Railway Infrastructure Assets/Tunnels

Applications

There are many ways of gathering, recording and processing information for both tunnel and track inspections and assessment. Visual inspection and desktop study can record construction changes and apparent distresses for both assets. Repeated laser profiling can map tunnel dimension with enough precision to detect deformation. Coring and drilling can reveal the lining thickness at specific locations. Using these 'traditional' methods the tunnel engineer can gain an overview of condition and can identify potential trouble spots, but more information is needed for well defined maintenance and repair strategies.

With track condition, historical information and inspection pits are often the only method of assessing maintenance requirements. This lack of sub-surface information has been tolerated in the past because compensation for unknown problems could be achieved by frequent lifting and lining of the track, which now proves an expensive and time consuming maintenance method. These costs to maintenance budgets could be limited by a more effective information on sub-surface condition and thus more focused expenditure.

In response to this need, non destructive techniques have been developed to determine sub-surface construction and condition over much larger sample areas, bringing the benefits of rapid data collection with minimal disruption to the track operator and no damage to the fabric. Such methods enable potential problems to be found before they reach the surface.

Approach

Effective investigation usually relies on a combined approach where several test methods may be used and client and investigation specialist exchange test data and historical information openly. Accurate an reliable analysis of NDI data often requires targeted exposure of the structure, for example using narrow drill holes and an endoscope in tunnels or trench pits in the ballast.

The most versatile and best proven NDI method for tunnel investigation is impulse radar, which has been used in hundreds of surveys in the UK, Europe and USA over the last decade. It is unique in offering rapid sampling of large areas to determine construction and condition information. It has only recently been applied to Ballast condition assessment in this country and the USA, where its ability to collect continuous sub-surface information at reasonable speed is being seen as a major step forward in track inspection.

Infrared thermography is also being increasingly used for rapid assessment and location of water and structural anomalies in tunnels, but its success is dependant on working in optimum thermal conditions.

Logistics

Radar surveys in tunnels are typically conducted from the hydraulic platform of a road/rail access vehicle enabling the survey team to manoeuvre the measuring head over the lining in a series of longitudinal or transverse profiles. Longitudinal surveys are typically conducted at 4 to 8 kph enabling multiple runs through most tunnels even during short track possessions. Surveys are conducted by experienced teams, using specialist equipment built to remain operational under harsh tunnel conditions.

Since resolution of detail and penetration to depth are a function of the frequency of the radar antenna, tunnels may be surveyed using a multi frequency approach where high frequencies are used for high resolution, for example of lining separation, or voided perp joints. Lower frequencies are used for mapping deeper features such as voids, timbers or structures beyond the tunnel lining.

A graphic summary of the type of information available from radar surveys is shown below:

Ballast-track investigations can be carried out using a system attached to an adapted HiRail vehicle for higher speed investigations or by towing antenna on foot or slowly behind a rail trolley for more detailed investigation work. Ballast thickness information, and variations in moisture content (possibly related to conditions associated with track settlement ) can be recorded continuously and related to chainage recorded from a distance measuring unit in the vehicle.

Limitations

The effectiveness of radar, is reduced by highly conductive materials such as:

Shotcrete treated areas where small gauge reinforcement shields hidden structure
Thick soot deposits are highly conductive and reduce penetration
Engineering "blue bricks" with a high ferrous content are conductive and reduce penetration and resolution.
High clay or slag content in the ballast material.

GBG Australia have undertaken investigations of: