Banking On The Surveyor
(Prepared for Geomatics World Showcase Supplement to be published September 2004)

Modern survey techniques and technologies are playing an increasingly prominent role in the monitoring and recording of embankment stability across UK rail networks, says Dave Appleyard of Leeds based Met Surveys.

Both topographical and geophysical investigation methods are now used, independently and in tandem, to provide accurate measurement data and site condition information that will inform monitoring programmes and future repair and re-development projects.
Land survey techniques are principally used to monitor ground movement on railway embankments over a period of time by measuring lateral and height variations of permanent ground markers or anchors. These are installed at locations where movement is anticipated: typically, at intervals of
Carrying out a Topographical Survey of Network Rail Property
2m from the base of a slope to the top. The permanent markers have anchoring spikes which grip the ground at low levels to ensure relative stability with the surrounding earth. A special adaptor is then screwed to the top of the permanent marker when monitoring takes place. A Leica mini prism assembly can then be screwed onto this adaptor to ensure precise repeatability.

The current eastings and northings of each point are measured using a Leica TCRA1101plus instrument from (typically) 3 PGM's which are installed well away from the area of instability. Auto Target Recognition (ATR) is utilised to minimize pointing errors and several rounds of angles are taken between the Reference Object and the monitored points.

Back at the office, a least squares program (STARNET) is used to establish the most likely position of each point. Weighting is given to the angular measurements (+/-1.5") which are of greater accuracy than the EDM measurements (2mm +/- 2 ppm). The current levels of each point are measured using a Leica NA2 with PPM from a site TBM or deep datum, achieving an accuracy of +/-0.3mm.

The frequency of each repeat survey depends largely on the nature of the perceived problem, for instance if there is visible evidence of movement then a monthly revisit may be necessary to monitor the situation.

These three dimensional coordinates are then entered into an Excel spreadsheet and the relative movement in easting, northing and level can be plotted against time. The direction / speed and acceleration of movement and hence the possible causes can then be evaluated.

In addition to topographical methods of monitoring railway embankments, geophysical (non-intrusive) techniques are used, together with traditional intrusive methods, to help determine the thickness and condition of construction material within a railway the embankment.

A ground penetrating radar (GPR) survey is carried out along a length of railway track to produce a 2D section of the sub-surface. GPR provides an image of the sub-surface structure from the reception of reflected electromagnetic impulses transmitted into the ground. Multiple GPR traverse lines can be surveyed, which when processed and interpreted, provide a 3D image of the railway embankment. By using different frequencies of antenna it is possible to image the ground to different depths.

However, the deeper the signal is transmitted into the ground, the lower the resolution of near surface features and smaller targets, so the choice of antenna frequency is crucial to achieving a valid survey result.

For example, if information is required on deep features such as depth to natural ground under a large embankment, then a low frequency antenna will be selected. However, this antenna will obtain less information on near surface features, such as the depth of ballast or the location of utility or drainage features. The frequency of the GPR system used and hence the depth of penetration and resolution must be determined by the specific objective of the survey.
A GPR survey by itself can identify where there are different sub-surface layers but will not necessarily be able to determine the material composition. The collection of samples of ballast material at various locations along a length of track using intrusive techniques provides information on the condition of the ballast and the underlying made ground.
Met Carrying out Geophysical Investigations of Network Rail Property using GPR, Ground Penetrating Radar Technology
Condition information obtainable from a track bed survey usually consists of the location and extent of voiding beneath the track bed as well as moisture variations and the extent of the mixing of the material at the base of the ballast. This information is then used to confirm areas that have been identified by the GPR survey as being in poor condition. The combination of the non-intrusive GPR survey and intrusive sample collection provides the client with accurate information between sample locations so that a picture of the overall sub-surface can be obtained.

By comparing the depths and condition of railway ballast material along a length of railway embankment using a combination of GPR and conventional intrusive techniques, the maintenance history of the track can be established, highlighting possible areas where there may have been stability problems in the past. Where there is an abnormally thick layer of ballast material compared to the surrounding length of track, it suggests that the track bed has continually been "topped up" with ballast material to keep the track level.

An area where ballast has continually been replaced may indicate that settlement has taken place in that location, which in turn suggests that the embankment may be suffering from instability and should be monitored using a combination of intrusive instruments such as piezometers, slip indicators and/or magnetic extensometers. If ground conditions are optimal, a geophysical survey can be undertaken to map natural ground interfaces and slip surfaces.

One relatively new use of geophysics along railway earthworks is in the location and determination of extent of animal activity: the monitoring of badger setts for example. Even knowing where setts are located within a particular embankment can aid interpretation of the current and future stability of the earthwork!

Established in 1991, Met Surveys is a top five UK surveying specialist by resource and turnover. The company works in close cooperation with consulting engineering practises, constructors and rail operators throughout the UK on a wide variety of projects such as monitoring the stability of railway embankments, bridge and platform constructions, and investigating site conditions and utility location to inform infrastructure re-development plans.