Dec. 12 project update: The south portal comes into focus

Posted on Dec 12 2016 4:11 PM

It's been nearly one year since the SR 99 tunneling machine resumed mining following the tunneling delay. We'll be highlighting a different aspect of the program each day this week to illustrate the progress we've made to date.

It all began in the south end. Before Bertha, before cut-and-cover tunnels and new ramps and temporary lanes, the land to the west of the stadiums belonged to the viaduct's southern mile.

A time traveler from 2008 would barely recognize the landscape that's now home to the SR 99 tunnel's south portal. Back then, in addition to the viaduct, the area was home to warehouses and large swaths of pavement. Freight heading to and from nearby Terminal 46 was frequently blocked by trains.

Photos of south portal location

Today, the south portal is a much different place. The south end of the viaduct has been demolished and replaced, and a new overpass has created an uninterrupted connection for freight. A temporary section of highway carries SR 99 traffic around the tunnel work zone.

The land where the viaduct once stood is now bustling with construction activity. Crews have built hundreds of feet of cut-and-cover tunnel through this area, and the future entrances and exits into and out of the tunnel are visible at the surface. It's now possible to walk from the start of the cut-and-cover section of tunnel near South Royal Brougham Way all the way to the ground beneath Third Avenue and Blanchard Street – a distance of nearly a mile and a half!

Drivers on SR 99 are now treated to views of the south portal operations building's glassy exterior, along with its signature yellow ventilation stacks. This building, along with a similar building at the north portal, will house the tunnel's lighting, ventilation and emergency systems when it opens to traffic.

The south portal work zone continues to be the main hub for tunnel construction. It's where materials are stored, including the curved concrete segments that make up the tunnel walls which crews lower into the launch pit using a large crane. From there, a special vehicle drives them to the tunneling machine for installation.

West of the main work zone, on a portion of the Port of Seattle's Terminal 46, crews load excavated soil onto a barge that takes the soil to a disposal site in Port Ludlow. The soil makes its way to the barge via a conveyor system that grows as the tunneling machine moves forward. More than one million tons of soil have been removed from the tunnel so far.

New bridge technology

As part of a separate contract, crews working for Interwest Construction are building the future northbound off-ramp to South Dearborn Street. But this isn't just any off-ramp.

By combining memory-retaining metal rods and a bendable concrete composite, the future off-ramp will become the first bridge in the world built to sway with a strong earthquake and return to its original shape.

The video we released last month explains how this exciting new technology works.

What's next at the south portal?

Construction at the south portal won't slow down anytime soon. Future contracts will be responsible for building the final highway and ramp connections that will allow us to open the tunnel to traffic. Until then, we're working hard to turn the rendering below into reality.

Rendering showing what is next at the south portal


Order: 1.5