Use Of An MASW Survey To Assess Flood Damaged Road – A Case History

An MASW survey was carried out over and around a road severely damaged by flood waters, and in the creek-bed below. The MASW technique was adopted because a significant amount of the site was accessible only by foot. The survey comprised 936 metres in six lines, on which 85 points were analysed for S-wave velocity profile. From the S-wave velocity, Young’s modulus and pseudo-N values were estimated, assuming density and Vp/Vs ratio and an empirical equation. While the data quality was reasonable and free of background noise, the analysis was challenging due to quick lateral variation of the S-wave velocity. The use of relatively fewer traces for lateral resolution results in a low signal-to-noise ratio, and identifying the fundamental mode Rayleigh wave in the frequency-phase velocity domain becomes difficult. To combat this, a number of mute parameters were tried to improve the dispersion images. The results show S-wave velocity sections with rapid variation and reasonably consistent confidence levels. The S-wave velocity sections were used to infer the depth to bedrock beneath the road, providing information on foundation depths on which gabion slope retention structures could be founded. Information on depth to bedrock was also used to inform how much a particular section of the road could be lowered, thereby reducing the height of planned slope retaining structures, and thus the overall capital construction cost of the project. The line along the creek bed was difficult to analyse due to the weak signal to the distant geophones, however the results provided a reasonably accurate interpretation of the thickness of fluvial sediments, which was proven during construction. Overall the results of the survey correlated well with boreholes drilled in areas where vehicular access was possible; following close correlation in those areas, the survey data was successfully extrapolated to other areas where access was only possible by foot.