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Workers will start digging the foundation in June for the first phase of’s three-block high-rise complex in the Denny Triangle, a spokesman says, and the buildings should be finished by late 2015 or early 2016.

The updated timetable was one of many details about downtown Seattle’s biggest development ever that Amazon’s architect, development manager and general contractor revealed in a presentation Thursday to Commercial Real Estate Women/Seattle.

Jeff Giuzio of development manager Seneca Group, who provided the construction schedule for the first phase, said the timetable for the second and third blocks is uncertain and “dependent on future Amazon head count growth.”

Amazon consultants told the city last year that the three blocks probably would be developed consecutively, at roughly two-year intervals. John Savo of NBBJ, the complex’s lead architect, said Thursday that the second phase may be built “sooner rather than later.”

The first block scheduled for development — between Sixth and Seventh avenues and Virginia and Lenora streets — will include a 37-story tower and a low-rise meeting hall seating up to 1,800.

Demolition and pollution-cleanup work is under way there now, Giuzio said. Crews also are relocating utilities from the now-vacated alleys in that block and the block to the north, said Brad Hayes of Sellen Construction, the building’s general contractor.

When construction starts, there will be 600 to 650 workers on site at peak times, he added.

More big numbers: 250,000 cubic yards of dirt will be excavated for the building’s foundation and six-level garage. The tower will be built with 6,000 tons of steel.

The entire three-block complex could hold 12,000 employees, Savo said.

It’s been a year since fast-growing Amazon first approached the city with plans to build 3.3 million square feet of offices on the three blocks bounded generally by Sixth Avenue, Westlake Avenue and Blanchard Street.

Amazon bought the three blocks from Clise Properties in December for $207.5 million.

While Amazon intends to occupy all the office space in all three towers, Savo said the buildings were designed to accommodate multiple tenants if circumstances ever change.

“It’s just being responsible as designers,” he said.

Amazon has options to buy more Clise blocks and has purchased one small parcel on a block just north of the site from another seller. But there’s no plan now to build more, Savo said.

While work begins on the Denny Triangle complex, Amazon continues to expand elsewhere. It already occupies about 2.8 million square feet in that neighborhood and South Lake Union, and has agreed to lease at least 600,000 more.

Eric Pryne: or 206-464-2231