| Offshore wind turbines(OWT)development and utilization is an important way to reduce traditional fossil fuels and the emission of carbon dioxide,and has become the direction of new energy development all over the world in the future.Monopiles are the most commonly used foundation form of OWTs.The lateral bearing behavior of monopile is the key of foundation design with the marine environmental load.With increase of fan power,the diameter of monopiles increases gradually to reduce the applicability of conventional design methods in large-diameter monopiles.Soft clay is widely distributed in China’s coastal areas.The strength and stiffness of soft clay are weakened under cyclic loading to reduce the lateral bearing capacity of monopile and to cause the excessive lateral deformation.Cement-soil mixing method can be effectively improve the mechanical properties of soft clay to enhance the lateral bearing capacity and reduce the cumulative deformation of monopiles.This method can be provided a new idea for the monopiles design of OWT.However,the design method of cement-soil reinforced large-diameter monopile is relatively scarce,and the enhancement mechanism of reinforced pile is not clear.Therefore,the lateral bearing behaviour of large-diameter monopiles with cement-soil is deeply researched by means of 1g laboratory tests,centrifuge tests,theoretical analysis and numerical simulation in this paper.The main research achievements are as follows:(1)The model tests of lateral bearing response of monopiles before and after cement-soil reinforcement under monotonic and unidirectional cyclic loading were carried out.The load displacement curves,internal force,cyclic cumulative displacement and cyclic stiffness of pile before and after cement-soil reinforcement are researched.The failure of cement-soil reinforcement piles is also studied.The mechanism of cement-soil reinforcement on lateral bearing and deformation of pile are deeply revealed based on the model test.(2)Centrifuge tests on the bearing capacity of large-diameter monopile before and after cement-soil reinforcement under monotonic loading were carried out.The load displacement curves,bending moment,soil resistance and displacement distribution of large-diameter monopile before and after cement-soil reinforcement are compared and analyzed.The p-y curves and pore pressure response of soil before and after reinforcement were investigated.The results show that the results show that cement-soil reinforcement effectively improves the properties of soil around the pile,enhance the initial stiffness and ultimate soil resistance,and then improves the lateral ultimate bearing capacity and flexural capacity of monopile.(3)Based on the numerical simulation of pile-soil and cement-soil interaction under lateral monotonic loading,the effects of reinforcement width,reinforcement depth,cement-soil strength and replacement rate on the lateral bearing capacity of monopile were analyzed.The results show that the reinforcement width and depth are the main factors affecting the lateral bearing capacity of monopole.There is a critical value of reinforcement width.When the critical value is exceeded,the improvement effect of lateral bearing capacity is not obvious.It is suggested that the factors such as reinforcement depth and width should be comprehensively considered in the design to optimize the cement-soil replacement rate in order to improve the lateral bearing capacity of monopile.(4)Based on the hyperbolic p-y curve model,considering the reinforcement and dimensional effect of soil around the pile,the initial stiffness and ultimate soil resistance enhancement coefficient were proposed.The modified expressions of initial stiffness and ultimate soil resistance are constructed.On this basis,the p-y curve calculation method of lateral bearing capacity of cement-soil reinforced pile was established.Combined with calculation cases,the theoretical calculation value of this method is in good agreement with the measured value,which verifies the rationality of this method,and this method can also be used to calculate the bearing capacity of unimproved piles.(5)Centrifuge tests on the bearing capacity of large-diameter monopile before and after cement-soil reinforcement under cyclic loading were carried out.The load displacement curves,cyclic cumulative deformation,cyclic stiffness,bending moment,cyclic p-y curves and pore water pressure around the monopile under different loading forms were researched.The prediction model of cyclic cumulative deformation and cyclic stiffness suitable for large-diameter monopile is established with the experimental data.The cyclic bearing mechanism of large-diameter monopile in cement-soil is deeply revealed,so as to systematically evaluate the shallow reinforcement effect of cement soil.The results show that the results show that cement-soil reinforcement can effectively weaken the cumulative displacement,increase the cyclic stiffness,and reduce the residual bending moment and the accumulation of pore pressure.The fatigue resistance of monopile is enhanced,which is beneficial to the cyclic loading of monopiles.(6)Based on the strain wedge method,the upper p-y curve of the rotation point is founded.Combined with the failure mode of large-diameter monopile,the lower part of the rotation point is equivalent to a rotation spring to construct a hyperbolic Mr-θR model.A simplified calculation model of horizontal bearing capacity of large-diameter monopile in saturated soft clay is established.On this basis,combined with the concept of secant stiffness attenuation of soil,a simplified calculation method of lateral cyclic response of monopile before and after cement-soil reinforcement is established with the secant stiffness attenuation models of soft clay and cement soil.The example analysis shows that the calculated value of the simplified calculation method is consistent with the measured value,which can better predict the cyclic internal force and deformation rules of cement-soil reinforcement under cyclic loading.Finally,the design method of cement-soil reinforcement range is proposed.It is suggested to take the resistance attenuation coefficient and the maximum bending moment depth as the upper limit of reinforcement width and depth.Considering the actual factors,the cement soil reinforcement scheme is reasonably optimized.(7)Combined with practical engineering cases,a scientifically formulate the construction scheme of foundation treatment in marine environment was proposed.The reasonable construction technical parameters were obtained by optimizing the construction process.By the numerical simulation and theoretical calculation,the rules of foundation reinforcement on lateral bearing capacity of large-diameter monopiles are investigated to evaluate the reinforcement effect for monopiles of OWTs,which has important engineering practice significance for the promotion of foundation treatment technology in marine environment. |
