Use of Time Domain Reflectometry to Determine the Length of Steel Soil Nails with Pre-installed Wires

Since time domain reflectometry (TDR) has been identified as one of the potential non-destructive methods for quality control of soil nailing works in 2003, further development work has been carried out. This report updates on the experience gained on the use of this technique to determine soil nail length. Various sources of uncertainty of a TDR test have been studied. They include human judgement in the carrying out of the test and interpretation of results, built-in error of the testing instrument, wire type, reinforcement size, and characteristics of grout sleeves. Among the sources of uncertainty, those related to human judgement and built-in error of testing instrument constitute the error directly in relation to the test. It is found that this test-related error is bounded by ±5% at 95% confidence level. Other major contributors of test-unrelated uncertainty include wire type and grout characteristics. In order to minimise the possible uncertainty of a TDR test, the installation details of the wire alongside a steel reinforcement has been standardized. Guidelines on testing procedure and interpretation of TDR test results are also given.