The Two Towers: The Caissons – The Doctor Is In

Previous posts in the New Narrows Bridge Series:

Introduction

History of the Narrows Bridges

The Bridge Foundations: The Caissons

The Caissons

The Narrows Bridge caissons, the foundations for the towers, underwent initial construction in the Port of Tacoma, and were towed to their location in the Narrows, where they were secured in place on the surface with a series of anchor cables arranged radially. These radial cables and flotation tanks in the caissons, were used to control the position and descent of these floating concrete islands, as layers of concrete were added to the top. A sharp cutting edge was built into the bottom, to ensure the caissons would reach bedrock to ensure stability when the weight of the towers was added.

The task was akin to building a 25-story building from the roof down, all the while holding its precise position in the alternating rip tides of the Narrows. The bottom of the caissons is configured to be a mammoth cookie cutter, with a sharp steel knife edge to cut through sediments to reach bedrock 50 feet beneath the bottom.

The process proceeds as follows: the caisson cutting edge fabricated in the Port is towed by tugboat to the site and secured to radial anchors positioned by GPS. Flotation chambers – steel drums filled with compressed air – are used to control the buoyancy.

Concrete is poured into steel forms built inside the perimeter of the box. The caisson becomes heavier and sinks into the water, position controlled by its radial guides. Dredge well tubes are used to evacuate the loose surface soil once the bottom is reached until the caisson sinks to its predetermined depth on bedrock.

Once the caisson is fully settled on bedrock, the flotation drums are removed by divers with acetylene torches. Concrete is then poured into the bottom of the hollow dredge wells, and the tops are then sealed. And this phase of the caisson build is now completed.

Next: we will examine the construction of the massive shore anchors.