Carmont – designing for success, working to fail
On 12 August 2020 a tragedy occurred on Scotland’s railways. A passenger train, already turned around due to blockages on the line, derailed. Three people tragically lost their lives.
After months of investigation the Rail Accident Investigation Board (“RAIB”) has released its formal report into the incident. The report details a number of failings, which led to the tragedy. This article considers two of those causes and some points that contractors and infrastructure groups may wish to consider.
The weather on 11 and 12 August 2020 was appalling. Landslides and flooding had crippled most of Southern and Eastern Scotland’s railway network. The RAIB noted that the rainfall at Carmont on the morning of 12 August 2020 rated as at least a 1 in 100 year event, and possibly a 1 in 144 year.
Sometime, most likely between 8:15 and 9:00 on 12 August 2020, gravel and other debris was washed onto the track near Carmont from a nearby drain. This completely buried the left hand rail and substantially covered the right hand rail (in the direction of travel). When train 1T08 reached the debris it derailed and struck a bridge parapet, which sent parts of the train down an embankment and scattered the remainder over the up and down lines.
The RAIB’s investigation focussed on what had caused the debris to be deposited on the track, and why the drain had failed.
The drain design
The point of derailment was at the end of a substantial cutting, constructed over a century earlier. Several landslips had occurred over the years, including one as early as 1915 and another as recently as 2008 which blocked both tracks. The 2008 landslide pushed Network Rail to start the works which led to the construction of a new drain in 2011/2012, along with other stabilisation works.
As designed, the new drain ran along the crest of the cutting, sloping gently down, before reaching the edge of the cutting and falling at a steep gradient (circa. 1 in 3) down to track level and then parallel to the tracks into a nearby watercourse. The drain was (for all but the last few metres of its length) formed of a perforated pipe, set in 20mm – 40mm diameter gravel so that it could capture the water running off from the fields above. As designed, the drain would have had sufficient capacity to deal with the heavy rain that fell on 11 and 12 August 2020. Any changes to this design should have been put to the designer and agreed before being made.
However, the contractor did not strictly follow the design and did not put its changes to the designer. While there were a number of deviations from the original design, the most critical was the addition of a bund running perpendicular to the drain on the steep section, which prevented water from spreading out and entering the drain along an extended length. This significantly increased the rate of water flow in a concentrated area near the bottom of the drain. The extra flow caused the overlying gravel to wash out of the drain and onto the track, burying the rails.
The as-built drawings
While not a direct cause of the washout, the RAIB also noted that the contractor had not provided as-built drawings on completion of the works. These are usually provided to allow routine maintenance to be undertaken, but also to allow the designer to ensure that the works have ben finished in line with the design. Without these drawings, a chance was lost to realise the risks of the redesign.
The lack of drawings may also have had a second effect. The drainage system (other than a section close to the track) was not entered into Network Rail’s Ellipse system and so was missed for routine maintenance and inspection. While the reason for this is unclear, it may have been due to the as-built drawings not being provided.
Tragedies of this kind are thankfully rare on Britain’s railways. Carmont serves as a reminder that the simplest of errors can have very significant consequences, and those consequences may not be known for years. It is inconceivable that anyone involved in the design changes, which incorporated the bund, had the slightest inkling that doing so could cause the drainage system to fail. However by not running those changes past the designer, as they should have done, it left the drain dangerously exposed to excessive water flow.
It must always be borne in mind that any system of this type is designed based on certain assumptions, predicted by models of both the environment and the system itself. Where a change renders those assumptions invalid the risk of serious consequences is magnified. Contractual processes for changes are put in place to avoid this very risk. By not following the change process in this case, not only was the drain unable to cope with the water flow, but lives were lost.
Contractual processes are incorporated into agreements for a reason. The designer is the expert. It has modelled the works and come up with a design that works. Any changes have to go back through the design process or bad things can happen once the works are finished. These are simply not processes that should be ignored. In addition, where following the design is critically important, consideration should be given to the use of a monitoring engineer. At Carmont the contractor was responsible for ensuring the works were properly constructed. It is possible that, if a monitoring engineer had been appointed, then the errors would have been picked up and dealt with at the outset.
Even after the works were finished. the chance to catch the problem before disaster struck was also lost for several reasons. At the forefront of those was the lack of as-built drawings. If these had been provided it would have been obvious that the drain had not been constructed as designed. This may have allowed the designer to spot the problem and avoid what happened subsequently. It is common in construction and infrastructure projects that the as-built drawings are delayed or not produced. Carmont again illustrates just how important these drawings are, and why they should be chased and procured for every project.
The Lord Advocate will now consider whether criminal proceedings will be brought against the various duty holders who may be considered to have been responsible for the accident. Whether the Lord Advocate’s decision is influenced by the fact that the contractor is no longer in business remains to be seen.
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