Model Test Study On Behavior Of Pile Group Subjected To Lateral Cyclic And Eccentric Loads

The foundations of large-scale offshore structures are constantly subjected to cyclic lateral loads with considerable magnitude during their service lives, which induced by winds, waves, currents, etc. They are likely to produce accumulated deformation under such load conditions, or even be damaged in extreme cases. On the other hand, owing to the stochastic directions of loads or the asymmetric shape of superstructures, significant eccentric lateral loads would also be transferred to the foundations probably. Both the lateral cyclic and eccentric loads would give rise to the complicated responses of pile group foundations, which have not been understood very well at present. Hence, it is of great theoretical and practical significance to conduct systematic researches on this issue, combined with the specific features of offshore engineering.Some different methods, such as1g large-scale model test, centrifuge model test and finite element numerical simulation, are adopt in this thesis to investigate the behavior of pile group with elevated cap subjected to lateral cyclic and eccentric loads, respectively. The main research works and results are as follows:(1) A series of large-scale model tests on single piles and pile groups were performed in saturated silts to investigate their behaviors under lateral cyclic and eccentric loads. The designs, as well as the procedures of these tests were detailed, which included the eccentric lateral loading test on1×2steel-pipe pile group, the eccentric lateral loading destructive test on2×2reinforced concrete pile group, the cyclic lateral loading test on3×3steel-pipe pile group and some other relevant single pile tests.(2) Based on the3×3steel-pipe pile group test, the lateral stiffness of the pile group, the internal forces and the load distributions of individual piles were discussed. Besides, a new empirical coefficient was introduced to evaluate the cyclic loading effect on the pile responses. The test results reveal that the lateral stiffness of pile reduces with the increasing number of loading cycles, and would be significantly affected by the previous loading history. The peak lateral load at pile head decreases with loading cycles in stable, developing, and failure pattern, respectively, which reflects the developing process of the soil around piles from elastic stage to plastic and failure stages. The lateral loads carried by each pile row constantly vary with cyclic loading and the leading pile row will undertake more and more loads. The pile group settles significantly in the test due to the cyclic lateral loading, as well as the strong constraint between the individual piles and the cap. The cyclic loading effect has a far greater impact on the responses of pile group than single pile.(3) Based on the1×2steel-pipe pile group test, the overall deformation of the pile group under eccentric lateral loads and the internal forces of individual piles were discussed. ABAQUS finite element simulations were also conducted to investigate the influences of the load eccentricity, as well as the layout of individual piles on the behavior of pile group. It is found that the internal forces of individual piles are quite different with each other within the group, and the pile closer to the loading point generally undertakes more shear force and torque. The ultimate torsional resistances of individual piles are apparently larger than the torsionally loaded single pile due to the deflection-torsion coupling effect. The torque applied on the pile group is shared by the torsional resistances and the shear forces of individual piles. With the increase of applied load, the contribution provided by shear forces gradually increases. The results of the numerical calculation show that, the load eccentricity would affect the internal forces of individual piles, rather than the overall deformation of pile group. However, the layout of individual piles would significantly influence the lateral stiffness of pile group.(4) Based on the2×2reinforced concrete pile group test, the progressive failure process of the pile group was described. The failure mechanism of overturning was also summarized by conducting numerical simulation. It could be concluded that the high altitude of loading point, the remarkable self-weight of superstructure and the cracking of pile shaft concrete together aggravate the overturning failure of pile group. The concrete pile group basically remains elastic during the initial loading stage, when the stiffnesses of individual piles are relatively large. Once the applied load exceeds a certain threshold, the pile shaft concrete begins to crack successively, which simultaneously reduces the stiffnesses of piles and increases the lateral deformation. The P-?effect induced by the self-weight of superstructure then gives rise to the rapid increasement of axial forces within the individual piles in the leading row, and finally leads to its eccentric compression failure, as well as the overturning of the structure.(5) A series of centrifuge model tests were conducted in sand to investigate the behavior of pile group subjected to lateral and eccentric lateral loads. The different responses of plumb and battered pile group were compared in detail. The results reveal that the lateral and torsional bearing capacities of battered pile group are much higher than those of plumb pile group. Under the eccentric lateral load, the differences between the displacements of individual piles within battered pile group are larger than those in plumb pile group. The battered pile group could more effectively resist lateral load than plumb pile group by taking full advantage of the axial capacity of individual battered piles. Numerical analysis shows that the lateral capacity of battered pile group would obviously increase with the increment of pile inclination.(6) Under the assumption of rigid cap, an analysis model of elevated-cap pile group subjected to multidirectional loads was proposed. A variety of different pile group effects and load coupling effects were summarized, as well as the corresponding computing methods. These effects should be carefully considered to determine the stiffnesses of individual piles in different directions. The general analysis process of pile group under multidirectional loads was also briefly summarized.