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HS2 cost cutting study reveals retaining wall design issues

NCE reports that the pilot study, which was carried out by Aecom, looked into how the designs of the bridges, viaducts, tunnel portals and retaining walls could be streamlined to reduce the cost of construction.

HS2 commissioned the report to ensure the project designs were the best value for money and the results have highlighted the impact of lack of ground investigation work on the design work undertaken so far.

Aecom engineering director Mark Raiss said around 12 main conclusions had been drawn from the study, which started in November last year. One was that design specifications for some of the structures had been incorrectly applied or misunderstood. The study also concluded that not enough design time had been spent on the smaller structures, which had pushed up costs. In addition, the study said that cheaper alternatives to the use of barriers on viaducts in the event of a train derailment should be considered. 

Last year HS2 chief executive Mark Thurston revealed to New Civil Engineer that the project has been facing increasing cost pressures ahead of the notice to proceed, the point at which the costs are agreed and construction of the line can start. The deadline for this was pushed back three months from February to June this year.

One example of where Raiss said the specifications had been misunderstood was where one requirement had been applied correctly in one situation and then “extrapolated” to apply to another with expensive consequences. However he said that the contractors, designers and HS2 working together had brought many benefits to the project.

“The specification for water tightness for retaining walls, this is good for tunnels [to make the concrete walls watertight], but when there’s rain on the other side of the retaining wall, why do you not want to make the ground water come through,” he said. “It’s just an example of one area where it’s a specification used on one area and has been extrapolated to another and added £20M to one structure.”

Raiss said the study found a lot of time had been spent designing the big ticket items, but not enough time had been spent on designing the smaller structures. But considering these would be repeated many times, this was an area where more effort needed to be spent.

The designers had not generally included much fat or contingency into the design which could otherwise have yielded cost savings, according to Raiss. The exception to this was in the case of foundations where there had simply not been enough ground investigation work carried out to design out the risk which had driven up pricing on these elements. However he said this had been inevitable at this stage due to the lack of access to the land at the time of the designs being carried out.

The study is one of a number of pilots being carried out looking at different aspects of the phase 1 design. The learnings will also be used to carry through to phase 2 of the line to improve the design going forward.

 

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