Harriet the tunnel boring machine on Tuesday began digging the second leg of the PortMiami tunnel — the westbound lanes of the under-the-bay facility linking the MacArthur Causeway with the port.

The giant rotating cutter head of the $45 million, German-built machine started slicing into the earth at the entrance of the second tunnel tube running from the port to the median of the causeway on Watson Island.

The eastbound leg was completed July 31, marking the halfway point of the $1 billion project designed to draw cargo trucks from congested downtown streets. Trucks clog downtown streets because it’s currently the only route they have to the port after exiting area expressways. The tunnel will provide the first direct link between the port and the expressways via the MacArthur Causeway, which is connected to Interstate 95 and State Road 836 via I-395.

The tunnel will consist of two underground tubes, each of which will carry a two-lane road between the MacArthur and PortMiami. The second leg is expected to be completed by April, said Chris Hodgkins, vice president of Miami Access Tunnel (MAT), the firm boring the tunnel. The entire project is expected to be completed by May 2014.

Digging of the second leg was originally scheduled to begin Oct. 19, but the start was delayed because of recent stormy weather.

“Essentially we’re on schedule,” Hodgkins said. “Harriet is beginning to slice into the dirt. We’ll be coming out back on Watson Island in April of 2013 and we will meet our … completion date of May 15, 2014.”

Once the project is finished, Harriet will be dismantled and its parts will be used in future tunnel boring machines.

Hodgkins on Tuesday led a reporter and a photographer from El Nuevo Herald on a tour of the recently-completed first leg, followed by a visit to the site at PortMiami where Harriet began boring the second leg.

Driving a Bobcat, a vehicle that resembles a large golf cart, Hodgkins traversed the completed eastbound tunnel — almost a mile in length— in minutes. The paved road inside the tunnel has yet to be built, but Bobcats and supply trucks drive on a smooth dirt deck that lines the bottom of the tunnel.

The eastbound tube starts at the MacArthur Causeway median and comes out on the port side, near Port Boulevard.