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Looking for trouble

Railtrack Scotland's asset management strategy is based on sound risk assessment and a pro-active, rather than reactive, approach to remediation, says Andy Sloan.

Railtrack's geotechnical asset is of a grand scale, with much of the network running through cuttings and supported on embankments. Failure of these man-made structures has safety and financial implications for the operation of the railway.

Understanding the risk posed by slope instability in financial and safety terms is of critical importance, and for such a wide and varied asset it is a huge task. Railtrack Scotland has been working intensively for the last four years to define the risks posed by embankments and cuttings and to implement a slope remediation policy.

Initial stages of the process involved data collection and analysis which was carried out using the technique of stereo oblique aerial photography (SOAP) (Ground Engineering April 2000). SOAP is a tool that can be used in the assessment of risk associated with natural and engineered slopes.

Photography from a low-flying helicopter provides stereo-photographic images of the slope. Detailed engineering examination of the stereo oblique photographs and geotechnical observations made from the air allow a subjective assessment of risk to be made.

SOAP has been used extensively and successfully in the assessment of slopes associated with the rail network in Scotland. But this was simply the first stage in a huge data collection and assimilation process.

Assessment covered a total of 5610 slopes the number of embankments and cuttings in Scotland's rail network.The SOAP process was geared essentially as a coarse sieve to identify high- and low-risk sites and about 10% fell within this category.This still represents a large number of slopes.Consultant Donaldson Associates, which was carrying out the risk assessment work, and Railtrack Structures under the control of Allen Lloyd, recognised that a specific asset management system had to be developed for embankments and cuttings.This plan had to allow for the targeting of funds to remediate high-risk slopes, while recognising that not all high-risk sites could be remediated in the immediate short term.

For the system to work effectively, the philosophy of dealing with both problematic slopes and slopes in general has to be pro-active, not reactive that is, identifying where problems are likely to occur and dealing with these in a timely manner, as opposed to reacting once a slope failure has occurred.

The latter philosophy, simply to repair slopes as and when they either fail or develop problems, was considered incompatible with Railtrack corporate philosophy, which emphasises the safety aspects of running the rail network.

And taking remedial actions in response to emergency situations ie in an unplanned way has economic implications too.

Each of the high-risk sites was visited and a second risk assessment carried out, this time in more detail and geared towards providing a quantitative basis for the comparison of risk posed by each slope.The result of this was a register ranking each of these sites in order of risk.

Remedial works on the highest risk sites started last year. Donaldson Associates designed and managed the construction of remedial works on 12 slopes. The works were carried out by Ritchies (the geotechnical contracting division of Edmund Nuttall), John Mowlem and MacKenzie Construction, under specific framework agreements.

A wide variety of slope stabilisation measures were required for the works. The assessments indicated that in general terms, the highest risk was posed by rock cuttings.

While large scale kinematic instability is unlikely (but not unknown), in most cases the chances of block fall are very high with severe consequences.

The major part of the remedial works has been on rock cuttings. The works have involved the placement of rock fall protection netting, rock bolting, rock anchoring, the use of sprayed concrete and construction of high impact rock fall protection fencing.

The second area of major risk is related to the presence of cuttings where embankments have been built running along cut slopes ('side long ground') and the problems this introduces in relation to the management of water.

Numerous cases of instability and several of the slope failures suffered by the railway are related to the saturation and washout of soil slopes in side long ground. Remedial works have included both passive and active support and the reprofiling and replacement of failed material.

Common to all slope remediation projects is the management of any run-off water.

Year two remedial works are now being designed.This involves 36 sites across Scotland.The scope again involves a high percentage of rock cuttings but also some major embankments.

Andy Sloan is director of Donaldson Associates.

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