Report No. : GEO Report No. 133

Report Title : Non-destructive Tests for Determining the Lengths of Installed Steel Soil Nails (2003), 54 p.

Author : W.M. Cheung

Abstract

Steel soil nails are used extensively for slope improvement works in Hong Kong. In order to enhance the quality control of the soil nail installation works, an in-house project has been conducted by the Geotechnical Engineering Office of Civil Engineering Department aiming at identifying and developing effective non-destructive means for determining the lengths of drillholes and installed steel soil nails.

The study has identified two techniques for determining the lengths of drillholes and five techniques for determining the lengths of installed steel soil nails. The former two techniques include methods of utilising a plastic tube and a rod, and the drilling process monitoring developed by the Hong Kong Jocky Club Research and Information Centre for Landslip Prevention and Land Development of the University of Hong Kong. The latter five techniques include the Sonic Echo method, the Mise-a-la-Masse method, Magnetometry, the Electro-magnetic Induction method and Time Domain Reflectometry. All these techniques have been investigated and tried out in the field. Merits and limitations of these methods are discussed in the Report.

Based on the study, Magnetometry and Time Domain Reflectometry have been identified as the most suitable methods for application in existing soil nailed slopes. For new soil nailed slopes, it is recommended to install plastic tubes or electric wires along with the steel nails and to allow the use of the Electro-magnetic Induction method or Time Domain Reflectometry respectively as a method for quality control testing. Further development work on the Electro-magnetic Induction method with a single pre-installed plastic tube and Time Domain Reflectometry without the need of an electric wire is also recommended.

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