Tunnels & Bridges Clearances


Rolling stock clearances to tunnels and bridges must remain within acceptable bounds for the safe running of vehicles to ensure that loads can be carried safely. Over time, these may change for various reasons. The track can move out of position when lifted for maintenance and because of ground instability and the tunnels themselves can deteriorate. There are only two main methods of measuring lineside clearances for tunnels and bridges: mechanical and with lasers. Mechanical measurements take a long time and are not even a feasible option if a detailed actual tunnel/bridge profile has to be recorded and matched against expected profile. A transition to laser technology in early 2000 proved highly successful but was still very slow and took nearly 20 hours to do a 1 km section. With the current technology, complete inspection can be performed at reasonable vehicles speeds where the speed has a inverse correlation with the data resolution produced. Current laser devices use “time of flight” technology to map out the asset. A periodically driven laser diode is use to send infra-red light pulses that are reflected by the surrounding structures and the echo is picked up by photo diode. The interval between the transmitted and received pulses is measured using a quartz-stabilised clock. A 360 degree laser beam is used to produce a complete profile of the surrounding area and generates a Lidar based point cloud output, where each point in the cloud shows the distance between the measured object and the sensor. The Lidar equipment is coupled with a linescan imaging system with LED lighting that records the tunnel ceiling and walls to generate a full image profile of the tunnel which is processed by high speed image analytics software to indicate surface damage, cracking and water seepage. Further sensors including thermal imaging may be used as needed to provide additional data on wall temperatures.

RV Diagnostics

Tunnel clearance measurement is performed by TrackVue S series combining information from track geometry, inertial sensors and LIDAR data. The position of the train within a tunnel depends on previously obtained track geometry parameters and a calibration/correction of sensor data to account for this is required before correct tunnel clearance measurements are generated. A 360 degree scan of the tunnel wall is undertaken which is corrected and compared with the expected ideal profile to determine regions of interest. The distance between ideal and actual profiles is thresholded to find clearance issues for the rolling stock.

The following measurements are made.

Tunnel Measurements
3D Tunnel profile giving a 360 degree view 3D point cloud data and review software, where each distance measurement is linked to track position along the direction of movement
3D Clearance Exceptions Start and end track position of each exception, its value with threshold applied, severity
Visual surface defects Start and end track position, position on tunnel, dimensions, severity
Water seepage and thermal defects Start and end track position, position on tunnel, dimensions, severity

Figure 1 shows a conceptual example of how lasers scan the tunnel environment and output corrected profiles for each measured position. The profiles are further thresholded to determine regions of interest with clearance issues that need further investigation. Figure 2 shows actual and expected profiles overlaid, and finally Figure 3 shows a clearance distance strip chart for all points on the wall measured, which are thresholded at user set values to identify those spots on the tunnel where measurements are below threshold line (exceptions).

RV Advantage

  • We can deploy solutions for higher speed inspections for clearances than possible with current solutions
  • We can deploy solutions for passenger vehicles if needed
  • Our measurements are corrected with data from track geometry and inertial sensors for accuracy