The Study Of Simulated Modal Used In Detecting The Ultra-shallow Railway Roadbed State

On-line reinforcement, testing and evaluation on railway bed is of great significance for the maintaining of railway system. The test based on the properties of elastic wave frequency response curve is a new method to evaluate the nature of bed and effect of reinforcement. The method has the advantages of bringing high efficiency and enabling testers to get parameters of medium dynamics. However, special researches should be made as to its theoretical foundation and practical effects.Starting from the concept of simulated modal, the author generally introduced the basic theory of staggered grid finite-difference algorithm used in simulated modal test analysis and numerical simulation. In light of the structures of railway bed and railway diseases, the author built a non-bridge and non-culvert bed model featuring soft rock strata for one and normal conditions for anther. The author also worked out the curves of natural frequency of vibration and of frequency response of bed structures under the two different conditions, with related formula of natural frequency of vibration of layered soil from the theory of Site Seismic Evaluation Theory and according to forward modeling records based on elastic double-wave field. The curves show that, compared with normal bed, the dominant frequency of the other kind of bed gradually becomes low.The elastic wave exploration system ensures the quality test of railway bed directly done on railway ballast surface. Relying on that system, the elastic wave tests were conducted on reinforced and unreinforced soft rock bed of heavy haul railways in a work area of Shuo-Huang Railway. Their frequency response curves match the profiles of earthquake image, which further proves the disciplines. And the method based on property analysis of frequency response is applicable, when normal seismic exploration and GPR can not accomplish the evaluation of railway bed containing very soft rock strata.The subject of the paper is the seven sub-subject of the National Hi-tech R&D Program—Continuous Test on Railway Bed (Program 2009AA11Z101belonging to863Program). The study was conducted on test of railway bed diseases from the angle of elastic wave dynamics. It expands the ways of test and evaluation of railway bed and provides support for application of simulated modal theory into practice.