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SLOPE ENGINEERING - Real time monitoring technology has proved invaluable for examining unstable slopes in South Korea. Devon Mothersille and Tony Barley report.

Some 70% of South Korea is covered in mountains and hills and constructing the national road network has meant extensive use of cut slopes. But difficulties arise again from a combination of extreme climate and heavy rainfall, which has created instability in many of these slopes.

Answering the challenge is the Kumoh National Institute of Technology (KIT) in South Korea, which has developed a reputation as a formidable research institution. In the field of ground movement monitoring it has pioneered use of optical fibre sensing technology, installed with other measurement transducers in high cut slopes, tunnels and structures within the country's highways network. KIT has also gained recognition as a centre of excellence for research in geotechnical engineering and the development of the real-time monitoring and warning systems for slope movements.

The system can provide early detection and response to slope movements to mitigate against loss of life, damage to services and disruption to transport routes.

Specially developed measurement transducers, called TRS sensors, quantify parameters such as displacement (translation), rotation, settlement (hence TRS). Rainfall levels, regarded as important in the analysis, are also recorded with a relatively high degree of accuracy and used in the stability analysis of the slopes.

These parameters have been used to help the design of remedial support measures and also provide early warning systems by monitoring preset trigger levels.

One of the principal features of the system is the use of computer technology to provide measurements in real-time, making full use of the internet for remote access to the data.More recently this has led to a novel approach to observing incipient slope failure using close circuit television with digital cameras.

Twenty six instrumented sites are being monitored in South Korea and these form a substantial database of information which has helped enhance the monitoring system.

The versatility of the instrumentation allows it to be easily adapted to existing equipment using the options of wire, wireless or radio frequency technology for data transmission and remote data analysis via the internet.

Analysis of data from Korean slopes has revealed that 64% and 30% of slope failures can be attributed to geological discontinuity and rainfall/groundwater respectively.The use of the TRS sensor has identified that failure modes can be attributed to rock falls (43%) and wedge failure (20%).

KIT has also developed a Distributed Strain Sensor using optical fibre as a linear sensing element.

Fibre optic sensors generally consist of a glass core, 50 micrometres in diameter, surrounded by a glass 'optical cladding' giving an outside diameter of about 120 micrometres.

The fibre is used to transmit information using infra-red or even visible light as the carrier (usually a laser).

The Distributed Strain Sensor provides strain measurements at any point along the fibre with great sensitivity (0.002%) and over long distances (over 10km).

The sensor is therefore a cost effective tool for highway structures since a single fibre may remotely monitor many kilometres of embankment or any other geotechnical structure such as deep basements, slopes or tunnel linings.

The application of the real-time monitoring system provides a opportunity to use new technology to solve critical safety issues in susceptible slopes. Where circumstances dictate, the proposed system can also offer cost effective solutions to the real time monitoring of geotechnical structures.

Slope engineering firm Single Bore Multiple Anchor (SBMA) is now marketing and commissioning the real-time monitoring system in the UK and elsewhere.

Devon Mothersille and Tony Barley are directors of SBMA and Geoserve Global.

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