Enhancement of the GPR method of railway trackbed investigation by the installation of radar detectable geosynthetics

For several years ground penetrating radar (GPR) surveys have been used to provide a rapid method of investigating the depth of the ballast/sub-grade interface. At the same time, rail industry geotechnical engineers are increasingly recognising the benefits of geosynthetic products specifically developed to suit the variety of needs and subgrade conditions encountered during repair and construction of the trackbed. This paper describes the recent development of a specialised geosynthetic, namely Terram PW5, which is uniquely designed to enhance remote monitoring by GPR. The benefits of providing a geosynthetic which is clearly detectable by radar are firstly, that it is easy to confirm after construction that the geosynthetic has been installed, and secondly that it facilitates the measurement of the depth of ballast above the geosynthetic at any time after construction. Terram Ltd and IMC Geophysics Limited have collaborated to develop a GPR detectable geosynthetic, and also a method of GPR detection, which improves the accuracy of depth of ballast measurement. A further application of radar detectable geosynthetics is monitoring trackbeds which are susceptible to subsidence, such as those running over karstic landscapes or old mine-workings, where gradual downward deflection of the reflecting stripe may be used to identify developing cavities. Other possible developments will be briefly discussed, regarding additional geotechnical monitoring functions which may be implemented by means of suitably designed geotextiles.