Experimental Study On Vehicle-mounted GPR Monitoring High-speed Railway Tunnel

High-speed railway construction is developing rapidly in China.Due to the speciality and concealment of high-speed railway tunnel,it is difficult to detect and identify the internal quality defects or diseases of lining structure.Fortunately,based on long-distance detecting advantage,the innovative vehicle-mounted GPR technique,as a fast,high-efficiency and extensive detection technology,can be used to detect and evaluate the quality or disease of high-speed railway tunnel by gaining the intuitionistic structure section image.It has broad application prospects.However&at present,the theory and practice of using the long-distance GPR detection technology to identify the conditions of the high-speed railway tunnel is still in swaddling clothes.So it has theoretical and engineering significance in protection of high-speed railway to study on long-distance monitoring of high-speed railway tunnel lining based on vehicle-mounted GPR theory.The vehicle-mounted GPR system for tunnel detection uses air-launched antennas,which enable this system work within a certain distance from the lining surface.During practical detection,there are diffusion(geometric)attenuation in the air while absorption attenuation in the tunnel lining and surrounding rock.This paper focuses on the two electromagnetic wave attenuation characteristics of the vehicle-mounted GPR.Based on theoretical statistics,numerical simulation and outdoor experiment,the absorption and attenuation discipline of electromagnetic wave in tunnel lining-surrounding rock is achieved and the geometrical attenuation influence on the detecting depth of vehicle-mounted GPR is analyzed,which provides the theoretical basis for detecting the lining structural conditions of high-speed railway tunnel.The main works(contributions)are summarized as follows:(1)Based on the two kinds of train-mounted conditions,this paper establishes the essential requirements and conditions for vehicle-mounted GPR detecting high-speed railway tunnel.By analyzing the long-distance vehicle detecting exclusions,the distance between antenna and lining surface is 3m?4m.(2)Based on the abundant data of existing tunnel detection,188 groups of data from 33 tunnels located in Xiangfan-Chongqing Railway,Xi'an-Ankang Railway and Yangpingguan-Ankang Railway are selected as the sample source of statistical data.And the least squares method is used to fit the electromagnetic wave attenuation coefficient of lining-surrounding rock.Further,under the condition of 95%confidence,the paper estimates the attenuation coefficient value of 300MHz antenna detecting tunnel lining-surrounding rock,0.867?0.9,which provides theoretical basis for the detecting depth of the high-speed railway tunnel using vehicle-mounted GPR system with 300MHz antenna.(3)The amplitude response attenuation curve and the effective detection depth of vehicular GPR-300MHz antenna are determined under the different test distance by conducting outdoor tests.Based on the radar images of the numerical simulation and the existing tests(3m?4m),the relationship between the monitoring distance and the detecting depth for high-speed railway tunnel,by the vehicle-mounted GPR system,is obtained from the angle of attenuation theory and the analysis of the experimental results:The corresponding effective detection depth is reduced from 2.61m to 0.98m when the monitoring distance gradually changes from 1m to 4m.Detailedly,when the antenna is located within the vehicle gauge locomotive in the high-speed railway,for 200km/h railway tunnel,the distance between vault and antenna is 2.85m-3.70m,minimum detection depth is about 1.0m?1.4m;for 250km/h dedicated-passenger railway tunnels,the distance from vault to the antenna is 3.25m?3.88m,the minimum detection depth is about 1.0m;for 350km/h dedicated-passenger railway double-line tunnels,the distance from vault to the antenna is not less 3.98m,the minimum detection depth is about 0.9m.Overall,the system can evaluate the tunnel lining-surrounding rock conditions of 200km/h single-line railway,while other high-speed rail tunnel detection depth is shallow,can only evaluate the lining layer state.(4)Aiming at the long-distance test condition of high-speed railway tunnel,the paper studies the signal enhancement technology of vehicle-mounted GPR,and puts forward the idea of superimposing multiple times in data collection,which amplifies the deep echo signal effectively and can compensate system for the diffusion loss caused by the increasing distance between the antenna and the tunnel lining.Further,through the outdoor experiment,the more the accumulative number is,the more the effect of the deep echo signal is enhanced.When the parameter is 4 and the antenna is 4m away from the lining surface,the detection depth can reached 1.5m,which can be used to evaluate the conditions of lining-surrounding rock for the high-speed railway tunnel(200?350km/h).However,the more the accumulative times,the slower the test speed,it is recommended that the stacking parameter is 4,not only should the requirements of the lining structure and the surrounding rock disease census been meet,but also a higher collection speed is maintained.(5)In view of the vehicle-mounted GPR system,this paper puts forward that the long-distance vehicular detecting conditions of high-speed railway tunnel,and the vehicle-mounted GPR long-range detection scheme,can be detected in full section for single-line tunnel and double-track tunnel can be unilateral reciprocating detection.Fundamentally,a set of antenna pedestal and fixed method with long-distance detecting high-speed railway tunnel is developed.The research results rely on the railway scooter that can enter the high-speed rail line as the installed object of the antenna pedestal,and the specific solution scheme is also given.In addition,this implementation measures can meet the needs of the rapid detection of subway tunnels,highway tunnels and other underground caves.Especially,in the implementation of on-site testing,which broaden the application scope of vehicle-mounted GPR system.This study presents a certain reference value of promoting the high-speed railway tunnel quality inspection&disease detection.And it can provide positive research direction for vehicle-mounted GPR monitoring of high speed railway tunnel.