The contractor building the new TriMet transit bridge over the Willamette River began Monday morning a 36-hour continuous concrete pour that will form the base of the bridge’s east tower.

Ross Island Sand & Gravel is providing the cement for the project that will pour 1,700 cubic yards to create an 18-foot-thick concrete base to support the tower in the Willamette River. It’s the company’s largest continuous pour in more than five years.

The pour began at 7 Monday morning and will continue until about 7 p.m. Tuesday, Sept. 25.

The 1,720-foot-long bridge spanning the Willamette River will have two 180-foot-high towers in the water and two other towers on land. The cable-stayed bridge will be about 75 feet wide and have 3.5 miles of cable.

The bridge is a link in the 7.3-mile, $1.49 billion Portland-to-Milwaukie light-rail line under construction in Southeast Portland and Clackamas County. It will carry light-rail trains, streetcars, buses and have two 14-foot-wide bicycle and pedestrian lanes.

The cement pour is enough to create a 4-inch-thick, 3-foot-wide sidewalk about 8.7 miles long. It requires 180 truckloads of cement that will require 60 people to deliver.