Alaskan Way ViaductThe Alaskan Way Viaduct section of State Route 99 has been a fixture of the downtown Seattle waterfront for almost six 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 were signaled 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 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. A 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 South Washington Street. WSDOT also imposed roadway restrictions that remain in effect today. Ongoing inspections have revealed other increased cracks, exposed rebar, and weakening concrete; all signs that the viaduct is aging and continues to deteriorate. |
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 Elliott Bay seawall. 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 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 were conducted to assess the seawall's condition. These investigations showed that the seawall's condition is worse than expected, and it 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 gribbles.
For more information about the seawall replacement, visit the City of Seattle website. |