Lateral Behaviour Of Single Pile And Composite Pile In Soft Clay

Pile foundations are widely used for supporting structures like high-rise buildings, bridges, transmission towers, offshore platforms and offshore wind turbine structures. These structures are generally subjected to relatively large cyclic lateral load, combined with a constant vertical load. The cyclic loading poses a challenge to design and construction of laterally loaded piles in soft marine sediments, which usually has a high water content, high compressibility, low strength and high sensitivity.Compared with the extensive exsiting research on the vertically loaded piles, relatively less attention has been paid to the studies on the interaction between soft clay and piles subjected to cyclic lateral loading. The influence of pile stiffness on the failure mode of pile-soil system remains unclear, and the design approches for laterally loaded pile in soft clay may be improved. In addition, there is a lack of systematic study on cyclic and post-cyclic behavior of laterally loaded piles, by considering effets of pile diameter, vertical loading, and localized ground improvement around the pile at shallow depth. For these reasons, a comprehensive study on the behavior of laterally loaded pile in soft clay was performed, through theoretical analysis, numerical simulation, full-scale field test and centrifuge modelling. The following key issuses have been addressed in this study:1. Full-scale field tests in soft clay were carried out to study the monotonic, one-way cyclic and two-way cyclic response of single pile. Based on the test results, the ultimate bearing capacities of single pile were interpreted based on various criteria. The effects of loading regimes on lateral cyclic responses of single pile were also studied. Moreover, the influence of jet grouting on improvement of lateral pile capacity was systematically evaluated.2. Based on the field test results, three-dimension finite element analyses were performed to analyse the progressive failure mechanism of both single pile and composite pile, incorporating the plastic-damage model. The failure mode and its effects on the pile stiffness, strength, pile-soil interaction and the reinforcement were investigated. Besides, numerical parameter studies were conducted to study the influence of various factors regarding the jet-grouting on the bearing capacity of the composite pile, leading to some suggestions for design and construction.3. Lateral monotonic and cyclic behaviours of pile in soft clay are studied through a series of centrifuge test. This section focuses on the analysis of the cyclic behaviour of rigid-flexible pile, aiming at the comparison between the rigid-flexible pile and rigid pile or flexible pile. Based on the PIV technique, the displacement field of the soil around the piles was obtained, revealing the "three-region failure mechanism" of soil around the rigid-flexible pile. According to the newly revealed failure mechanism, the cyclic p-y curves were compared and assessed. In addition,the effectiveness of the reinforcement of shallow soil surrounding rigid-flexible pile were demonstrated, by comparing the cyclic resposes of a single pile (such as the accumulation of pile-head displacement, degradation of stiffness, response of bending moment, failure mechanism and p-y response).4. According to the cyclic triaxial test results of kaolin clay, the parameters of a hypoplastic model for clay were calibrated. Based on the calibrated hypoplastic model, the influence of pile-soil stiffness on the failure mechanism of surrounding soil is analysed and improved p-y curves for monopile were deduced. Numerical parameteric studies were then carried out to investigate the effects of pile diameters on monotonic, cyclic and post-cyclic responses of monopiles.5. A series of centrifuge model tests were carried out, with the aim to reveal the influence of vertical loads on the lateral behaviour of single pile. Through the tests, the influences of drainage conditions, over-consolidation ratios of clay, magnitudes of vertical loads on monotonic and cyclic performance of the pile were also assessed.