Research On Model Tests And Numerical Simulation Of Single Pile In The Loading Form Of Top Pressure And Bottom Support

At present,the research on load transfer of single pile and stress distribution in soil around pile under different loading forms is not perfect.In this paper,based on the research background of top-pressed pile and bottom-supported pile,the laboratory model test and numerical simulation of single pile under the form of top pressure and bottom support loading loading modes are taken as the main means.And the transformation coefficient of positive and negative friction of the pile body,the load transfer model and the stress distribution in the soil of the pile are studied.Firstly,through indoor model test,the load displacement curve,pile body axial force curve,pile side friction curve,pile side friction resistance and pile soil relative displacement curve under different loading modes are obtained,and the analysis and explanation are carried out.Secondly,in view of some problems existing in the current research at home and abroad,the transfer problem of positive and negative friction resistance of pile body,the load transfer function model and fitting problem,and the stress change in soil around pile under load are studied respectively,and some scientific conclusions are obtained.Finally,the model test is simulated by the FLAC3D numerical simulation software,and the simulation results and the test results are mutually verified.Through the indoor model test and numerical simulation of the top-pressed pile(without soil under the pile),the bottom-supported pile,the top-pressed pile and the self-balanced test pile,the conclusions were as follows:(1)The indoor model test of the top-pressed pile(without soil under the pile),the bottom-supported pile,the top-pressed pile and the self-balanced test pile in the silt is studied,and the axial force distribution of the pile is related to the loading position.The pressure distribution of the top-pressed pile(without soil under the pil)is similar to that of the conventional top-pressed pile,and present the shape that upper is large and bottom is small.The bottom-supported pile is similar to the axial force distribution of the self-balanced upper pile,and present the shape that upper is small and bottom is large.The top-pressed pile(without soil under the pile),the bottom-supported pile and the upper section of the self-balanced pile all have"sudden failure" when in the ultimate bearing capacity.The maximum of pile shaft resistance appears at the length of 1 to 3 piles from the loading point of the pile.This phenomenon does not vary according to the top pressure or the loading direction of the bottom support.(2)The different loading positions significantly affect the distribution of the friction resistance of the pile side.The side friction of the bottom-supported pile mainly occurs in the middle and lower part of the pile,and the pile side friction resistance of the soil top-pressed pile(Traditional piles subjected to vertical compressive loads)mainly occurs in the middle and upper part of the pile.For the traditional top-pressed pile(with soil under the pile),the lateral friction resistance of the pile at different depths is different from the relative displacement curve of the pile and soil,which is close to the middle part of the pile,showing lateral resistance hardening,and the two ends of the pile show weak lateral resistance soften.The side resistance of the bottom-supported pile shows a weaker hardening phenomenon in the whole pile length range.Under different loading modes,the pile side frictional resistance is different,and the required relative displacement of piles and soils is different.The displacement of the top-pressed pile is greater than the displacement of the top-pressed pile(without soil under the pile),and the displacement of the top-pressed pile(without soil under the pile)is greater than the displacement of the bottom-supported pile.In addition,the negative friction of the pile in the silt is about 0.85 times the positive friction of the pile.,that is to say,the conversion coefficient of pile negative friction resistance is 0.85.(3)Further research shows that the relationship between the lateral frictional resistance and the relative displacement of the pile under different loading points and loading modes in the silt is well in accordance with the hyperbolic function model.Using the linear function of deformed hyperbolic function to fit the ?-s curve has higher fitting degree than using the hyperbolic function model directly to fit the ?-s curve,that is to say,the accuracy of linear fitting is higher.By two methods fitting the measured single pile side frictional-relative displacement of pile and soil curve,the fitting degree of ?-s curve is higher than 0.98.In addition,it is found that when the measured data and the function model directly fit the accuracy is not high,the linear fitting formula can significantly improve the determinable coefficient,further improving the accuracy of the fitting.Therefore,for engineering tests or theoretical derivation,it is recommended to use the linear fitting method after hyperbolic deformation to analyze the load transfer curve.(4)The stress variation in the soil around the pile during the loading of the pile is analyzed The additional stress in the soil increases with the increase of the load on the pile body,and shows a linear distribution.Under the same load,the additional stress of the top-pressed pile is greater than the bottom-supported pile.The additional stress variation in the soil at the distance of 1.7 d from the horizontal direction of the pile body is relatively large,and the additional stress change at the distance of the pile body 4.5 d is very small,only 1/14 to 1/10 at the 1.7 d.In the depth direction,the additional stress in soil of the pile in the top-pressing mode present the upper part is large and the lower part is small,which is opposite to the bottom-supported mode whose the upper part is small and the lower part is large.The top-pressed pile shows the loading effect on the soil in the range of 4.5 d,and the bottom-supported pile shows the unloading effect.Under the condition of self-balanced test pile,the loading effect of the lower section pile on the bottom of the pile is greater than the traditional static load test.(5)Under the same conditions and loading methods,the numerical simulation results show that the ultimate bearing capacity of the pile,the distribution of friction on the side of the pile and the stress distribution in the soil around the pile are close to those obtained by the laboratory model test,and the error is within a reasonable range.The numerical simulation results verify the accuracy of the indoor model test results.