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Wealth of GI data key factor in Crossrail’s GBR success

The scale of investment in ground investigation data has been cited as one of the main factors in the successful use of geotechnical baseline reports (GBRs) on Crossrail.

The statement was made by GCG partner John Davis during a joint British Geotechnical Association and British Tunnelling Society evening meeting last night, which he delivered with Transport for London principal engineer Mike Black.

The lecture, entitled Crossrail’s experience of geotechnical baseline reports, aimed to give the client’s perspective of the use of GBRs on the scheme.

“It is my view that GBRs were successful on Crossrail in terms that they worked well,” said Davis. “They presented a good deal that balanced cost certainty with risk transfer.”

Davis said that the project invested in the region of £50M to £60M in ground investigation over a 20 year period, which he said that the ground presented “few surprises” as a result. He added that the fully engineered design enabled the GBRs to specifically address the planned construction techniques which also contributed to the success of GBRs, as well as successful amendment of the NEC3 form of contract.

Black outlined the history of GBRs from their first use in the US in 1997 to the later adoption of the concept in the UK, as well as the approach taken to development of GBRs by Crossrail. “GBRs clearly define the ground a contractor can expect to encounter,” he said. “They establish what is foreseen and what is unforeseen and triggers a compensation event.”

GBRs were used on 32 Crossrail contracts, which covered ground improvement and piling work as well as tunnelling for which GBR were originally developed.

Davis estimated that the cost of underground work on Crossrail has a value of £4.25bn. He said that despite the high value of the work only £15.2M was paid out through 132 valid compensation claims made under the GBR process.

“I don’t think that is many considering the number is spread across 32 contracts,” he said. “Only six were valued at over £1M, with the most expensive costing £1.6M. The mean value of claims was £139,000.”

According to Davis, 72 of the compensation events were related to obstruction, 23 were due to bentonite loss on diaphragm wall panels (the contractor claimed for each panel rather than the overall issue) and 16 claims related to contaminated ground. “Only nine related to actual geotechnical issue,” he said.

Davis added that there were claims for issues that fell outside of the GBRs, which came to a total cost of £10.8M. “Of these 23 events, most were due to obstructions,” he explained. “Four were due to ground conditions, water ingress or unexpected movement of third party assets. The remaining two were higher cost claims - £1.6M for ingress of ground into a tunnel and £2.4M relating to TBM obstructions.”

Davis added that 182 compensation events were rejected under GBRs but none of these were taken to adjudication, which he believes further demonstrates the successful application.

Black called for contractors to discuss whether their experience of GBRs on Crossrail had been positive in order for the industry to build on the knowledge gained during the work.

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