Mackinac Bridge Project Reports

A collection of geological information gathered as part of the process that led to the building the original ‘Big Mac’, Mackinaw Bridge.

This product is the compilation of following documents :

Item # MBP-01: Proposed Mackinac Straits Bridge Preliminary Report, O. H. Ammann, D. B. Steinman, G. B. Woodruff, 1951.

The construction of a bridge across the Straits of Mackinac with construction methods which have proven successful on other large bridges in entirely feasible. The location of a bridge directly northward from Mackinac Point is more suitable than other locations which had previously been proposed. It has been definitely established that the rock formation nderlying the Straits has much greater strength than necessary to resist the moderate pressures which would be mposed upon it by the structure, even under severest combination of ice and wind forces. A bridge designed for two lanes of traffic in each direction is recommended. It will be adequate for a reasonable number of years to come. The proposed design provides for the heaviest vehicular loadings specified by the American Association of State Highway Officials.

Item # MBP-02: Pier Foundations for a Proposed Bridge Across the Straits of Mackinac, C. P. Berkey, S. Paige.

Despite the complex history and unusual geological structural conditions indicated at this site, there is no good reason to condemn it as impracticable. It is absolutely necessary, however, to make suitable exploratory borings to determine the actual physical condition of the ground that has to support this great proposed structure. There is ample experience in the field of construction for the treatment of any questionable ground that may be discovered. The foundation thus treated, if necessary, in our opinion would be amply strong enough to carry the moderate loads that will be imposed upon them.

Item # MBP-03: Mackinac Bridge Final Geological Report, J. C. Rosenau, 1956.

Available geologic data were analyzed, 51 borings were observed, the voluminous reports and the tests were conducted to leave no doubt as to the condition of the rock formations directly beneath the piers; that the bedrock areas explored are well cemented and formed into substantially one mass containing no significant voids and no caverns. The rock tested has much greater strength than needed to withstand the moderate pressures to be imposed by the structure, even under the most adverse climatic conditions.

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