Published Thu 1st Sep 2022
Construction and modification of tunnels requires continuous convergence monitoring in order to verify the temporarily fragile stability of the structure in order to maintain safety and ensure construction progress. Conventional, mainly mechanical or geodetic observations can be obstructed by construction operations and infrastructure. Hence, autonomous wireless optical displacement sensors prove to be versatile, easy to handle and reliable for permanent and instantaneous condition monitoring.
Efficient yet safe construction progress can only be maintained if permanent knowledge of structural integrity is ensured. Many modern tunnelling projects practise the “observational method“ in which a parameter-based model is continuously populated with a range of ground and construction data including.
Unlike manual surveys that can be configured taking into account the current obstructions or obstacles, permanent systems must remain operational despite frequent temporary changes in the jobsite constellation. This complicates the use of long-distance total station observations and wired extensometers.
Wireless condition monitoring (WCM) systems have been found to be well-suited to this application, particularly as they can be moved as the tunnelling face progresses and extended to form multi-sensor systems.
Martorell and Costa Blanca Tunnels, Spain
As part of the Mediterranean Rail Corridor project, the Spanish Rail Infrastructure Operator ADIF needed to upgrade three old tunnels in the Catalonia region for use by bigger, faster trains.
Main contractor Dragados was tasked with strengthening the linings, lowering the track and installing overhead electrification. By lowering the invert the overall tunnel geometry was altered and there was a need to closely observe whether the redistribution of loads would jeopardise structural integrity. Conditions for long distance optical observations were expected to be very dusty, with sight lines blocked by construction machinery and materials. The use of total stations was therefore rejected. Dragados and its assigned monitoring consultant INSTOP opted for a Senceive WCM system utilising laser distance sensors (ODS) in a triangular arrangement, i.e. a node at each spring line and one at the crown. The monitoring team opted to use a combination of Total Station and wireless technology in order to provide a level of redundancy and cross-checking. A key advantage of the wireless ODS in this application was the shorter distances being measured across the tunnel as opposed to the long-distance measurement of prisms located along the tunnel length. This proved less likely to be obstructed by construction equipment in the busy site conditions.
Installation of wireless monitoring instrumentation is fast and straightforward. Instruments can be pre-configured off-site and settings can be modified remotely after installation. At Martorell and Costablanca Tunnels an installation rate of four profiles sections per hour was achieved. Two communications platforms are available. The 2.4 GHz FlatMesh™ platform enables high frequency reporting and multiple remote access options. The 868 MHz GeoWAN™ was chosen here however, because it works over greater distances and there was a need to connect nodes with Gateways located up to 1.2km away.
In addition to the convergence observation, longitudinal settlement was observed using wireless tilt nodes fixed to horizontal beam chains to measure vertical displacements which could be accumulated along the beam length to represent a mechanical levelling stretch.
Because a number of high-rise buildings were located above Martorell Tunnel there was a need to monitor ground movement during the works. Vibrating wire rod extensometers were installed in boreholes and connected to 4-channel nodes - thus integrating them with the rest of the wireless monitoring system.
Chopping Sodbury Tunnel UK
A similar task was encountered at the 4 km Chipping Sodbury Tunnel in south-west England. The 1902 completed brick-lined arch-profile underwent renovation and trackbed lowering to increase clearance and install overhead electrification. Beam chains with tilt nodes were installed. Due to the close proximity of the sensors the versatile 2.4GHz Flatmesh™ communication platform was employed. This provided high frequency reporting and enhanced reliability due to a degree of redundancy.
The monitoring at the three Spanish tunnels and at Chipping Sodbury tunnel are examples of successful multi-sensor wireless monitoring programmes. They highlight the speed of installation, reliability in challenging conditions and low maintenance needs of the technology as a means of mitigating construction risk.