Crews worked at the scene where a gondola tower on Blackcomb Mountain partially collapsed on Tuesday. (Darryl Dyck/Canadian Press)

Whistler Blackcomb Ski Resort had been warned of potential structural failure to its gondola towers if water seeped into them and turned into ice, according to documents obtained by CBC News on Thursday.

On Tuesday, a ski lift tower on Blackcomb Mountain broke in two, injuring 13 people when several of the gondola cars fell to the ground, Dozens of others were stranded inside the remaining cars.

The damage to the Excalibur Gondola tower was likely caused by "ice jacking," a situation where the water inside the tower tube froze, expanded and caused the tower splice to rupture, a spokesman for the ski resort has said.

A similar tower, made by Doppelmayr CTEC Ltd., snapped in half almost two years ago on a ski hill near Kellogg, Idaho, and the manufacturer had issued a safety alert to all its customers, including Whistler Blackcomb.

The three-page safety alert warned that "accumulated water within tower tubes can have catastrophic effects upon structural integrity."

"Routine annual maintenance activities must include a close visual inspection of all tower components including the tower base," said the alert, issued in December 2006.

That report also detailed an audio test to check for water in the towers by striking them with a hammer in several ascending places and listening for a change in tone, indicating there may be water inside.

Warren Sparks, executive vice-president of Doppelmayr CTEC Ltd., says the company will issue a formal warning about the risk of gondola tower failure next week after completing its investigation. (CBC)

"We've relied on that safety system now for many years, and it's worked for us well. But obviously, it did not work for us here at Whistler," said Warren Sparks, executive vice-president of Doppelmayr.

"A huge amount of the safety relies on the ongoing operation and maintenance skills of the owners, Sparks said.

Whistler Blackcomb received the safety alert in February 2007 and all towers made by the Doppelmayr were checked, said Christina Moore, the ski resort's public relations and communications manager.

"Our records show complete compliance with the Doppelmayr test protocol," Moore said in an e-mail reply Thursday.

"The recommended test protocol appears now to be insufficient to deal with towers containing concrete. The Excalibur Gondola has four towers containing concrete, including Tower 4," she said, referring to the one that partially collapsed.

'Ice jacking' was also blamed for the collapse of this gondola tower, made by the same manufacturer, almost two years ago on a ski hill near Kellogg, Idaho. (Doppelmayr CTEC Ltd.)

"We are now working with the lift manufacturer and the B.C. Safety Authority to modify and improve the test protocol to include towers containing concrete."

Doug Forseth, senior vice-president of operations at Whistler Blackcomb, said Wednesday the gondola tower passed a safety inspection by the B.C. Safety Authority in October and no problem was spotted.

But Jason Gill of the safety body confirmed when it conducted its last inspection of the collapsed gondola tower in October, it did not perform an audio check that might have revealed whether water was inside.

"No, we do not do this on a regular basis. We do periodic inspections," Gill said.

Sparks said Thursday that Doppelmayr will issue a formal warning next week after completing its investigation. In the meantime, companies have the information they need to prevent a similar incident, he said.

"Our customers already know to check for this water and if they detect it, they're supposed to drill a hole to drain it out," Sparks said.