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The object of this chapter is not to provide the bridge-builder with either a complete specification for building piers of all kinds or full directions as to sinking them under all possible circumstances, but to indicate to the designer, first, how to determine the best kind of piers to use at any proposed crossing, and, second, how to proportion them. Textbooks on substructure do not generally cover this ground, but deal mainly with masonry specifications and methods of sinking piers. The reader who desires to learn anything concerning piers which is not given in this chapter is referred to Baker's "Treatise on Masonry Construction" and Patton's "Practical Treatise on Foundations."
In determining the layout of spans for any important crossing, the first question to settle is what method of pier-sinking to adopt, for upon this will depend to a certain extent the span lengths.
The three principal methods in common use are as follows:
1. The Coffer-dam system.
2. The Pneumatic process.
3. The Open-dredging process.
The use of coffer-dams is, or should be, limited to crossings where the bed rock is not more than fifteen feet below the ordinary stage of water, and where there is no great, sudden rise anticipated. This method always figures low in the preliminary estimate, but is generally found to run much higher when the total cost of the finished structure is computed. The author nearly always discourages his contractors from attempting to use this method; and thus far his experience
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