Research On Lateral Loadbearing Capacity Of Large Diameter Cement-Improved Soil Composite Steel Pipe Pile

Large-diameter cement-improved soil composite steel pipe pile is a new type of composite pile foundation,which is combined with large diameter steel pipe pile and shallow cement-improved soil.It is mainly used in offshore soft soil foundation and has a broad development prospect in the field of offshore wind power.Under complex Marine environment conditions,wind load,wave and tidal action will exert huge lateral load on pile foundation,which brings great challenge to the construction of single pile foundation on soft soil.However,there are few researches on the lateral load-bearing capacity of the new pile foundation.Therefore,this paper takes cement-improved soil composite steel pipe pile as the research object,and adopts the method of combining laboratory test,numerical simulation and theoretical analysis to study the load-bearing capacity of the composite pile foundation.The main research contents and conclusions are as follows:(1)The mechanical properties and related influencing factors of cement-improved soil were investigated by triaxial shear test and unconfined compressive strength test.The results show that there are two failure modes of cement-improved soil,and the strength is affected by cement content and curing age.Based on Lianyungang Marine clay,a prediction formula for unconfined compressive strength of cement-improved soil considering cement content and age is proposed.The characteristics of pile-soil contact interface were investigated through large-scale direct shear tests in laboratory.The results show that the interface cohesion is mainly affected by the cementation between cement soil and steel plate,and the stronger the cement-improved soil is,the greater the cementation effect is.(2)The lateral bearing capacity of large diameter cement-improved soil composite steel pipe pile in soft clay was studied by laboratory model test.The changes of lateral bearing capacity of composite steel pipe piles before and after reinforcement were compared and analyzed,and the influences of different reinforcement ranges and methods(machine stirring and precast model)on lateral bearing capacity of composite steel pipe piles were explored.The results show that the lateral bearing capacity of steel pipe piles is significantly increased after the cement-improved soil is adopted.The reinforcement of shallow foundation by machine stirring is reliable and effective,and has good reinforcement effect.When the reinforcement width is the same,the reinforcement depth has little effect on the ultimate lateral bearing capacity of single pile,and when the reinforcement depth is the same,the reinforcement width has great effect on the ultimate lateral bearing capacity of single pile.(3)Based on laboratory model tests,the effects of cement-improved soil range and cement-improved soil strength on lateral bearing capacity of single pile are compared and analyzed by numerical simulation.It is found that the reinforcement width has a great effect on the lateral bearing behavior of steel pile,and has an approximate linear relationship with the bearing capacity of single pile.There is an optimal reinforcement depth and cement-improved soil strength,and the lateral bearing capacity of single pile is no longer improved significantly when the range exceeds it.(4)Based on the Winkel foundation beam theory,a modified p-y curve calculation model was established,and the key parameters in the formula,initial stiffness k and ultimate soil resistance p_u,were reasonably modified.The p-y curve calculation formula suitable for cement-soil composite steel pipe pile was deduced,and the effectiveness of the calculation method was verified by practical case.