Research On Bearing Capacity Mechanism Of O-cell Test Pile

The O-cell test pile method is a method for testing the bearing capacity of piles.The load box is placed in the pile body,and the high-pressure oil pump is loaded into the tank for oil filling.It is suitable for large-tonnage piles or on-site conditions.At present,the O-cell test pile method has been applied in the fields of industry,civil construction engineering and traffic engineering,and has achieved good results and certain economic benefits.However,the theoretical research of O-cell test piles lags far behind engineering practice.Based on the static equilibrium analysis,the relevant specifications are given for the O-cell test pile bearing capacity conversion formula.However,engineering practice shows that the end resistance of O-cell test piles tends to be lower than that of top pressure piles under the same conditions.The existing conversion formula ignores the unloading effect of the negative frictional resistance of the upper pile of the O-cell test pile on the pile end plane,and the loading effect of the positive frictional resistance of the pile on the pile end,resulting in the bearing capacity of the O-cell test pile.When the top pile is converted,the bearing capacity of the pile end is underestimated,which affects the accuracy of the position of the balance point.In this paper,starting from the different working conditions of O-cell test pile and traditional vertical compression static load test pile,the problem of low bearing capacity of O-cell test pile in engineering is solved by Mindlin stress solution and Meyerhof pile end resistance formula.Firstly,the loading and unloading effect of the pile end plane is proposed and the calculation method is given.The pile side frictional resistance is regarded as the concentrated force.Based on the Mindlin stress solution,the pile-soil body is divided into n units in the vertical direction to obtain the interaction equation of each unit,and the additional stress generated by the pile side frictional resistance on the pile end plane is calculated.This additional stress is introduced into the Meyerhof ultimate pile end resistance formula to obtain an improved formula for calculating the ultimate pile end resistance.Second,the calculation model of the fracture surface of the O-cell test pile was established.The upper pile of the O-cell test pile is the bottom pile.In order to construct the equation of the fracture surface of the upper pile foundation,the angle between the fracture surface and the pile wall is introduced in the equation of the new fracture surface,and the Meyerhof deep foundationfracture surface shape is adopted in the lower section.According to the position of the balance point,the possible conditions of combining the fracture surfaces of the upper and lower piles are divided into three types,and the formula for calculating the ultimate end resistance considering the influence of the fracture surface of the upper pile is derived.With reference to the estimated bearing capacity of the upper pile calculated by geological data,the approximation method is used to obtain reasonable parameters,and the shape of the fracture surface of the O-cell test pile can be calculated.Thirdly,an improved method for bearing capacity transformation and balance point determination of O-cell test piles is proposed.Calculate the end resistance of the jacking pile and the O-cell test pile considering the loading and unloading effect,and use the ratio of the two as the end resistance correction coefficient to improve the existing conversion formula.When determining the balance point,the equilibrium point is first estimated according to the empirical formula method,and then a new equilibrium point is determined according to the corrected O-cell test pile end resistance.The balance point determined according to the improved method can make the upper and lower piles closer to the simultaneous failure state,and the improved formula is used to calculate the bearing capacity of the O-cell test pile,which is slightly improved compared with the existing empirical formula,and is closer to the top-loaded pile.