Comment

22.11.16

Geophysical condition assessment of railway earthworks

Source: RTM Oct/Nov 16

Dr Shane Donohue from Queen’s University Belfast discusses the findings of a recent project that used time-lapse monitoring of climate effects on earthworks using surface waves.

A recent DfT review on infrastructure resilience highlighted the importance of maintaining climate-resilient infrastructure, and specifically recommended that some geotechnical asset owners “maintain a strong focus on trialing newly available condition monitoring technologies”, and improve their “ability to identify and anticipate slopes that will fail and target remedial work as efficiently as possible”. 

Geophysical methods, when used as complementary tools together with traditional geotechnical investigations, are ideally suited for earthwork investigations. 

In addition to being non-invasive, several of these techniques are cost-effective and rapid, allowing detailed 2D and 3D volumetric information to be obtained, with repeated observations permitting temporal variations to be measured. This level of spatial and temporal detail cannot be captured using boreholes or other forms of geotechnical investigation. 

Geophysical methods are particularly useful if the measured properties are calibrated with relevant geotechnical parameters. One such geophysical approach, MASW (or Multichannel Analysis of Surface Waves), is fast becoming established as a reliable approach for imaging small strain shear stiffness (Gmax) distributions of geotechnical structures. Gmax measurements have been shown to be very sensitive to changes in pore water pressure, which directly affects the stability of earthwork slopes. 

The GEOphysical Condition Assessment of Railway Earthworks (GEOCARE) project was recently funded by the Engineering and Physical Sciences Research Council to investigate the use of MASW for monitoring climate-related variations in the mechanical properties of railway earthworks. 

A test embankment located near the village of Laverton on the Gloucestershire-Warwickshire Steam Railway line was instrumented with a weather station, along with a series of soil moisture and pore water pressure sensors and subjected to MASW monitoring over a 16-month period. 

It was found that MASW asset monitoring has the potential to provide internal asset information on climate induced pore pressure and small strain shear stiffness variations. Such observations indicate the potential for repeat MASW surveys to be used as a basis for long-term asset condition and deterioration monitoring, potentially informing maintenance and remediation decisions. 

In addition to monitoring the mechanical properties of the ground at targeted sites, MASW can also be adapted to enable rapid data acquisition over significant distances by towing a landstreamer behind a person or vehicle (road/rail) and recording seismic data at regular intervals. In this way several kilometers of earthwork may be investigated per day, enabling zones of low stiffness/soft material that may cause serviceability issues to be identified. This information can then be used for identifying suitable monitoring locations or it could directly inform remediation strategies.

FOR MORE INFORMATION

The research paper ‘Time-lapse monitoring of climate effects on earthworks using surface waves’ can be accessed at: 

W: http://library.seg.org/doi/full/10.1190/geo2015-0275.1

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