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Orders from on high

Stereo oblique aerial photography offers a rapid method of analysing a large number of slopes systematically and provides an effective way of prioritising work on Scotland'srailway network.

A typical railway network stretches over hundreds of kilometres and includes many thousands of natural and manmade slopes. Issue 2 of a new Railtrack directive on the safe asset management of embankments and cuttings, GC/RT5151, comes into force this month.

Under it Railtrack is required to assess the condition of all slopes over 3m high, and any slope less than 3m in height that is known to be a problem.

This has big implications for Railtrack's regional operating 'zones' and could be very good news for geotechnical companies providing infrastructure asset condition appraisal services.

As owner of the nation's rail infrastructure, Railtrack generally holds good information on known problem slopes. But until now, maintenance work has tended to be reactive.

This old way of working is not compliant with GC/RT 5151, which advocates a more proactive approach to slope management.

Under the directive, each Railtrack zone is required to prepare an inventory of cuttings and embankments and develop an asset management strategy. Methods of achieving this are left to the individual zone.

Railtrack Scotland has taken a radical lead by calling in the services of consultant Donaldson Associates, which has set up an oblique aerial photography-based risk assessment package which it is using to catalogue the 1800km of track in Scotland.

Donaldson has been developing and refining its aerial photography skills over the last three years, and to date has assessed 4000 slopes in Scotland. Of these about 5% are identified as requiring some further investigation. Essentially use of aerial photographs helps Railtrack satisfy GC/RT5151 and filters out the need to visit 95% of the slopes.

In essence it is a process of looking at the whole network and then saying to Railtrack, 'These are the slopes you need to be worrying about.'

'The assessment is subjective and there is no penetrative geological investigation, ' says Donaldson director Andy Sloan, who heads the company's Scottish operations from Glasgow.

'Railtrack understands risk associated with railways and we understand engineering risk associated with slopes.'

Assessment is not a pure geotechnical risk analysis, but is focused on assessing risk to the track (a geotechnically unstable slope set back from the track would be a low priority for Railtrack).

'It's about sharing knowledge of risk to produce engineering designs that are appropriate to the client's needs, 'says Sloan.

Donaldson has spent considerable effort establishing the most appropriate technology and working practices. The aim is to achieve consistent and high quality results from which practical engineering judgements can be made.

The company uses professional photographers experienced with both stereo photography and working from a helicopter, and a professional processing laboratory.

It also charters its helicopters from one company, so that the pilots will be familiar with the process. Donaldson has two geotechnical engineers dedicated to the project, both of whom accompany each flight.

Armed with topographical and geological maps the engineers undertake an initial scoping flight to assess the photographic requirements and establish a flight strategy.

Co-ordination between pilot, photographer and engineers is vital to the success of the flight. Adjacent photographic frames require overlap to achieve stereo pairs, and so the helicopter speed in relation to the height and time interval between frames is critical.

Flying height varies from 15m-50m, although there are various flight restrictions, chiefly in urban areas, which sometimes preclude using the method.

The process generates an immense amount of information at a tremendous rate and keeping track of it is essential.After a flight, the films are catalogued and developed overnight.

It is important that the engineers who accompany the flight are responsible for interpreting the photographs.

As Donaldson's Helen Monahan explains, 'you might think that when you are looking at hundreds of kilometres of track that it would be impossible to recall small details about individual slopes - but it's astonishing how much detail the photographs trigger.'

The next step is for Monahan and colleague Neil Davidson to systematically rate each section against a checklist of factors such as height, slope angle, vegetation, drainage, evidence of failures, previous repairs and adjacent structures.

Each slope is then placed into one of four categories from one, where failure is anticipated within three months, to category four for low risk slopes.

'It is essentially a risk assessment based on engineering judgement, 'explains Sloan.

The aim is to produce a top 200 ranking of priority slopes. These will be reinvestigated with site inspections by Sloan and Railtrack's structural engineers Alan Lloyd and Grant Lisk to determine a pecking order for work.This too is done by helicopter, but simply as a means of transport to the often remote and inaccessible locations.

This year Donaldson will look at 600km of track, almost completing assessment of the entire 1800km of track in the Scottish network.

Sloan stresses the importance of Railtrack's involvement and commitment to the approach.

'The method is also cost effective compared with other techniques for producing a similar level of information, 'says Sloan.'To assess one slope by helicopter is very expensive, but to look at 800 is very cost effective.'

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