The Two Towers III: The Caissons

Previous posts in the New Narrows Bridge Series:

The Two Towers I: Introduction

The Two Towers II: History of the Narrows Bridges

The Bridge Foundations: The Caissons

Types of Suspension Bridges

Our journey begins with building and placement of the caissons—the foundations of the new bridge.

Suspension bridges are an ingenious application of engineering. They are surprisingly old—the oldest suspension bridge still operational in the world, the Wheeling Suspension Bridge across the Ohio River in West Virginia —was built in 1849.

Suspension bridges come in two different designs; the elongated “M” shape, and the “A” shaped design called a cable-stayed bridge. The engineering is quite different between them. The weight of the bridge deck on a suspension bridge is borne by two different types of forces: compressive–the downward pressure on the bridge towers, and tension–the transfer of weight horizontally along ropes, chains or cables. In the cable-stayed bridge, the weight of the deck is borne primarily by the tower (or towers), as each radiating cable bears the tension of supporting only a small section of the bridge platform. The beautiful Millau Viaduct in southern France is a stunning example of this “A” shaped design.

Cable-stay suspension bridges require taller towers and greater number of towers as the length of the span increases, and therefore are better suited for shorter distances, over land or non-navigable water. The more conventional M-shaped suspension bridge, such as the Tacoma Narrows, the Verrazano Narrows, and the Golden Gate, can bridge much greater distances between towers–and therefore longer spans–by transferring much more of the weight to the shore as cable tension, as well as to caissons which support the towers. In order to secure the cables and resist the enormous tension forces generated, the cables are embedded at either end of the span in anchorages, typically massive concrete structures or embedded rock.

One of the earliest and most famous suspension bridges was the Brooklyn Bridge, built between 1869 and 1883, seen above, connecting Brooklyn and Manhattan. Between 20-30 men died during construction, many from “caisson disease”, now known as the bends or decompression sickness. This resulted from poor understanding and management of barometric changes while diving.

We will discuss the anchors in a subsequent post, but first let’s look at the caissons.

The Caissons

The Narrows Bridge caissons 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 air drums are removed by divers with acetylene torches (An amusing incident about this process I’ll mention during the upcoming tour). 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.

In the next session we will examine the construction of the massive shore anchors. See you then!