Transport geotechnics and geophysics

The BGS is working with industry and academia to develop geoscience tools for strategic railway planning and performance monitoring that includes:

Efficient rail performance requires that constant and level rail track geometry be maintained. The geotechnical properties of the subgrade — the native material or bedrock underneath the track — exert huge control over the performance of our railway network.

Efficient running of the existing network and plans to upgrade operational and abandoned infrastructure are high on the agenda. Now, more than ever, the rail network urgently needs fundamental research into the performance and properties of the subgrade particularly within superficial geology and aged-compacted clay engineered structures.

While much data can be gathered using traditional site investigation methods, the BGS is actively involved in developing new techniques to measure and assess subgrade properties for the benefit of the whole network.

Route evaluation and infrastructure assessment

From our previous work we have developed a range of standard practices related to routine route evaluation, engineering geological assessment, geotechnical and geophysical investigation of earthworks and subgrade conditions. 

We also manage a number of research sites where we specialise in the development of new technologies to monitor processes and performance of earthworks and the subgrade.

Geophysical monitoring of track and subgrade

We have undertaken a range of invasive and non-invasive surveys at railway sites including:

  • borehole drilling and core sampling
  • ground penetrating radar (GPR) surveys
  • resistivity surveys
  • static and dynamic cone penetration resistance tests
  • surface wave surveys

At one of out test sites, the materials from 0.4 m–0.8 m formed the original engineered ballast pavement constructed in the 19th century and GPR surveys show this pavement to be extensive across the embankment.

It appears as a strong continuous reflector at approximately 0.5 m depth along line one on the west side of the embankment, but as a highly disrupted reflector along line three between the rails.

This disruption is possibly a recent phenomenon related to the line taking very high train loads.

The original ballast comprises 'hand pitched stone' overlain by 'granite' chippings', Bidder (1900), which on inspection, are coated with a layer of soft, red-brown clay. This clay-coated interface was coincident with high moisture content in contrast to the above materials and is the cause of the strong reflecting boundary on the GPR profiles.

Profile from test site

Engineering geological assessment along the East Coast mainline

Map showing the East Coast mainline route laid over  bedrock and superficial geology.

Stiffness profile; Peterborough south

Along the East Coast mainline route, stress-controlled models produce equivalent stiffness; depth profiles from the site geology provided by borehole logs.

The models were developed from material testing in our laboratories and can be used to assess the effect of changing ground moisture levels that could arise from climate change.

Borehole logs from within some cuttings can show simple soil and rock sequences.

South of Peterborough (Figure 3), there is a sequence of overconsolidated clays and mudstones from the Oxford Clay and Kellaways Formations.

An overconsolidated clay layer extends to 4.3 m and is underlain by a mudstone. Both layers are of low to medium moisture content, so the low moisture stiffness curve would better represent the stiffness profile at this site.

At the Peterborough site shear moduli greater than 20 MPa are modelled at depths greater than 1 m, reaching 40 MPa at 1.5 m depth, however, these values are far lower than those calculated for the overlying ballast, being in the range 60–100 MPa.

[Gunn et al. 2003. Predicting subgrade shear modulus from existing ground models. NDT & E International (36), 135–144.]

3D geospatial models for route appraisals

Geological and engineering geological linear route assessment related to track geometry.

Rail network maintenance programmes can be improved with better integration of geological data from regional to site scales.

Attributed data now held in 3D geospatial databases can be combined in rating schemes designed to identify areas where ground conditions are difficult and likely to cause subgrade problems.

Studies undertaken in Manchester, London and Glasgow, where 3D models already exist, show the relevance of this approach for linear route appraisal.

These studies produce:

  • detailed geological sections along railway routes
  • identify problem soils at outcrop and subcrop
  • produce data applicable at regional and site scale
  • use the latest digital terrain models
  • correlate problem soils directly with track geometry
  • selectively evaluate geotechnical properties along the route to identify sections of poor track
  • give engineering geological assessments
  • enable assessments of made ground

Research and partnerships

The BGS is leading research teams in the management of railway research sites where new methods are evaluated. Partners include:

Our core research focuses on combined geophysical and geotechnical surveys to investigate heterogeneity within earthworks and the subgrade.

