Demolition to resume on old Nalley Valley Viaduct

Demolition of the original eastbound State Route 16 Nalley Valley Viaduct in Tacoma will resume on Friday, according to the Washington State Department of Transportation (WSDOT).

WSDOT and contractor Mowat Construction suspended demolition work on Jan. 11 after a bridge support column, called a “tetrapod,” fell onto the shoulder of South Tacoma Way. Per WSDOT’s standard practices, a safety buffer was in place while this section of the original viaduct was being demolished and no injuries occurred. The demolition work is part of WSDOT’s I-5/SR 16 Eastbound Nalley Valley project.

Following the incident, WSDOT and contractor Mowat Construction reviewed the project’s demolition practices and procedures. Mowat determined that the incident was caused by cutting the girders that connect the tetrapod’s legs before removing enough concrete. The top-heavy weight of the tetrapod allowed it to lean and ultimately fall onto the roadway below.

Revised demolition practices include removing more concrete from the tetrapods before separating them from the bridge deck and doing additional inspections during the demolition process. WSDOT believes that these changes will provide safe demolition practices for the four remaining tetrapods. The old viaduct had 13 tetrapods in total.

The weight of the fallen tetrapod damaged local water and gas lines that were subsequently repaired by utility crews. The roadway shoulder and an adjacent lane were also repaired and repaved. All costs associated with the utility and roadway repairs will be paid by the contractor. The incident did not affect the overall project schedule.

Motorists can expect continued lane closures and realignments on both South Tacoma Way and South Center Street as work on the project continues.