Adopting a simplistic approach, railway infrastructure comprises two component groups:1. Dynamic items
– the track and its supports, which experience dynamic displacements during the passage of rolling stock.2. Essentially static items
– where changes in adjacent structures or the geotechnical environment cause displacement to other adjacent static engineering components. The track can also experience these displacements.
The first group require movements monitored at short time intervals (minutes, even seconds) together with three-dimensional monitoring of their absolute, static location to determine when maintenance is required. The dynamic components are one-, two- or sometimes three-dimensional and best measured with accelerometers.
The ‘static’ location is best determined by the use of automatic total stations (figure 1). It is the rail part of the track that is important and the most effective method is to fit reflecting prisms to key parts of the structure and to the underside of both rails with Pandrol clips (figure 2) to determine cant, slew, gauge-widening etc.; monitoring the movements of sleepers has been tried and whilst this gives a good indication of twist and settlement the results can be difficult to interpret.
The key ‘static’ infrastructure components of the railway system are embankments, cuttings, stations, bridges and tunnels. The main movements of interest are usually the deformations transverse to the long axis of the railway. This reduces many projects to a two-dimensional situation and a combination of inclinometer and settlement units can provide the essential understanding of the displacement field, particularly if a mesh system can be set up and the displacements of the nodes used to determine the shear stain pattern.
Very few current situations (see figure 3 above) are green-field projects; most involve the impact of new works on existing, often complex units of infrastructure (e.g. Blackfriars, Oxford Circus). Measuring absolute stresses or loads in existing soil/structure interaction situations is virtually impossible as the zero load origin cannot be ascertained; only load changes can be measured and these are likely to be qualitative rather than quantitative. Displacements are real quantifiable measurements but only from the time of installation, the convergence measurements in new tunnels are a similar case.
If a two-dimensional mesh system within the surrounding soil is in place prior to construction of a tunnel or any other soil/structure interaction problem, then a meaningful set of absolute displacement and shear strain patterns at working load can be obtained (figure 4). This strain pattern can identify the developing mechanism that would eventually lead to failure. A back analysis can then be confidently based on this identified mechanism (an upper bound analysis).
Predictive analysis for this derived data has to be found by numerical analysis as classical upper and lower boundary assessment and provides no displacement data.
Numerical analysis is fraught with assumptions, the choice of a soil model and its basic parameters, and the ever-complex interaction with water, seepage and consolidation. An interesting potential use of instrumentation would be to set up idealised field situations – embankments, cuttings for instance – and back analyse the instrumentation behaviour to determine at least some of the soil parameters.
The more conventional uses of instrumentation are the following:1. For record purposes
. Checking that the displacement changes measured comply with prior experience or do not exceed acceptable numerically obtained displacements.2. Observing changes in ‘near real’ time
. To control the influence of new adjacent engineering work (e.g. excavation, loading, grouting etc.) on a critical existing structure.3. Monitoring key points
which have been shown to be critical from previous experience or numerical analysis in identifying the safety of the on-going work in order to minimise risk while still allowing rapid progress and economic savings.
The important local changes within structures are tilting, bending and cracking; figure 5 shows a typical crack unit. Figure 6 shows some typical tilt units (two- and three-axis units are also available).
Probably the most sensitive areas of railway infrastructure are tunnels, particularly old ones under urban areas, where new work by the rail owner or other utilities has to be constructed adjacent to or even threading between existing tunnel structures. The existing facilities need to remain fully operative, be maintained and safe. The existing working tunnels need continuous monitoring of stability and clearances throughout the running day without access by personnel. Under operating conditions the output data can be very difficult to untangle and to identify all the quantitative magnitude of the individual influences.
Railways are very aggressive environments and considerable care and planning must be carried out with all the other interested parties engaged on the system so that all items, connections, monitoring units and personnel involved are never vulnerable and the data obtained has not been ‘corrupted’ by other occurrences within the railway system as a whole. These can range from high voltage spikes or discharges to small impacts from ballast and other items.
Railways are tough environments for instruments – even ruggedised ones, and any installations need to be regularly checked and maintained. It is essential that this component of instrument monitoring is appreciated at the initiation of a project or contract and all sides understand the implications and the costing. If there is a clear understanding and co-operation of all parties of how the data can help the project in allowing sensible modification and improvement in working practices, then considerable benefits result.