Infrastructure in 2014: Balancing risk and cost
Geotechnical failures present big risks to people, infrastructure and budgets, so managing assets to mitigate against the need for emergency action should be the preferred option, but it is one few clients are opting for.
The UK’s geotechnical assets are running on borrowed time, according to CH2M Hill technical leader for tunnels and earthworks Roger Moore, speaking at the Slope Engineering and Geotechnical Asset Management conference organised by NCE sister magazine Ground Engineering last month. “Last year was the second wettest on record and, although it was extreme, the conditions are not unprecedented,” he said.
“The wettest [year] on record was 2001/2002, so we had 10 years of stable ground conditions, and 2012 showed that our geotechnical assets are running on borrowed time,” says Moore. “Climate change suggests that we will have more extreme weather in the next 50 years. This should be ringing alarm bells for asset managers - either they can ignore it and deal with the failures, or try to understand the change and proactively manage our assets to prevent failure.”
While the need to understand the condition of an asset in order to make decisions regarding maintenance seems simple, there are appears to be a general lack of awareness of the impact that it can have on lowering risks and cost.
“There seems to be an acceptance that you should wait for failure before acting to resolve a problem,” says Uretek managing director Roland Caldbeck.
The reluctance to invest before failure comes partly because making smaller investments to reduce the risk may not completely remove it, and Caldbeck reports that many clients are looking for remediation to deliver zero risk.
“People need to understand that, for some problems with geotechnical assets, smaller, more frequent action may be more beneficial than one big, costly intervention,” he says. “Phased work means that failure is prevented, and these operations can be completed quickly so the infrastructure is only closed or restricted for a short period.”
“There seems to be an acceptance that you should wait for failure before acting to resolve a problem”
Roland Caldbeck, Uretek
According to Caldbeck, in some situations a permanent solution may not always be necessary. “We recently undertook some ground stabilisation work below some rail sidings where washing of trains had resulted in a loss of fines and ground subsidence,” he explains. “The lease on the depot only had another three years to run, so our client only needed a solution to allow the rail lines to remain operational for that period, not the next 100 years.
“The other alternative considered for the site was to completely rebuild the slabs below the affected rail lines and reinstall the track, which would have been a lengthy process and cost 40% more than the shorter term solution of polymer injection could offer.”
Caldbeck adds that the use of smaller, remedial action that is originally intended to delay the need for major investment can actually mean that the high cost of a permanent solution can be delayed or avoided completely. He cites the example of the Burma Road freight rail line in Glasgow, where Uretek has been working for the last year. The line had subsided by approximately 150mm due to tidal wash out from the River Clyde, and line speeds were restricted to 8km/h. “We have carried out four phases of polymer injection at the site, and each time the track has remained stable for a longer period, to the point where the multi-million pound full fix of the site using conventional methods may not be necessary,” says Caldbeck.
The concept of geotechnical asset management is not a new topic – the Geotechnical Asset Owners Forum (GAOF) will be celebrating its 10th anniversary in 2014, and remains focused on drawing together knowledge on condition monitoring and failure prediction. According to GOAF chairman and Network Rail route asset manager (Kent) Derek Butcher, who also spoke at last month’s conference, the group has identified a number of topics that still need to be addressed. As well as asset degradation and remote monitoring, one of the key topics for GAOF going forward is the impact of third party assets.
The impact of third party geotechnical asset failure on UK infrastructure has hit the headlines over the last 18 months, with Network Rail taking on responsibility for repairing a failed slope at Hatfield Colliery in order to reopen the rail line and Dorset County Council footing the bill to repair the failed slopes above the Beaminster Tunnel.
Infrastructure owners may be aware of the risks presented by neighbouring geotechnical assets, but Caldbeck says they are often reluctant to step in and to take action before failure. “Undertaking repairs to reduce the risk means that the liability could transfer to the infrastructure owner, and many are not able to take on this added cost, despite the potential for both risks and costs being lower in the long term,” he says.
“We have been involved in a case where a disused tunnel runs below a major road and water flow into the tunnel was causing ground subsidence to the road and services above. But because the problem is not impacting on the tunnel owner they will not take any action.”
Settlement in one section caused a sewer in the road to burst, and Uretek was brought in to help create a stable platform below the rebuilt sewer that could mitigate against further ground movements caused by wash out in the tunnel.
“Undertaking repairs to reduce the risk means that the liability could transfer to the infrastructure owner, and many are not able to take on this added cost”
Roland Caldbeck, Uretek
Caldbeck believes that because many geotechnical assets are hidden from view below ground, clients do not fully appreciate how much they rely on them until failure occurs. “We often see highway engineers who are very focused on the condition of the surface but have very little knowledge of the geotechnical condition of the sub-surface layers,” he says. “In the last few years we have carried out more and more ground penetrating radar surveys of roads to help provide insight into problems at depth before they are visible at surface. This gives engineers the opportunity to intervene at an earlier stage and helps highways departments to justify the investment of a more proactive approach.”
While predicting when geotechnical failure will occur is still almost impossible, it is clear that condition monitoring and maintenance is key to understanding which structures present the highest risk. “Unpredictability is still an issue but use of risk allows investment to be spread more thinly and enables asset owners to focus on maintenance to mitigate failure rather than failures absorbing a larger part of budgets,” says Butcher.