2025 West Best Projects
Award of Merit, Specialty Construction: Concrete Technology Corp. Graving Dry Dock Caisson Gate
Concrete Technology Corp. Graving Dry Dock Caisson Gate
Tacoma, Wash.
Award of Merit
Submitted by Moffatt & Nichol
Owner/General Contractor Concrete Technology Corp.
Lead Design Firm/Structural Moffatt & Nichol
After more than 100 gate docking and undocking operations across more than 45 years, Concrete Tech Corp.’s Tacoma dry dock was in need of a replacement caisson gate, a 150-ft-long floating concrete structure that serves as a watertight lock at the dry dock entrance.
CTC decided to manufacture the new caisson at its own dry dock, constructing it using high-performance concrete with low chloride penetrability and galvanized reinforcing steel. The replacement caisson is about 150 ft long, 16 ft wide and 25 ft tall.
At docking, the caisson is floated into position, ballasted with seawater to allow it to sink into a seat at the dry dock entrance, creating a watertight seal and allowing the seawater to be pumped from the dry dock.
During construction, the team faced three critical engineering challenges: maintaining precise exterior dimensions to ensure proper sealing with the existing dry dock, managing floating stability during various ballasting conditions and executing a successful launch with limited water draft.
The team designed the caisson as a lightweight precast prestressed concrete structure with ultra-thin walls capable of resisting over 20 ft of hydrostatic pressure while remaining buoyant.
