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Cause & cure

Cover story; Boston Artery

What went wrong on the Big Dig and how the programme managers reacted

Unforeseen Ground Conditions

The biggest cause cited for the £900M overrun on the Boston Artery project is 'unforeseen ground conditions'.

Two thirds of the city's ground consists of man made 'backfill' hiding unknown obstructions which caused major difficulties as Big Dig contractors set about excavating 10M m3 of earth.

Massachusetts Turnpike Authority director of construction Joe Allegro explains: 'We've encountered obstructions such as old granite sea walls, piles and timber. Mapping of utilities was not very good and rerouting pipes that never showed up has been very expensive.'

Asked why it was not possible to be more accurate in their original ground inspection, he replied: 'Geotechnical surveys can tell you the obstruction is there but can't tell you what it is. This is one of the oldest cities in the US and a lot of the old plans of the city don't exist anymore.'

Comprehensive ground surveys on an excavation of this size were not possible, he claimed: 'There is no industry standard for checking the ground conditions on longitudinal tunnels. The ground was tested every 100ft. If we had tested it every 10ft, the process would have taken 10 times as long and been 10 times as expensive.'

Some of the most difficult ground was encountered when excavating the underground approach to Ted Williams Tunnel to Logan Airport, underneath the Fort Point Channel. Ground on which concrete immersed tube tunnels were being laid was found to be soft jelly-like clay. Bechtel and Parsons Brinckerhof opted to mix the ground with low density soilcrete to strengthen it.

Original project engineer for the immersed tube tunnelling, Mike Bertoulin of Parsons Brinckerhof says of the £1.25bn tunnelling operation: 'No one had done soil mixing to this scale before. To claw back time on our schedule we speeded up the casting and moving of the tunnel tubes.'

Speeding up the project in other areas typifies the general programme management approach of accelerating construction to keep the Big Dig on time. 'Time extension claims as much if not more than the extra cost of accelerating construction and the public get better value for money if the project is on time,' says Bertoulin.

Bechtel and PB have reacted to the slipping schedule by focusing resources 24 hours a day, seven days a week, on 'critical path' contracts which, if delayed, would have an expensive knock on effect in delaying other contracts. Alternatively they move resources away from a contract that has been delayed and speed up another area to make up the time.

The 'soft' skills

Construction on the Big Dig was slowed down after noise level agreements struck with Boston neighbourhoods were redrawn because of complaints from residents. Among the biggest irritations was the need to move an estimated 541,000 truckloads of dirt out of central Boston.

Big Dig spokesman Terry Brown says: 'Bechtel and Parsons Brinckerhof had to redraw the agreement for a lot of neighbourhoods and pay for extra mitigation measures. In some neighbourhoods we've paid for insulated windows, in others we've had to stop using jack hammers after 10pm. We can't use dirt trucks after 10pm because of complaints about the back up alarms.'

Extra mitigation has slowed progress on the project and cost an estimated £125M.

'We have learned from experience that it wasn't good enough to have a uniform specification for dealing with these issues,' says Brown. 'There was no rule book for the soft skills needed to keep a thriving metropolis open while digging directly underneath it. Bechtel and PB had to organise the kind of massive public outreach effort that you don't learn in engineering school and that's been a major lesson of this project.'

The new contract

With Big Dig managers overwhelmed by change orders to contracts, Bechtel and PB are in final negotiations with client MTA to extend their programme management role for another two years. The new contract, which will keep them on the Big Dig until 2002 with an option until 2004, is due to be signed soon. Bechtel and Parsons are expected to be paid an extra £160M.

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