| The prominent problem faced by China's high-speed railways from large-scale construction to long-term security and stable operation is the dynamic performance evolution mechanism and laws of the infrastructure during its operation.As an important part of the railway line,the long-term stability of the subgrade under repeated dynamic loads is the basis for ensuring the smoothness of the track structure.The design theory of the pile-net composite foundation used in the design and construction of high-speed railway subgrade is not perfect,and the design method has not been incorporated into the specification.The mechanical properties and transfer law of low embankment with pile-net composite foundation under long-term dynamic load need to be studied.The main research content of this paper is as follows:1.Configure "transparent soil" as a "visual" subgrade test materialDue to the invisibility of subgrade and ground,the microscopic feature of the displacement field inside the subgrade and foundation cannot be obtained through these tests,resulting in different scholars' different understanding of the soil arching effect and failure mechanism,and thus the proposed bearing capacity formula results are different.As a new kind of geotechnical material,its unique transparency can solve the invisible problem of the interior of the foundation and the subgrade,and provides the conditions for the study of the microscopic characteristics of the pile-net composite foundation.This paper has carried on the experimental analysis to the physical mechanics performance of the transparent soil,the result shows that the transparent earth can replace the natural sand in the road and the railway subgrade engineering visual model test research.2.Study on the "power" soil arch problem of composite foundation with pile netsAt present,the research results of pile-net composite foundation are mainly concentrated on the static loading conditions.In view of the lack of research on soil arch under the action of "dynamic load",the dynamic load characteristics(amplitude,frequency)are studied in this paper for power transmission and soil arching effects.The influence of the factors such as the dynamic load characteristics,embankment height,and geogrid layers on the soil arch was revealed by the high-strength squall line test section,the indoor model test,and the particle flow software simulation.3.Design a small high-speed rail pile network composite foundation test modelThe "transparent soil" material was used as a kind of filler material for high-speed railway foundations and subgrade models to design a test model with a size similarity ratio of 1:50.Due to the "visualization" of the transparent soil,soil particle displacement and stress strain were analyzed by photographing and PIV software.Avoid measurement distortion due to sensor size effects,sensitivity,etc.Study on the space mechanism of microscopic soil arching effect.The power transmission and other laws of this model are consistent with that of the prototype high-strength rifling test section,which verifies the reliability and applicability of the model.4.Design and build a dynamic loading systemIn terms of test equipment,most of the dynamic load-loading devices currently use hydraulic servo and cylinder type,and the frequency range is 0-20Hz.The transparent soil test is generally a scaled test.The actual frequency of high-speed rail 5Hz is 30Hz in the 1:50 scale model.Some devices cannot meet frequency requirements.According to the demand of output,frequency and waveform of high-speed rail small-scale model tests,a dynamic load system with exciter-power amplifier-signal generator-impedance head-thrust rod-reaction rack as main components was designed.Low cost,can be promoted and patents have been granted.5.Numerical analysis of discrete elements establishes the connection between"stress arches" and "displacement arches"Establish a bridge from microscopic to macroscopic perspective of the "interaction force between soil particles and relative displacement ? shear stress ? soil arch" and reveal the load transfer mechanism of dynamic soil arches.Based on the PFC2D particle flow software,a corresponding program was developed to simulate the "arch structure" of the pile foundation by discrete elements.From the aspect of macro-mechanics,the contact force,principal direction of stress and stress distribution under static and dynamic loads were analyzed.And soil arch influence factors.6.Theoretical derivation of dynamic soil arch calculation methodThe structural mechanics arch structure analysis method was applied to the soil mechanics dynamic soil arch,a dynamic soil arch model was established,the theoretical derivation of the reasonable arch axis and catenary equation was carried out,the dynamic soil arch calculation formula was proposed,and the use of Taylor expansion simplified The calculation method is convenient for engineering applications.Based on this theory,the applicability of the subgrade design standards in Germany and the British was analyzed.This paper starts from the practical data of the high-speed rail line,visual model test,discrete element numerical analysis and theoretical derivation.Based on the"dynamic load" conditions,the formation,development and destruction mechanism of the dynamic soil arch are studied and the power is proposed.The calculation method of soil arches unifies the understanding of microscopic to macroscopic mechanism and macroscopic phenomena of dynamic soil arches.It provides theoretical support for the dynamic soil arch design and long-term maintenance operations of high-speed railway pile-net composite foundation structures. |
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