original article: Something New in Bridges | story by Ray B. McMinn | photo Kyle M. Walker | Ford Times April 1953
U.S. Highway 10, also known as the North Pacific Highway, once ran along the shore of Lake Coeur d’Alene in the vicinity of Blue Creek Bay. It was scenic, but the road had so many sharp curves and steep grades that driving it was neither safe nor pleasant.
The solution to the problem was to build a bridge across the bay, but the bay was an unusual one that required something special in bridges. The width of the bay was only 1,300 feet, but there was 80 feet of water over 215 feet of mud before solid rock was found.
A suspension bridge was designed, and then a fixed lever pontoon bridge, but each would have cost $2,000,000 and could not be financed. Engineers finally decided to erect piers to support several steel spans.
A new type of pier was conceived to solve the Blue Creek Bay problem. The pier consists of two elements. The first is the four steel caissons, or pipes, which, extending from the bridge deck down to solid rock bearing, support the bridge. The second element is the underwater towers which extend down from the deck through the water and into the mud for a short distance.
The principle involved in the design is this: the long caissons have to be supported laterally. The towers form the bracing system to stabilize the caissons above the mud line, and the great depth of mud supports the caissons laterally below the mud line.
The key to construction of the towers and of the placing of the caissons is that no work need be done below the water level. Workmen need not even get their feet wet unless it is raining. This method of construction amounts to building the bridge from the top down.
The towers are 120 feet high, comprised of four stories. Erection proceeds as follows: The bottom story is erected above the water, being supported by barges. It is then lowered into the water, and the second story erected on top of it. The third and fourth stories are placed similarly. When the tower is complete it is sunk into the mud a short distance by jetting.
When the towers are complete and set in correct position the four caissons are set in the corner cylinders of the tower, which guide them down to the mud line. The caissons are sunk into the mud down to the rock by jetting. The caissons are then drilled into the rock to key them and secure solid bearing.
When the caissons are in final position they are thoroughly cleaned out and a steel column inserted for their full length. The final stage is to fill the remaining space in the caissons with concrete.
The greatest depth to which the caissons were carried was 363 feet—probably a greater depth than that of any other bridge foundation in the world.
The cost of constructing the bridge was $867,000, saving more than a million dollars over doc other type of bridge which could have been built.
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