Research On The Suction Determination And The Bearing Behavior Of Suction Caissons

Suction caissons have been widely used as foundations for offshore wind turbines due to easy installation, reuse and saving-cost. A suction caisson is open at the bottom and closed on the top, like an upturned bucket. It first penetrates the seabed to a certain depth under its self-weight with outlet valves on the top open to make water inside the caisson escape. Suction is then produced by pumping out water encased with all outlet valves closed, driving the suction caisson to penetrate to the designed embedded depth. This paper mainly studies the determination of suction range and the formation mechanism of soil plug during suction-assisted installation, the vertical bearing capacity,the subsidence behavior of the mud-line under the uplift load and the failure mechanism of horizontal loaded suction caissons in sand. The theoretical analysis results are verified by the model test results and the field data borrowed from the published literature. The main contents and conclusions are drawn as follows:(1) In chapter 2, a modified slip-line field is proposed to calculate the tip resistance of the skirt in terms of the slip-line theory. The minimum suction is determined by using the static equilibrium method;then the maximum suction is obtained by using the mechanism of sand piping. In addition, the maximum penetration depth of the caisson is determined in terms of the maximum and minimum suctions.(2) The critical suction will lead to plastic failure of soil at skirt tip level, then the soil particles exterior of caisson at the skirt tip level will be driven into the interior of caisson, and this will increase the height of soil plug. The critical suction is obtained by establishing an equation of the effective stresses on the soil inside and outside of the caisson wall at the skirt tip level in terms of the slip-line theory.(3) Vertical bearing capacity of suction caisson is mainly composed of friction resistance, bearing capacities from skirt tip and soil inside the caisson, and its value is directly affected by diameter, height and embedded depth of the suction caissons. Aiming at researching the vertical bearing capacity of suction caissons, The relationships between shaft friction, tip bearing capacity and embedded depth are researched and the analytical formulae to calculate the bearing capacity of suction caissons is proposed.(4) Based on the reversed bearing capacity failure mechanism of suction caissons subject to the up-lift load, the range of plastic zone around the caisson is analyzed. Then the analytical expression of subsidence range of the mud-line is developed by calculating the boundary of the plastic zone according to the reverse bearing failure mechanism, and the subsidence depth of the mud-line is solved based on the volume conservation of plastic flow soil.(5) A series of model tests are carried out to find out the failure mechanism of the lateral loaded suction caissons in sand. It can be found that the caisson failure mechanism comprising a wedge failure near the mud-line in front of caisson, an arc of the logarithmic spiral failure surface near the mud-line at the back of caisson and a hemispherical failure surface in the lower layer soil. The center of rotation of the mechanism is located at 0.67 embedded depth of suction caissons. The uplift and the subsidence range of the mud-line in the direction of 0 degrees from loading direction are cos? times of the uplift and the subsidence range in the main deformation plane. Based on these deformation characteristics, the shape of the three-dimensional failure surface is obtained, which can be used in upper bound analysis to solve the horizontal limit bearing capacity of suction caissons.