The existing slab is already showing signs of failure and there is concern that the tunnel walls may become unstable when the existing slab is removed. The tunnel of approximately 470m demonstrates high levels of ground water which further raises the concern over the tunnels stability when the ground works take place.

MONITORING OBJECTIVE

The structural health monitoring is to provide 24/7 observation of the tunnel walls behaviour, identifying any changes the walls verticallity, which would suggest a loss in the structure’s stability and present risk of structural failure during the ground works to replace the slab track. In addition there is a desire to understand the water saturation variation in the ground behind the tunnel walls. In order to measure and change in the distance between the tunnel walls that does not reflect in a change in their verticality, the absolute position of the walls is to be periodically measured before and during the works..

INSTALLATION CONSIDERATION

SENSORS: To ensure the tilt sensors are monitoring the tunnel walls overhall behaviour and not just that of the specific brisks the sensors are attacehd to, the tilt sensors were to be installed on tilt-beams. However there is not 1m of vertical wall clearance without obstructions.

COMMUNICATION: The tunnels curvature and length provides a challenge for the sensor to gateway wireless connection, as line of site is not possible to all 100 sensor locations from the tunnel mouths.

POWER: There is no mains power available onsite and tunnel sits significantly lower than the surrounding dense foliage.

MONITORING SYSTEM

• 100no. Triaxial Tiltmeters

• 2no. Analogue Nodes

• 4no. Vertical Vibrational Displacement Sensors

• 2no. Communications Gateway

OUR COMPONENTS

• 100no. Custom 700mm Wall Tilt-beams Prism Fittings

• 2no. Gateway Cabinets

• 2no. Unvented 200kPa Piezometers

• 2no. Solar Power Systems

• 2no. Solar Panel Mounts

• 2no. 30m LoRa Antenna Extension Kits

• 1no. 25m Solar PV Cable Extension Kit

The custom 700mm tilt-beams enabled the tiltsensors to be installed at 10m intervals along both tunnel walls. Custom prism fittings were added to the tilt-beams to allow for the prisms to be mounted to the tilt-beams themselves and prevent the need for additional drilling and installation of these. Due to the length of the tunnel and the number of sensors being installed, two gateways were required for the project, each requiring power. Two 200w/100Ah solar systems were supplied to support the installation of the gateway at either end of the tunnel and keep the system online 24/7. To extend the LoRa wireless communication antennas in to the tunnel at either end to reduce the distance from the antenna to the further sensors, two 30m LoRa antenna extension kits were supplied to enable the installation of the antennas 30m away from the gateway location.

PROJECT NOTES

The lead time to supply the components was less than 3 weeks with products requiring specific customisation. The monitoring system installation was completed over several weeks due to limitations for track possession. The low elevation of the tunnel's high embankments and tree line presented a challenge for the location of the solar panels, most notably at the northern end of the tunnel.

Our 30m LoRa antenna extension kits provided a simple solution to the wireless communication range, and our 25m solar PV cable extension kits allowed for the 200w solar panel at the southern end of the tunnel to be installed above the tunnel entrance, while the power cabinet and gateway were installed adjacent to the tunnel entrance.

As the available sunlight at the northern mouth of the tunnel was particularly limited, it was proposed that the gateway and solar power system be installed approximately 200m further north of the tunnel on the south facing wall of another rail bridge. The increased range of the gateway at the southern end of the tunnel, due to the LoRa antenna extension kit, meant that the majority of the sensors were about to connect to this gateway and the remaining sensors at the northern end of the tunnel would have line of site to the relocated gateway and solar power system.

CONCLUSION

Our ability to customise our tilt-beams and supply them in a very short time frame provided an ideal solution to the limit space for installation and the need for 100 prisms to be installed. The prisms were pre-fitted to the tilt-beams significantly reducing the installation time and associated costs.

The solar power systems supplied provide ample power for the monitoring system

SUPPORT

We attended the site post installation to provide advice on the location of the solar power system to maximise the power generated. We recommended the northern solar system and gateway be moved to the bridge location as detailed above and assisted with the relocation of the southern solar panel to its position above the southern tunnel mouth.