Study On Bearing Characteristics Of Vertical Stress Of Super Long Pile

The super long pile foundation has a good effect on the bearing capacity of the superstructure and the settlement of the upper structure.For the building with poor geology or large load,the super long pile foundation has good engineering and economic benefits,and is being widely applied in the engineering field.However,the theoretical research on the bearing capacity of super long piles is lagging behind the actual engineering.The mechanical characteristics of super long piles are still not very clear.There are many factors that affect their bearing capacity,which need to be studied carefully.Designers often analyzed in accordance with the method of ordinary piles,and design it by experience or the method of numerical simulation.It will cause the bearing capacity of design is not consistent with the actual limit.So the researches of bearing capacity of super long pile characteristics and influence factors of super long pile has become urgent.Basing on the theory of load transfer method which its distribution function is hyperbolic form,the force principle of super long pile has been expounded,the constitutive equation of the method is founded;The compression coefficient of pile body is introduced to get the method of calculating the compression of super long single pile body,and that becomes the settlement of the top of a single pile.The settlement of group pile can be obtained by method of equal pier and settlement ratio.The basic theory of the finite element method is introduced,and the concrete application of the finite element method on the stress of the super long pile is also discussed.A appropriate similarity ratio has been selected to carry out the indoor model test of bearing capacity of super long pile,and a series of conclusions are drawn: Before the ultimate load,the settlement curve of the pile top of the super long pile is relatively slow.the settlement is mainly composed of compression of pile body,pile tip settlement accounts for only a small part.The axial force and lateral friction resistance of the pile are asynchronous.The axial force decreases along the depth.The lateral friction is "Hump type" distribution when the load is lower,as the load continues to increase,the center of gravity is constantly moving to the end of the pile.At this time,the lateral friction resistance of the upper part of the pile is no longer increased.In general,the lateral friction resistance and the relative displacement curve of pile and soil,another name is called distribution function curve,is in the form of hyperbola.The softening of pile and soil will occurre when it goes to the maximum lateral friction resistance.For the different segments of the pile,the expression formula of distribution function curve is also different.The limit relative displacement of the part near the end of the pile is smaller,but the ultimate lateral friction is greater.The vertical bearing capacity of the super long pile is mainly provided by the lateral friction resistance.The end friction is only a small part.The vertical force model of super long pile is established by the finite element simulation software midas gts-nx.The result of its bearing capacity is obtained.Comparing this results to the model test results,it is found that they are more consistent.By changing the different parameters of the pile and the soil,the influence of pile length,pile diameter,elastic modulus ratio of pile and soil,cohesive force of soil and internal friction angle of soil to stress characteristics of super long pile is studied.It conclude that:The increasing of the pile diameter and the elastic modulus of the soil can increase the ultimate bearing capacity of the pile,it can also reduce the settlement of pile top.However,the increase of pile length can increase the ultimate bearing capacity,and the settlement of the pile top will be increased.So,the building which is required for the settlement of the pile top should be checked before using.The increase of cohesive force of soil can improve the bearing capacity of super long pile partly.The internal friction angle has little effect on its ultimate bearing capacity.