Time-lapse Monitoring of Geotechnical Properties of Heritage Earthworks by Means of Near-Surface Seismic Techniques

Abstract:

Surface wave (SW) method and P-wave refraction tomography are widely spread methods for the characterization of the near-surface. We applied these techniques to a 1-year time-lapse monitoring of the geotechnical properties of a heritage earthwork at risk of failure, a stretch of the embankment of the Gloucestershire-Warwickshire railway in Laverton, UK. Like a significant part of UK railway network, this line was built in the early 20^th century without modern construction standards. Poor maintenance and the increase in extreme weather events due to recent climate change have further compromised its stability.

The aim of this monitoring campaign is to assess the capability of non-invasive and repeatable geophysical methods to measure temporal changes of mechanical parameters of earthworks.

MASW (multichannel analysis of SW) and P-wave refraction data were repeatedly acquired along a 100 m line on the crest of the embankment, every other month from July 2013 to July 2014. Smaller scale seismic data were also recorded on the flanks of the embankment. Sensors measuring climate –temperature, precipitation, solar radiation - and geotechnical parameters – water content, suction – were installed at various locations and depths within the embankment. Moreover, penetrometric data were acquired, soil samples were analysed.

SW data were analysed in terms of phase velocity and attenuation. Hodocrones, dispersion and attenuation curves show a limited but continuous seasonal change. SW dispersion curves and P-wave travel times were separately inverted for V_S and V_P models with a laterally constrained and a tomographic approach, respectively. The V_S and V_P sections describe the temporal variation of seismic properties of the embankment, consistent with the climate trend. Such results were jointly interpreted with data from field sensors and cone penetration testing. This calibration stage provides a geotechnical model that explains the temporal variations of seismic velocities.

MASW and P-wave refraction have proven to be adequate in depicting the seasonal change of seismic velocities of a heritage earthwork. The joint interpretation of geophysical data with sensors, sample and penetrometric data provides a geotechnical model explaining the variation of mechanical parameters of the soil with time.