Faced with an unusual measurement scenario, ALPES TOPO, a survey company with expertise on large construction sites, engaged the technical assistance of Leica Geosystems in the setting up of an innovative monitoring system. Leica Geosystems’ monitoring experts provided a solution with wireless geotechnical sensors – WiSen wireless sensors, part of the portfolio of Leica Geosystems, part of Hexagon.
Automatic monitoring of two towers under construction
With approximately 15 stories and currently standing at one-third of their final height, the towers are at a critical phase requiring reliable and continuous information regarding the control of their structural stability. The towers are each designed with central structural cores, supporting the floors of the different levels, which contains the stairs and the elevator shafts.
ALPES TOPO performs regular manual optical precision surveys; however, due to project requirements for constant data it’s client required, they installed a real-time monitoring system for the central core of each tower.
The central cores of the towers, located at the heart of the structures and several metres behind the façades, make it difficult to traditionally monitor the buildings as construction progresses. As a setup of cabled sensors on this site is not practical either due to the construction. Thus, a hybrid of optical and wireless geotechnical measurement techniques was proposed to enable both the automated and manual data collection with optimum efficiency.
The first test with wireless sensors at 2.4GHz frequency (WiSenMeshNET) demonstrated that the structure of the walls and floors was blocking the transmission. The wavelength of the radio was unable to penetrate the structure to communicate between the sensors.
A network of wireless sensors
WiSen recently developed a new range of wireless sensors equipped with 868MHz radio (WiSenMeshWAN), the frequency of the radio signal is lower than the 2.4GHz models, but it has the advantage of greater penetration of solid objects and greater range between sensors.
Quick tests showed the sensor communication succeeded between five vertical levels of the structure. The devices use a hub to communicate data called SmartGateway, this was installed on the ground level and easily accessible to the project team. This facilitated communication with sensors located within the structure, via low-frequency data transmission.
However, even with this technology, the sensors positioned on the façades were located behind too many obstacles to achieve direct communication.
The WiSenMeshWAN sensors have a great advantage of creating a radio mesh automatically between them to communicate data. Therefore, there is not only a connection between the SmartGateway and each independent sensor but meshing from SmartGateway to sensors and then from sensors to other sensors, enabling sensors with no direct communication to send data.
Thus, the sensor located in the middle of the fifth floor is able to communicate with adjacent sensors on the same level located on the façades and with the other sensors located another five floors above.
By exploiting this simplicity of implementation and the power of wireless mesh technology solutions, ALPES TOPO designed an installation configuration that made it possible to directly measure the cores of the two towers.
The system is easy to configure directly in the field using a smartphone and will continuously expand during the construction of the towers with the regular addition of sensors as the construction progresses.
The completion of the tower construction is scheduled for the end of 2020, but the system will remain in place for several years. With the battery lifespan of the WiSen wireless sensors capable of exceeding 10 years of operation, based on a measurement every 30 minutes, no intervention on site will be required for the maintenance of the equipment.
A complete portfolio
WiSen connected sensor solutions allow you to monitor any type of structure in real time.
The WiSen portfolio of connected sensors includes tiltmeters, combined tilt and distance meters, crack meters and connected interface nodes which you can connect to Vibrating Wire (VW), RS485 or analogue sensors, such as strain gauges, load cells, in-place inclinometers, piezometers, temperature sensors, environmental sensors, and more.
Leica Geosystems supported ALPES TOPO using this wider portfolio of measurement sensors and monitoring techniques to create a bespoke solution building on their existing use of technology and combining measurement techniques for a Total Monitoring Solution.
Thanks to Corentin from ALPES TOPO for his invitation during the setting up of the monitoring solution.
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