyancy of the foundation at the beginning of the sitting stage in clay are summarized.3.The numerical study on the adhesion force of the mat foundation is carried out by using the subroutine,and the effects of consolidation time,external load,uplift rate,foundation area,clay thickness and over consolidation ratio on the adhesion force were analyzed.The results show that,the adhesion force has a linear relationship with the degerr of consolidation.The larger the initial load on the mat foundation,the greater the excess pore pressure generated in the soil,and the higher the dissipation efficiency of excess pore pressure.Under the same degree of consolidation,the adhesion force increases with the increase of external load,and the relationship between the two is basically linear.When the initial load and consolidation degree are the same,the adhesion force per unit area increases slightly with the increase of the bottom area of the structure.When the initial load and consolidation degree are the same,the adhesion force per unit area increases with the increase of the normalized uplift rate,but when the normalized rate increases to the point where it can be considered that the uplift process is undrained,the adhesion force per unit area tends to a constant value;The adhesion force per unit area decreases with the increase of the thickness of clay under the foundation.The over consolidation ratio has little effect on the adhesion force per unit area.To sum up,the main influencing factors of adhesion force are soil strength(pre-load,consolidation time)before uplift and drainage conditions(drainage path,uplift rate)during uplift.Finally,based on the above research,the formula for calculating the adhesion force of is improved.