Design Of An Automatic Monitoring System For Shield Tunnel Bed Stripping Disease And Analysis Of Measurement Results

Shield tunnel monolithic track bed stripping disease is an important factor affecting the stability of subway traffic and operational safety.In order to study the evolution of stripping disease under train vibration,an automatic field monitoring programme for stripping disease was developed and measured based on numerical simulations in a section of the Chengdu Metro where more stripping disease occurs.Combining numerical simulations with actual measurement results to analyse the development pattern of stripping disease under dynamic train loads.The main elements are as follows:(1)The existing engineering data and site research were collated and counted,combined with grouting material tests to briefly analyse the principles and influencing factors of stripping disease,and identified three aspects of monitoring under train operation:Variation of wheel-rail vertical force on track structure,stripping volume and settlement pattern of shield tube.(2)Using the bending moment difference method as a theoretical basis,ANSYS numerical simulations were used to determine the key measurement point for wheel-rail force measurement at the upper surface edge of the rail bottom at the span centre of the track structure,and combined with dynamic and static analyses to demonstrate the feasibility of static calibration.The calibration coefficients for key measurement points were obtained through indoor static calibration tests and the design of a wheel-rail force monitoring method was completed.(3)ABAQUS was used to calculate the structural response of shield tunnels under dynamic train loads.Based on the laws and range of values obtained from the calculations,a peel crack gauge scheme was designed and the accuracy and range of the sensor was selected.The seamometers are arranged from near to far from the expansion joints in the track bed,so that the changes in stripping can be measured at different distances from the expansion joints,facilitating comparative analysis at a later stage.(4)Based on the image processing techniques used in disease monitoring,an image monitoring scheme for tube settlement combined with a laser pointing device has been designed.Based on the frequency of axle action and the measurement accuracy required,a CCD industrial camera and matching lens were selected.This camera set has a frame rate of 36Hz and a measurement accuracy of 0.1mm at 5 megapixels.(5)The design of the signal transmission system and data acquisition system is completed.The electrical signals of the wheel-track force monitoring and stripping volume monitoring sections are transmitted over long distances using current,which is then converted to voltage for data acquisition,and the image signals of the tube settlement monitoring section are transmitted directly using super category 6 network cable.(6)Field monitoring work was carried out to measure the stripping disease of all trains passing through the monitoring section during the operating period for one day,verifying the feasibility of the monitoring system.The monitoring results show that the train wheel track droop force is around 80-100kN and the train load is concentrated in the low frequency part.The amplitude and frequency of stripping cracks do not show obvious periodicity under dynamic train load,and the stripping volume is significantly larger at the measurement points closer to the expansion joints of the track bed,which is in line with the numerical calculation results and field research rules.Both stripping and tube settlement reach their maximum at the beginning of the load action,with the amplitude oscillating back as the load continues to act,eventually approaching the initial value.At a train load of 97kN,the maximum stripping crack opening is 0.37mm,corresponding to a maximum tube settlement of 2.3mm,and the monitoring results are comparable to the numerical simulation.(7)Combining numerical simulations and field monitoring results,the main rule for the development of stripping disease under dynamic train load is that when the load enters the monolithic track bed,the other end of the bed will buckle and produce the maximum stripping throughout due to uneven forces.As the load continues to move,the amplitude of the stripping decreases.In addition,the unsynchronised settlement of the track bed and the tube under load is one of the causes of the tension on the contact surface and hence the stripping.The sides of the expansion joints are the weakest locations prone to stripping disease.