Bearing Capacity Analysis And Prediction Of Large Diameter Belled Pile

With the improvement of economic level,large-diameter belled piles have been widely used as the foundation for such structures as high-rise-buildings,factory building,transmission towers and marine structures.However,due to the complexity of its bearing mechanism,the theoretical research on large-diameter belled piles has not kept up with the application level,and no unified calculation method has been obtained in the determination of bearing capacity and deformation calculation.Moreover,the large-diameter belled pile is often affected by wind and wave loads when it is applied to marine structures,which will affect the stability and deformation of belled pile.Therefore,this paper studies the bearing capacity of large-diameter belled pile by means of field test,theoretical analysis and numerical simulation,and puts forward a simple and accurate formula for calculating the ultimate bearing capacity of large-diameter belled pile,and analyzes the stress characteristics of the soil around belled pile under the action of wind and wave.It is hoped to be helpful for the design of large diameter belled piles in coastal engineering.The main contents of this paper are as follows:(1)Based on the field load test results,this paper analyzes the influence of design factors on the vertical bearing capacity of the pile,such as the diameter of the enlarged bottom(D),the length of the pile(L),the ratio of the length of the pile to the diameter of the pile bottom(L/D),the ratio of the diameter of the pile bottom to the diameter of the pile shaft(D/d),and compares and analyzes the difference between the vertical bearing capacity of the belled pile and the equal diameter pile under the same site conditions.The results show that when the pile diameter and length are the same,the vertical bearing capacity of belled pile is much higher than that of equal diameter pile,and the debris at the bottom of pile will greatly weaken the bearing capacity of belled pile;when the large-diameter belled pile is subjected to vertical load,it behaves as friction end bearing pile,and its end resistance accounts for more than 65%of the vertical ultimate bearing capacity.For a given soil condition,the increase of pile length or bottom diameter is helpful to improve the vertical ultimate bearing capacity of the pile,and the optimal L/D ratio for the base diameter to play a more effective role in improving the bearing capacity is about 3.2.(2)Based on the field load test data,the ABAQUS three-dimensional finite element model of large-diameter belled pile is established to further analyze the influence of pile diameter(d),deformation modulus of bearing stratum(Eb),ratio of pile side soil modulus to bearing stratum modulus(Ec/Eb)on the bearing capacity of large-diameter belled pile.At the same time,based onπ theorem,the ultimate bearing capacity formula of large-diameter belled pile is put forward,and the predicted results are compared with the measured results.The results show that increasing the pile diameter can improve the ultimate bearing capacity of large-diameter belled piles,but the increasing range is small,and the increasing part of bearing capacity is mainly provided by friction;the change of bearing stratum can significantly affect the vertical bearing capacity of large-diameter belled pile;under the same conditions,the larger the modulus of bearing stratum is,the better the bearing capacity of belled pile is;with the decrease of Ec/Eb value,the increasing speed of ultimate bearing capacity of belled pile will slightly increase;the ultimate bearing capacity formula based on π theorem can accurately predict the ultimate bearing capacity of large-diameter belled pile,and the error between the calculation results and the field load test results is less than 10%.(3)Through the finite element software,the ABAQUS three-dimensional finite element models of large-diameter belled pile and equal diameter pile are established respectively.Considering the influence of the rotation of the principal axis of soil stress around the pile on the bearing capacity of the pile foundation,the stress characteristics of the soil around the pile under the wind and wave load are emphatically analyzed,and the law of the rotation angle of the principal axis of soil stress around the pile and the pile with the same diameter is compared and analyzed.The results show that the deviation stress and principal stress direction angle of the soil around the upper pile change obviously when the belled pile is subjected to wind wave load,and the influence of dynamic load on the soil around the belled pile is more obvious in the middle section.With the increase of the distance between the soil around the pile and the belled pile,the rotation amplitude of the principal stress direction angle around the pile decreases gradually until it decreases to a relatively stable value;The variation law of the angular amplitude of the principal stress direction of the soil around the belled pile and the soil of the equal diameter pile is different.