We're also developing methods and introducing new technologies that can underpin new proactive approaches for evaluating infrastructure condition and monitoring the processes affecting performance. We've achieved this by having a presence at several research sites including:

  • Leominster Railway Station; originally part of the Shrewsbury and Hereford Railway and now managed by Arriva Trains
  • East Leake Embankment; originally part of the Great Central Railway and now managed by the Great Central Rail (Nottingham) Ltd
  • BIONICS; a compacted clay embankment research facility managed by the School of Civil Engineering and Geosciences, Newcastle University

Leominster Railway Station

At Leominster our team our team pioneered new techniques to measure dynamic loads from rail traffic.

A vibration amplification phenomenon was detected that has significant implications for the subgrade problems that develop due to repeated traffic loading.

Increased vibrations detected deep within the subgrade are shedding new light on the processes by which fine-grained material is transported into the ballast.

East Leake embankment

Investigation at the East Leake Research Site.

New caption

At East Leake, Nottinghamshire, our team is developing new techniques to characterise the condition and performance of engineered embankments.

We have combined traditional site investigation techniques with new advances in resistivity surveying and continuous surface wave surveys, and are pioneering new applications of microtremor studies to investigate the distribution and variability in the geotechnical properties of engineered fill.

The photo (Figure 7) shows the field set-up for a continuous surface wave (CSW) investigation of ballast and earthworks stiffness.

These surveys provide shear wave velocity and small strain stiffness depth profiles of the ballast and underlying subgrade or earthworks. We have also undertaken multichannel analysis of surface wave (MASW) surveys at East Leake and have used both MASW and CSW to develop 3D models of embankment stiffness.

The 3D model (Figure 8) shows the effect on embankment heterogeneity caused by the end-tipping construction method and changes in fill materials. A central lens of harder materials comprising siltstone, sand and gravel can be seen as the green, higher stiffness zone (circa 100 MPa) within softer clay and mudstone fill seen as purples and blues (40–80 MPa).

These interfaces between engineering properties have a very dramatic effect upon track geometry, affecting travel speeds and ride comfort. In this case, the stiff lens causes significant problems and results in speed restrictions of 10 mph.

[Gunn et al. 2011. Embankment stiffness characterisation using MASW and CSW methods. Proceedings 11th Int. Conference Railway Engineering, London.]

Automated time Lapse Electrical Resistivity Tomography for Monitoring Embankments (ALERT-ME)

The ALERT technology uses permanent in-situ electrode arrays and permanent or semi-permanent instrumentation.

The system can be interrogated remotely from the office by wireless telemetry (satellite, GPRS, GSM/3G, internet) to provide volumetric images of the subsurface correlated with surface movements in real time, thereby eliminating the need for expensive repeat surveys or inspections.

This non-invasive technology will monitor the internal physical condition of engineered earthworks using diagnostic imaging methods, analogous to those used in medicine, such as computerised tomography scans (CT) or magnetic resonance imaging (MRI).

Real-time monitoring

Latest processing developments enable us to measure the movement of individual electrodes within the monitoring arrays. ALERT-ME demonstrates the application of ALERT technology for the real-time monitoring of strategically important and at risk earthworks (i.e. embankments and cuttings) within the transport network.

We aim to aid strategic planning and design of low cost, targeted preventative maintenance to ensure the long-term stability of earth structures.

This strategy will lead to significantly more efficient use of scarce maintenance resources and underpin sustainable construction within the UK's transportation network.

High-resolution volumetric information

Resistivity sensors are combined with geotechnical and environmental point sensors to aid the in-situ calibration of the resistivity images. The great benefit of resistivity imaging is that it can provide high-resolution volumetric information on subsurface structure, and when used in time-lapse mode, provides differential images to monitor changes in the moisture content of earthworks.

Combined with measurements of movement of the array electrodes we can now begin to understand the cause and effect between sub-surface processes, geotechnical property change and the degradation of internal condition leading to surface displacement, i.e. the 'whole' failure process and not just the final stages.

Data is transmitted, processed, stored and displayed using dedicated BGS servers and a secure web-portal. The entire process from data capture to visualisation on the office PC is automated and seamless — allowing unprecedented time-series images to be captured with no manual intervention.

This will provide the basis of a commercial web-based bureau service which will enable asset owners to remotely assess the physical integrity of important earthworks 'on demand'. The resulting resistivity images and associated calibration data will enable threshold moisture levels to be established to give early warning of instability.

