CLIENT: CROSSRAIL / NETWORK RAIL
How the use of wireless monitoring systems can be used to
monitor trackbed infrastructure.
Overview
During the tunnelling works associated to the Crossrail project, track infrastructure at Abbey Wood Station Upgrade was required to be monitored for displacement movement associated to the impact assessment for ground movement from which a zone of influence was determined.
To be kept informed of any impact on the track infrastructure during the construction stage and transition of the TBM Network Rail commissioned a monitoring regime. Monitoring was to be carried out in accordance with NR Standard NR_L2_CIV_177, and trigger intervention levels set in accordance.
Due to the construction work next to the live NR property, weekly inspection was initially performed by the contractors to measure the cant and twist of rail tracks. The conventional method is using total stations in conjunction with prisms attached to the railway tracks. However, the current optical solution was strictly relying on ‘line of sight’ with constant track access requirement during manual monitoring periods. Once the data was collected, the site surveyor had to manually upload it to the system and create a report as request.To undertake this automatically two total stations would have been required be deployed to cover the area of concern and required sufficient power and installation area which was restrictive.
Solution
It was decided that both tracks would be monitored for cant, twist, and longitudinal profile settlement along a 90 m length of track. The monitoring contractor opted for a wireless monitoring solution rather than an optical survey method because more site visits would have been required, for installation and periodic maintenance associated with cleaning prisms or rectifying prisms which could be knocked off track. The use of WiSen high precision Omni Tilt Sensors because of their robustness and suitability for tough conditions at sites where construction and maintenance activity is taking place was decided as the most suitable monitoring solution and would also ensure worker exposure time to railway hazards was reduced .
The tilt sensors were placed at standard 3m centres on the track closest to the area of potential movement and risk mitigated to 6m centres in the zone further away. A monitoring duration of approximately 16 months was planned.
A total of 45 nodes with one solar powered 4G gateway were installed in a single shift to provide a wire and mains-power free solution. Sensors are fitted with antenna lids to ensure protection resilience to rail traffic and other activities.
Outcome
Multiple alert levels were set up for Network Rail and would be distributed via email upon a trigger alert breach. Data was made available in near real-time for authorised users on WiSen’s easy to use and remotely accessible Web Portal Software, along with the generation of an Auto Report containing a daily summary of movement.
As a pre-track condition survey was undertaken via traditional techniques the client imported the absolute cant and twist values such that the system was reporting both relative and absolute, cant & twist including twist ratio.
The high precision (0.0051mm/m on a normal 1.435 track gauge) along with a high frequency of measurement of the tiltmeters proved to be superior to that through an optical solution and provided the monitoring contractor and client evidence and assurance they had selected the correct monitoring solution for this project.
