Railway formation condition assessment using seismic surface waves

Abstract

The demands of railway transport have been changing over the 150 years of existence of this type of transport in South Africa, specifically the performance requirements of the formation to cater for new traffic requirements. As such, it is important to assess the condition of this vital part of a railway track. This dissertation covers a research project conducted on two railway lines in which measurements of ground vibration were conducted in order to perform geophysical analysis and characterise the formation based on the results obtained. Measurements were taken on a 26 ton axle load track (Coal line, at Bloubank) and on a 20 ton axle load track (at Amandelbult) in South Africa. Planning and implementation of several test procedures to characterise track formation require considerable effort to minimize the impact on railway operations. Coupled with track occupation and the destructive nature of some of the test procedures, it is relevant to investigate alternative testing techniques to address the issues stated above. The use of surface waves for geotechnical characterization of sites is increasing worldwide. Applications to railway engineering have so far been limited to light load, high speed lines to minimize the use of poor geomaterials with reduced Rayleigh wave velocity. Four sites were identified where trains are operated at heavy loads, with the formation condition varying from poor to good. Seismic testing (geophysical) and conventional testing (deflection measurements) were performed at the identified sites. Seismic measurements were recorded using geophones as receivers, coupled to an amplifier and a computer. The source of the seismic events was the trains operating on the track and a hammer for impact testing. For the deflection measurements, the Remote Video Monitoring (RVM) technique was adopted. Dispersion analysis of the ground vibration experimental data was conducted using the multiple receiver method. The main conclusions reached with the analysis indicated that: __ Dispersion analysis had a good correlation with the formation deflection analysis; __ Phase velocity can be used as an indicator of the quality of a certain site; __ There are limitations when using trains as the energy source in terms of the generation of excitation frequency, which greatly reduces the phase velocity information in individual layers in the formation (i.e. wavelengths are not short enough).