The Alaskan Way ViaductThe Alaskan Way Viaduct section of SR 99 has been a fixture of the downtown Seattle waterfront for over five decades. Today, SR 99 continues to be a main north-south route through the city, carrying 20 to 25 percent of the traffic traveling through downtown. However, its days are numbered. Time, daily wear and tear, salty marine air, and some sizeable earthquakes have taken their toll on the structure.
Studies in the mid-1990s showed that the 1950s-era viaduct was nearing the end of its useful life. The viaduct’s age and vulnerability was apparent by crumbling concrete, exposed rebar, cracking concrete, weakening column connections, and deteriorating railings. In early 2001, a team of design and seismic experts began work to determine whether it was feasible and cost-effective to strengthen the viaduct by retrofitting it. In the midst of this investigation, the 6.8 magnitude Nisqually earthquake shook the Puget Sound region. The earthquake damaged the viaduct, forcing the Washington State Department of Transportation (WSDOT) to temporarily shut it down.
Post-earthquake inspections of the viaduct revealed both good and bad news concerning its condition. The good news was that the viaduct survived the 6.8 magnitude earthquake. The bad news was the earthquake caused damage to the viaduct’s joints and columns, further weakening the structure and revealing its severe vulnerability. The team of experts concluded that it was not cost-effective to fully retrofit the majority of the viaduct; rather, the viaduct would need to be rebuilt or replaced.
Immediate repairs were made to four viaduct sections in the Pioneer Square area near S. Washington Street. WSDOT also imposed roadway restrictions that remain in effect today. These restrictions are for vehicles that weigh over 10,000 pounds. They include reduced travel speeds for large vehicles (from 50 miles per hour to 40 miles per hour) and require large vehicles to use only the right-hand lane of the viaduct. Ongoing inspections have revealed other increased cracks, exposed rebar, and weakening concrete; all signs that the viaduct is aging and continuing to deteriorate. |
The SeawallShortly after the Nisqually earthquake, a 100-foot-long by 10-foot-wide section of the Alaskan Way surface street settled, raising concerns about the condition of the Alaskan Way Seawall (Seattle Times 2001). The seawall holds the soil in place along Seattle's waterfront. The seawall also holds the Alaskan Way surface street and many utilities in place. The viaduct's foundations are also embedded in the soil held back by the seawall. If the seawall were to fail, sections of the viaduct, the Alaskan Way surface street, and adjacent structures and utilities could collapse or become unsafe.
Further investigations have been conducted to assess the seawall's condition. These investigations have shown that the seawall's condition is worse than expected and needs to be replaced. The seawall continues to deteriorate despite regular maintenance by the City of Seattle. Soils underneath the roadway moved and liquefied during the Nisqually earthquake. Liquefaction is what can happen to loose, wet soils when shaking motion from an earthquake causes the soil to turn into a quicksand-like condition.
In addition, marine organisms called gribbles have been eating away at the timbers that support the seawall. Inspections have shown that substantial portions of the seawall's timber support structures have been weakened or destroyed by the gribbles.
The Nisqually earthquake highlighted the inevitable fact that the viaduct and seawall are nearing the end of their useful lives, and it’s time to replace them.
For more information about the Alaskan Way Seawall, visit the City of Seattle Web site. |