Resistance Capacity And Deformation Behevior Of Suction Tripod Piles In Saturated Sand Under Inclined Pulling Load

In recent years,with the further development of offshore oil and gas industry,the application of suction pile foundation in subsesa engineering in the world especially in China is rapidly growing.Due to its outstading advantages such as feasible construction,high capacity and reusable,the suction pile is widely used as anchoring foundation for various offshore structures;the pile foundation is usually composed of steel cylinder covered by top.As the continuous development of oil and gas into deep-sea areas,the new floating structure requires even higher capacity of traditional suction pile foundation that makes the single pile geometry and weight greatly increased,and results in great difficulty of transportation and installation with additional project costs.The proposed suction tripod piles foundation can effectively improve the overall inclined pulling capacity of the pile foundation,with the better rigidity of the foundation,and foundation configuration becomes more reasonable and stable,less weight,easily transportation and installation,then the construction cost is reduced significantly,and is particularly suitable as anchoring foundation of deep-sea floating structure.As the subsea foundation of the floating structure,the suction pile foundation is mainly subjected to the inclined pulling load on the side of the pile body.At present,the research on the resistance and deformation behaviors of the tripod pile foundation under this kind of loading has just started,and it is hard to find any accepted theoretical system and calculation method,and the actual engineering application also faces so many unknowns.For this reason,this thesis focus on the large-scale indoor model tests of suction tripod pile foundation installed in the large geotechnical tank of TYUT,to find out the resistance and deformation behaviors and failure mechanism of the single pile and tripod pile foundation in saturated sand under static inclined pulling load,and furthermore the ABAQUS finite element analysis software is used to extend the indoor model test results,some valuable results are obtained and summaried,which provides a theoretical basis and technical support for the application of the tripod pile foundation in practical engineering.The main research results are summarized as follows:(1)In order to apply inclined pulling load in the indoor large-scale model test under 1g condition,the oblique load device was self-developed(having won two national invention patents).The deformation along the length of single pile and tripod pile under the inclined pulling load were collected by ShapeAccelArray displacement meter.The results of indoor model test indicate that the inclined pulling capacity of tripod pile foundation is higher than three times that of single pile foundation under the same conditions.The resistance capacity of the tripod pile foundations with different aspect ratio(L / D = 7.5,6,4.5),load angle(? = 15 °,30 °,45 °)and pile spacing(B / D = 5,4,3)were further analyzed.The results show that the inclined pulling capacity of the tripod pile foundations will be reduced with the increasing of the load angle,the decreasing of the aspect ratio(L/D)and the decreasing of the pile spacing(B/D).(2)Based on the results of model tests,the failure criterion of tripod pile foundation is proposed.That is,when the maximum displacement of the pile reaches the range of 0.02-0.04 times the width of pile foundation,the tripod pile foundation goes into the failure stage.(3)Through the model test results of the pile displacement,the deformation and failure mode of suction pile foundation in saterrated sand under the inclined pulling load are detailed analyzed.When the load angle is less than 15 °,the horizontal displacements of the tripod piles and single piles are greater than the vertical displacements,and the foundation failure mode are in the horizontal failure mode.When the load angle is greater than 45 °,the vertical displacements increase obviously more than the horizontal displacements,and tripod piles and single pile foundation are all in vertical failure mode.When the load angle is 30 °,the failure mode can be considered as the failure mode bewteen horizontal and vertical failure.Under different failure modes,the soil around the pile presents different forms of cracks.Under the horizontal failure mode,the distribution of cracks in the soil in front of pile is narrow and extends far.And under the vertical failure mode,the distribution range of the soil cracks is wide and the extension length is short in front of the piles.(4)With the aid of finite element software ABAQUS,the elastic-plastic models of single pile and tripod piles with the aspect ratio of 7.5 under inclined pulling load were tested and the finite element analysis matches well with the experimental results.The different pile spacing and loading point were analyzed and the results show that with the decrease of pile spacing,the movement tendency of tripod pile foundations gradually approaches the single pile foundation.The inclined pulling capacity of the tripod pile foundations render parabolic distribution with the change of loading point,and the optimal loading point is located at(0.6~0.667)L,which proves the rationality of the location of loading point in model test.(5)Combined with the results of indoor model tests,the theoretical analysis of the single pile foundation subjected to the inclined pulling load is carried out by using the limit equilibrium method.The failure envelope of the single pile foundation is given and the formula of its ultimate inclined pulling capacity was proposed.The concept of equivalent diameter of tripod pile foundation was proposed,which can be used to treat the calculation of inclined pulling capacity of tripod pile foundation into that of single-pile foundation with large diameter.Furthermore,the failure envelope diagram of tripod pile foundations under different working conditions was proposed,and the formula of its ultimate inclined pulling capacity is obtained.The experimental verification shows that the above two formulas have higher accuracy.