The causes of error that led to the Carmont derailment22 Jun 22
Deviation from the planned design and a failure of communication were among the root causes of error that led to the 2020 derailment of a train in Scotland and the loss of three lives, delegates at GIRI’s online webinar heard.
RAIB inspector Nick Bucknall explained the events that led up to the accident and the findings of his subsequent investigation, which concluded that had the design of a drainage system been implemented as intended, the accident may not have happened.
The incident occurred on the morning of 12 August 2020 when a train travelling at 73mph struck debris that had washed onto the track at Carmont in Aberdeenshire, after extreme rainfall. The train derailed and struck a bridge; the power car fell off the bridge, and carriages were scattered over the line and down the embankment. The conductor, train driver and a passenger died. All six remaining passengers were injured.
The RAIB analysed data from on-train recorders, weather stations, CCTV, and voice communications. Inspectors also examined the track, the bridge, and the washout debris and surveyed the land above the railway, including ground water and soil characteristics. As part of the investigation, the design, construction, and maintenance of the drainage system was examined, and Nick focused his presentation on these findings.
The rainfall had caused debris to be washed onto the track from a steeply sloped section of a drainage trench. The trench contained a perforated pipe that had been installed to address a known problem with the drainage and the stability of the cutting.
“There was severe weather the morning of the accident,” said Nick Bucknall. “It has been calculated that this was a rare event expected at that location once in 100 years.” But he said the flow of water could have been safely accommodated, had the drain been constructed in accordance with the designer’s requirements.
The two-phase construction of the drain was a critical factor. “Phase one within the railway boundary was constructed in May 2011. The other section was constructed in autumn 2012 by a largely different team because a legal agreement with the landowner wasn’t in place in May 2011.”
The work was carried out by framework contractor Carillion, and the company employed Arup as its designer.
The RAIB report identified a number of things that were not built in accordance with the design. The main changes that impacted the event were the construction of a bund and the absence of pipes connecting existing drainage into the new system at the intended catchpit.
“The design made no reference to a bund,” said Nick. Although Carillion’s subsequent liquidation meant no records for this work could be found, the investigation established that the 20m-long bund was constructed around the same time the drain was built and its effect was to concentrate water flow into the funnel feature on the surface of the lower part of the drain above the catchpit.
A second issue was that the catchpit was not connected into existing drainage as the designer had intended. “For reasons not clearly established, this catchpit was built outside the railway boundary in the second phase of the work. The team that built the second part of the drain were unable to find anything to connect into the new catchpit, and so it was built without connections.
“Had the connections been made, our calculations show that the flow of water on 12 August 2020 could have been accommodated. If the connections had not been made and there was no bund, there would have been some washout in the lower part of the drain, but as a shorter section it would have involved less material with less risk of obstructing the railway.”
In addition, the RAIB investigation found no evidence that important differences to the design of the drainage system, including the bund, were referred back to Arup or Network Rail for consideration and approval. “There were no as-built drawings, and no handover or CDM pack has been found by Carillion or Network Rail.”
Nick’s presentation was followed by discussion of the causes of error and lessons that might be learned in the wider industry. Among the issues highlighted were the lack of record keeping and a failure in the handover and CDM process. The maintenance team had no record of the existence of the drain, and so the upper part of the drain, where the failure initiated, did not appear to have ever been inspected.
The lack of as-built drawings and the importance of not deviating from the design unless it had been signed off by the designer were also picked up on. “Question the design by all means, but don’t just amend it,” said one delegate. “If you ask those questions and are told not to change it, don’t change it. If you do change it, get that agreed. Contractors are paid to build the design, not to change it at a whim.”
Another attendee pointed to the piecemeal way the drain was constructed, which creates additional interfaces and increases the risk of errors. This highlights the importance of good communication, as GIRI director Cliff Smith said in his closing remarks. “Communication needs to go both ways between designers and contractors on site. This is a lesson we need to learn throughout the industry, and it all comes back to culture.”
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