New York ? As many as 20,000 slaves and free blacks who helped build New York’s economy from docks to warehouses will be honored with a memorial near their burial ground.

At a groundbreaking ceremony Wednesday, African drumbeats accompanied the unveiling of the $3 million design by Rodney Leon, a Yale-trained architect who has lived in West Africa.

The monument’s design – a spiraling, sunken court made of granite from Brazil and Canada – includes symbols and hieroglyphics inspired by Leon’s time in the Ivory Coast. Jutting up from one side will be a slender, 24-foot-tall “ancestral chamber” meant to represent “the soaring African spirit embracing and comforting all those who enter,” he said.

The colonial-era cemetery where the slaves were buried is nestled between lower Manhattan high-rise buildings, near City Hall and adjoining the building that houses the New York offices of the FBI.

Closed in 1794, the five-acre burial ground was forgotten as a construction landfill eventually buried it 20 feet underground. When the cemetery was rediscovered during construction of a federal office tower in 1991, community pressure prompted the government to abandon the project.