| This paper mainly researches and discusses problem of breakout force that affects lift of caissons in YOKE project, the main content can be summarized as follows:(1) The ultimate bearing capacity of foundation based on Mohr-Coulomb strength criterion did not analyze effect of intermediate principal stress on yield and failure of soil so that the disparity between results obtained using Mohr-Coulomb strength criterion and those from experimental data are produce. In the paper, The ultimate bearing capacity of foundation based on unified strength theory is established and a unified solutions are obtained. The corresponding solutions of different materials can be got from it. The unified solution can not only be use to fit the properties of the materials with different tension-pressure strength but also those with equal tension-pressure strength, Results show that once make full use of the properties of the materials so to reduce supports, which is of important significance for engineering.(2) In this paper, the breakout t force required to extract partially embedded objects from cohesive sediments has been investigated experimentally. On the basis of theoretical approach, the formula for calculating the breakout forces of round-bottomed and flat-bottomed objects from ocean floor sediments has been obtained through experiment. Taking in to account the sediment characteristics, the formula is applid to various kind of ocean sediment.(3) With example of "CFD11-1&CDF11-2" project of Tianjin Bohai oil company of COONC, the purpose is to determine the lift force so as to get the YOKE up from sediment. The method of calculation is statistical and experimental method. The theoretical model of calculation method is Skempton model. The calculation results show that the maximum breakout force is about 4173kN, which means that it needs about 11115kN lift force , or9449kN lift force while 170ton ballast water is drained, to get the YOKE up from sediment.
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