Seasonal variation in rainfall results

This movie clip shows how seasonal variation in rainfall results in wetting and drying fronts migrating through the embankment via phases of infiltration and exfiltration. Midlanders may remember the summer of 2007 breaking rainfall records and being the wettest ever recorded.

This resulted in the embankment being maintained in a fully staturated, 'winter' condition during the summer months. This has implications for stability not only with respect to raising moisture content and material plasticity but also in relation to mobilising soluble mineral phases.

Preventive maintenance

The system will inform proactive strategies for targeted, preventive maintenance. The prototype ALERT technology has been installed on a section of Great Central Railway (GCR) embankment, near East Leake.

An advisory panel comprising transport sector stakeholders provides steerage and help to assess the cost-benefit of this approach compared to existing railway and waterway inspection procedures.

For more information go to ALERT-ME

Selected publications

Reeves, H J, Kessler, H, Freeborough, K, Lelliot, M, Gunn, D A, and Nelder, L M.  2005.   Subgrade geology beneath railways in Manchester.   8th Int. Conf. Railway Engineering, London.

Gunn, D A, Nelder, L M, Ghataora, G, Stirling, A B, Konstantelias, S, and Burrow, M.   2006.   Geophysical properties of the railway subgrade at a site in Leominster.   Journal of the Permanent Way Institution, 124, (3), 131–135, July 2006.

Chambers, J E, Wilkinson, P B, Gunn, D A, Ogilvy, R D, Ghataora, G S, Burrow, M P N, and Tilden Smith, R.   2007.   Non-Invasive Characterization and Monitoring of Earth Embankments Using Electrical Resistivity Tomography (ERT).   Proc. 9th Int. Conf. Railway Engineering, London.

Chambers, J E, Gunn, D A, Wilkinson, P B, Ogilvy, R D, Ghataora, G S, Burrow, M P N, and Tilden Smith, R.   2008.   Non-Invasive Time-lapse Imaging of Moisture Content Changes in Earth Embankments Using Electrical Resistivity Tomography (ERT).   Proc. 1st Int. Conf. Transportation Geotechnics, Nottingham, Aug 2008, 475–480.

Gunn, D A, Reeves, H, Chambers, J E, Ghataora, G, Burrow, M, Weston, P, Lovell, J M, Tilden Smith, R, Nelder, L M, and Ward, D.   2008.   New geophysical and geotechnical approaches to characterise under utilised earthworks. In: Ed. Ellis, E., Yu, H.S., McDowell, G., Dawson, A. and Thom, N. Advances in Transportation Geotechnics.   Proc. 1st Int. Conf. Transportation Geotechnics, Nottingham, Aug 2008, 299–305.

Gunn, D A, Haslam, E, Kirkham, M, Chambers J E, Lacinska, A, Milodowski A, Reeves, H, Ghataora, G, Burrow M, Weston, P, Thomas, A, Dixon, N, Sellers, R, and Dijkstra, T.   2009.   Moisture measurements in an end-tipped embankment: Application for studying long term stability and ageing.   Proc. 10th Int. Conf. Railway Engineering, London.

Gunn, D A, Ogilvy, R, Chambers, J, Meldrum, P, Haslam, E, Holyoake, S, and Wragg, J.   2010.   The first trials of the British Geological Survey's new ALERT-ME system for monitoring embankments using resistivity imaging have thrown up some fascinating results.   Ground Engineering, Sept 2010, 12–14.

Gunn, D A, Ogilvy, R, Chambers, J, and Meldrum, P.   2010.   ALERT?ME — New technologies for embankment warning systems. Rail Technology Magazine, Oct/Nov 2010, 80–82.

Chambers, J E, Gunn, D A, Meldrum, P I, Ogilvy, R D, Wilkinson, P B, Haslam, E, Holyoake, S and Wragg, J.   2011.   Volumetric imaging of earth embankment internal structure and moisture movement as a tool for condition monitoring.   Proc. 11th Int. Conf. Railway Engineering, London.

Gunn, D A, Raines, M G, Chambers, J E, Haslam, E, Meldrum, P I, Holyoake, S, Kirkham, M, Williams, G, Ghataora, G S. and Burrow, M P N.   2011.   Embankment stiffness characterisation using MASW and continuous surface wave methods.   Proc. 11th Int. Conf. Railway Engineering, London.

Contact

For further information please contact Dr David Gunn or Dr Jonathan Chambers