Experimental Study On Bearing Behaviour Of Deep Foundations In Soft Clay

Energy development and transportation are two important components of China's infrastructure development.Offshore wind turbines and cross-river bridges,as representative engineering projects,have placed higher requirements on the foundation(e.g.large diameter monopile and open caisson foundation)due to their large size and susceptibility to complex loads,such as wind,waves and wave forces.In addition,foundation construction is more difficult than that on land because of more complex offshore geological conditions.Studies on the bearing behaviour of large-diameter mono-pile foundations of offshore wind turbines and caisson foundations of cross-river bridges can reveal their soil-structure interaction mechanism,and provide scientific support for improving the ultimate bearing capacity and long-term service performance of the foundation,which brings great significance to China's Maritime Power Strategy and the economic development strategy of offshore areas.This paper studies the mechanism of interaction between soil and two typical deep foundations based on the large diameter mono-pile foundation of a Wind Farm in Zhejiang Province and caisson foundation of Oujiang Bridge through field and centrifuge model tests and provides theoretical guidance and basis for engineering designs.The specific work is briefly described as follows:The field installation and static load test of large-diameter steel pipe piles at the site of the wind farm are introduced in detail.The installation process of pile foundation,and the characteristics of monotonic compressive,tensile as well as lateral loadings are systematically studied.By comparing the results of dynamic test and static load test,it is found that there is a certain deviation(about 60%)in the evaluation of the vertical ultimate bearing capacity of the pile foundation measured by the dynamic test method.For axially loaded piles,the unit shaft resistance qs does not change with the shear strength of the soil in the same soil layer.When the applied load approximates the ultimate bearing capacity of the ultimate pile foundation,the lateral friction resistance in the cohesive soil layer shows a significant strain softening effect.By comparing the prediction results of several existing design methods,the applicability of each method is discussed.For piles under lateral loading,the p-y curve in the API method greatly underestimates the stiffness of the pile-soil lateral interaction,which makes the results conservative.A series of centrifuge model tests on the lateral load characteristics of a single pile with the same diameter as the site(D=2m)are carried out.By analyzing the response of the mono-pile foundation in soft clay under lateral monotonic loading,the applicability of the API method is evaluated.The cumulative displacement of the pile top cycle and the attenuation of the soil resistance around the pile are studied.It is revealed that the pile top displacement attenuation index increases with the cyclic amplitude,and the lateral soil resistance significantly weakens when the load level is large.Centrifuge model tests are carried to study the stability of the open caisson foundation under the lateral load(e.g.typhoon,wave)and the interaction mechanism between the soil and structure.The responses of pore pressure and earth pressure are also investigated.It is found that the soil in the passive zone produces significant excess pore pressure during monotonic and cyclic loading,and the earth pressure increment is also accumulating continuously,while the active zone exhibits the opposite characteristics.The pore pressure increment of the passive zone increases with the load amplitude,and the increment of the deep soil is lower than that of the shallow soil.During each stage of loading,the excess pore pressure tends to stabilize after 10 cycles.When the load amplitude is large enough,the soil in the active area and the sink is likely to disengage slightly.In addition,due to the disturbance and softening effect on the soil during the cyclic loading process,the second monotonic loading stiffness is significantly lower than the first one,which is consistent with the performance of pore pressure.This study provides the theoretical basis for the stability analysis of caisson foundation and the establishment of cumulative displacement/rotation calculation model under lateral loading in practical engineering.