-- SHORELINE

Interstate 5 in North King County has been witness to a daily parade of heavyweight commuters the past few weeks, as oversize trucks haul huge slabs of concrete from Portland to Metro's new bus barn during off-peak traffic hours.

The slabs of reinforced concrete - up to 85 feet long - soon will be the roof for the bus facility. They'll support a 2-acre park/playfield and a single-level transit operations building.

Until all 258 slabs are delivered, probably in the next week or two, the semi-trucks will be plying the I-5 corridor before

6:30 a.m. and between 10 a.m. and 3 p.m. every weekday, said engineer John Vaughn.

Delivery of the slabs is a sign that construction of Metro's North Operating Base is progressing on schedule, according to Metro spokesman Earl Rice.

A tunnel under I-5 and an interchange just south of the 175th Street exit was completed late last year. The interchange will be used by the 225 buses stationed at the new base as well as the employee vehicles leaving and approaching the barn from north and south of the area, Rice said.

Since work began in August, Howard S. Wright Construction of Seattle has planted the foundations for the 14-acre multilevel facility, Rice said. Walls are now being erected for the parking area, and then the roof supports - those slabs of reinforced concrete - will be put in place.

The bus barn, located between North 163rd and North 165th streets just west of I-5, is slated for completion in April 1991, with buses to begin using it the following summer, Rice said. Estimated cost when completed will be about $32 million.