DTSS used as early warning system for land subsidence
This application of Distributed Temperature and Strain Sensing (DTSS) technology as an early warning system for land sliding or subsidence was examined through a field test at a hillside in Korea. The results demonstrated the potential of the DTSS to monitor land integrity and areas at risk of subsidence. This case study was carried out in conjunction with Sensornet’s partner company SOAM Consultants Ltd based in Korea in mid-2005.
Land structure can be deformed with the passage of time by a lot of causes (for example: earthquake, ground condition). The deformation of the land can be estimated by strain measurement. If strain measurements can be repeated at many points in a structure, we can consider a counter-plan for the safety of the structure. However, if strain gauge rosettes are used for strain measurement, it is very difficult to attach many rosettes to the structure and also to measure strains simultaneously at so many positions. Meanwhile, strain measurement using an optical fibre will be more suitable for practical purposes, since strain measurements can be provided at every meter along the optical fibre attached to the structure. In this study we report an experiment demonstrating the reliability, performance and application of the Sensornet (DTSS) to monitor ground movement.
igure 1: Comparison of DTSS and strain gauges
LAND SLIDING AND SUBSIDENCE APPLICATIONS
The behaviour of rock and/or ground can be verified using distributed strain monitoring. After installation of a DTSS cable underground, periodic strain monitoring can be carried out. A counter-plan for safety can be established for any detected changes in strain. Similar measurements have been undertaken in embankment dams (please refer to the dam case studies for more information). Fig. 2 shows photos from strain monitoring of land sliding and subsidence that was performed at a hillside in Korea. Extremely strong, lightweight, rugged cables, designed for optimal strain transfer to the fibre were used and clamped on the subsurface at a depth of about 50cm.
Because land sliding and/or subsidence could not be expected within a short period in the test area, strain changes were artificially induced by a pushing tool and a weight drop tool as shown in Fig. 2. The results of DTSS measurements are shown in Fig. 3. A pushing tool caused the effect of land sliding, the corresponding strains increased up to 1,500µe depending on the pushing degree. It was expected that dropping a weight would induce slight subsidence. It actually resulted in 1,400µe strain increase. It has been proven by other experiments that the DTSS cable used here can measure up to 28,000µe strain, or 2.8%, for short periods of time.
Figure 3: The results of DTSS measurements
for land sliding and subsidence applications
Since the DTSS is able to measure ground movements over a 10km length of cable, it is anticipated that the DTSS system will be an efficient and economical warning system for land sliding and/or subsidence. Distributed strain sensing can provide a cost effective solution to subsidence monitoring.
To discuss your particular requirements please contact Sensornet or visit SOAM at www.soamkim.com for more information.