Concrete Technology Corp. Graving Dry Dick Caisson Gate
Tacoma, Wash.
Merit Award
Presented by Moffatt and Nichol
Owner/General Contractor Concrete Technology Corp.
Leading design/structural company Moffatt and Nichol
After more than 100 gate coupling and uncoupling operations over more than 45 years, Concrete Tech Corp.’s Tacoma Dry Dock. needed a replacement caisson gate, a 150-foot-long floating concrete structure that serves as a watertight lock at the dry dock entrance.
CTC decided to manufacture the new caisson in its own dry dock, constructing it with high performance concrete with low chloride penetrability and galvanized reinforcing steel. The replacement caisson is about 150 feet long, 16 feet wide and 25 feet high.
At berth, the caisson is moved into position, ballasted with seawater to allow it to sink into a seat at the drydock entrance, creating a watertight seal and allowing seawater to be pumped from the drydock.
During construction, the team faced three critical engineering challenges: maintaining accurate exterior dimensions to ensure a proper seal with the existing dry dock, managing floating stability during various ballast conditions, and executing a successful launch with limited water draft.
The team designed the caisson as a lightweight prestressed precast concrete structure with ultra-thin walls capable of withstanding more than 20 feet of hydrostatic pressure while remaining afloat.
