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Explosives used to demolish Port Mann Bridge pedestals

0 93 Infrastructure

The ongoing demolition of the 50-year-old Port Mann Bridge, the old-timer that lies adjacent to its shiny new sibling, is moving forward.
Explosives used to demolish Port Mann Bridge pedestals

The ongoing demolition of the 50-year-old Port Mann Bridge, the old-timer that lies adjacent to its shiny new sibling, is moving forward.

“We need to remove the eight in-water bridge pedestals. Seven will be demolished by explosives, one will be cut into pieces,” said Greg Johnson, spokesman for Transportation Investment Corporation, the provincial agency responsible for the $3.3 billion Port Mann-Highway 1 project.

Demolition of the old Port Mann Bridge is required in order to complete construction of the new bridge to its full 10-lane width.

The process is complicated – dismantling is a more accurate description of what’s involved – and requires removal of the bridge’s superstructure, substructure, piers and footings down to the Fraser River.

Because the Fraser River under the bridge is a sensitive fish habitat and fish transit area from the beginning of March until June, no on-water bridge work takes place during that time of year.

“There were two blasts in early February 2014 and the next five explosions will take place during the low-risk window for fish, which resumes in June,” Johnson said.

“There will be a total of seven blasts and they will all be timed around the fish window.”

In order to minimize the disturbance to the fish, controlled blasts will take place at slack tide, when there is no tidal movement in either direction in the river.

“We set off small blasts in advance to scare the fish out of the area and then use the smallest charges possible,” Johnson said.

“And, we wrap bubble curtains around the pedestals, in order to minimize the effect of the blasts.”

Demolition of the original Port Mann Bridge began in December 2012.

“It is being dismantled in the reverse order in which it was built,” he said.

“It is a precise and thorough process, with as much engineering required as building the new bridge.”

The dismantling process began with the piece-by-piece removal of the metal barriers, signposts and lamp standards on the portions of the old bridge that overlapped with the new one.

This was followed by the removal of layers of asphalt and concrete on the northern section of the bridge deck and the steel support structures.

Next to go were the girders of the bridge deck.

They were cut into pieces, lifted off by a crane and put on barges, which carried them down the river for disposal.

“The north approach has been removed and the majority of the south approach has been dismantled,” Johnson said.

“Some girders and piers are the only parts remaining.”

On the ground on the Coquitlam side of the bridge, the piers have been dismantled by either saw-cutting or tipping.

“Where possible, the on-land piers were felled and then broken up for removal once on the ground,” Johnson said.

“The same process is underway on the Surrey side right now.”

There are two river pedestals that will not be removed.

“The one by Tree Island will remain in place, to help anchor the tip of the island against future erosion,” Johnson said.

“And, the pier on the south shore of the Fraser River is (required to maintain the) stability of the dike near the CN rail tracks.”

The last piece of the old bridge to go will be the main span arch.

“The first cut will take place in the spring,” he said.

“It will be cut into pieces and lowered onto barges.”

Because the arch is under such a large amount of tension, it needs to be braced during this process.

“Crews have started construction of two towers, 200 feet tall on the bridge deck, in order to brace the main span and remove the tension from the arch,” Johnson said.

“In order to be as precise as possible, engineers have been consulting the original blueprints of the bridge.”

Dismantling of the bridge will be finished by the end of 2014.

“In addition to having to work around the fish window, the major challenge we faced was keeping noise to a minimum, in order to avoid disturbing our neighbors on either side of the bridge,” said Johnson.

About 18,000 tonnes of steel and 90,000 tonnes of concrete are expected to be collected in the $40 million demolition project.

Johnson said that whenever possible, contractor Kiewit Flatiron will reuse and recycle materials from the bridge.

by Peter Caulfield

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