A $100 million plan to fix the leaning Millennium Tower in San Francisco has been submitted to city officials for approval.

The plan calls for engineers to install 52 concrete piles to transfer a portion of the building's weight from its existing foundation to bedrock 250 feet below.

The 58-story skyscraper sits on a 10-foot-thick concrete mat foundation held in place by 950 reinforced concrete piles. Source: victorgrigas / CC BY-SA 4.0The 58-story skyscraper sits on a 10-foot-thick concrete mat foundation held in place by 950 reinforced concrete piles. Source: victorgrigas / CC BY-SA 4.0The 58-story skyscraper sits on a 10-foot-thick concrete mat foundation held in place by 950 reinforced concrete piles, which do not reach bedrock. Since its opening in 2009, the building has sunk 18 inches, leans 14 inches to the west and 6 inches to the north.

News reports said that the Millennium Tower Homeowner's Association and Mission Street Development agreed to a plan to retrofit the foundation with new concrete piles, hoping to stop the sinking and reverse the tilt. The plan needs approval from both the city and county of San Francisco.

If approved, contractors will install 22 new perimeter piles along Mission Street and 30 along Fremont Street, each measuring 24 inches in diameter and weighing 140,000 pounds. The engineers who designed the plan said the new reinforcement on the north and west sides of the tower will keep it from sinking too far and straighten the tilt.

The new plan outlines $30 million worth of work, but the entire upgrade adds $70 million to the price tag and would take 18 months to complete, news reports said.

In April, engineers proposed to drill piles down to bedrock to stabilize one side of the building. The approach would allow the other side to continue to sink until the building straightens itself. The job was estimated to cost from $200 million to $500 million, roughly as much as the $350 million it cost to build the tower in the first place.