Research On Uplift Bearing Capacity Of Piles In Soft Clay

With the development of national economic construction,the use of uplift piles is becoming more and more extensive.Based on the study of bearing capacity of uplift piles at home and abroad,this paper summarizes the calculation method of pile bearing capacity,carried out the indoor model test of small scale pipe piles in soft clay,and analyzed the characteristics of uplift and bearing capacity of different diameter piles in soft clay through the finite element software ABAQUS.The formation and dissipation of negative pore water pressure and the factors of affecting the negative pore water pressure are discussed.The main contents and conclusions are as follows:1.The theoretical calculation of the bearing capacity of the pile foundation can be divided into three categories: one is the normative method,such as the port engineering pile foundation specification,building pile foundation technical specification and API specification;the second is semi-empirical method,such as Meyerhof method,Das method and inverse truncated cone model calculation method,the third kind is limit equilibrium method,such as Chattopadhyay theory,Shanker theory,Kotter theory and He Siming's horizontal slice method.These algorithms consider that the uplift capacity of pile consists of two parts: one is the net uplift capacity,the other is the pile's weight.Most of theoretical calculation method neglect the contribution of the negative pore water pressure,resulting in the calculated value is less than the measured value of the uplift capacity.2.The small scale model test was carried out to study the variation law of the uplift load with displacement,the axial force of pile,side friction resistance of pile and soil pore water pressure.The results show that the axial force of model pile decreases uniformly along the depth when the uplift load is small,and the axial force decreases rapidly from the top to the bottom of pile when the uplift load increase.The pile-side friction resistance is smaller in the upper part of the pile,and the lower part of the pile is larger.The negative pore pressure increases with the increase of the uplift displacement.When the displacement reaches a certain value,the negative pore pressure reaches its peak value,and then the negative pore pressure decreases with the increase of the uplift displacement.The development trend of negative pore pressure over time is similar to that of uplift load,and the peak value of uplift load coincides with the time of negative pore pressure peak.3.The experimental procedure was simulated with finite element software ABAQUS and was calibrated with theoretical results and experimental observations,the bearing behavior of the pile and the production and development of the negative pore water pressure are studied.The finite element analysis shows that the negative pore pressure of the pile and soil contact surface is the largest during the upward pulling process.The development rule of negative pore pressure is increases at first and then decreases,when the negative pore pressure reaches the peak value,the pile end is separated from the soil.The negative pore pressure increases with the increase of pile diameter.With the increase of pile diameter,the generation and dissipation rate of negative pore pressure decreased.When the pile diameter is constant,he peak value of the negative pore pressure increases with the growth of the pull rate,but the peak displacement does not change.