ENGINEERS HAVE blamed sand or clay inclusions within a diaphragm wall panel as the likely cause of a major leak that closed Boston's £10bn Big Dig tunnel last month.
Water is reported to have 'rushed' into the northbound lane of the cut and cover tunnel near South Station, one of its deepest points.
Repair work took two days to complete, although a quick temporary fix meant the tunnel was operational during peak periods the day following the incident.
Crews have since carried out a permanent repair by injecting grout into the area of the leak.
Engineers from lead design contractor Bechtel/Parsons Brinckerhoff believe sand or clay got into the concrete when the tunnel's diaphragm wall panels were being poured a decade ago.
One engineer said that in the area of leak the construction team encountered sand layers at the bottom of the wall, and that there were only a few other panels with similar characteristics.
But he observed that it was unusual for a leak to take so long to manifest itself.
Tunnel operator Turnpike Authority has ordered an immediate review of other panels and described the leak as 'unacceptable' Chairman Matthew Amorello said, 'taxpayers and toll payers are not going to be paying for repairs or forensics' The Big Dig replaced the elevated Central Artery of Interstate 93 with underground tunnels through downtown Boston. It also connected Interstate 90 - the Massachusetts Turnpike - to Logan International Airport, and added the Ted Williams Tunnel beneath Boston Harbour.