Gallatin then turned to Benjamin Henry Latrobe for assistance in this matter. It is believed that Latrobe studied under John Smeaton, builder of the famous Eddystone Lighthouse. My impression is that Latrobe looked upon Smeaton as a mentor. If it is true that Latrobe studied under Smeaton, therein lies an interesting contrast. Whereas the challenge of the Eddystone Lighthouse was to adhere a masonry structure to a small rock inundated by harsh seas, Latrobe faced an equally difficult but somewhat different challenge – to secure and “float” a masonry structure on an ancient deposit of soft blue clay. Dr. Fazio’s article covers Latrobe’s design process and evolution in detail. For the sake of this discussion, we should concentrate on Latrobe’s final lighthouse design and focus on the foundation, sub-structure, and steps that Latrobe took to reduce the weight of his tower.
In addition to the lighthouse design, the soil conditions of Frank’s Island are of equal consideration. Frank’s Island was deemed to be a suitable location for a masonry lighthouse on two different occasions. According to Dr. Fazio’ article, in 1806 a man named Louis De Mun took soil samples from several possible locations for the proposed lighthouse. Although Frank’s Island was not his first preference, he concluded:
“I like the ground of the island well. It will bear anything that can be put upon it. The alluvium of which it consists, - Blue clay - is the alluvium also of our Atlantic Rivers, and like the clay on the Mississippi, the deeper you dig the harder it becomes. - The foundation of a stone building may be laid deep, and yet put up on piles, - for the clay is perfectly watertight.”
At this point, I as a blogger have to admit my own limitations… I am not a civil engineer, nor am I a geologist familiar with the alluvium that forms the Mississippi Delta. Therefore, I cannot begin to suggest how accurate De Mun’s conclusions are by today’s science. It would be great if a qualified individual could step in and offer an educated opinion.
In 1813, as reported in Dr. Fazio’s article, Frank’s Island was once again selected as the location for the Mississippi River Lighthouse by a group of three representatives, including Latrobe’s son, Henry. This decision appears to have been based mostly upon the accessibility and navigability of the Northeast Pass and not upon the soil conditions of the island. The final documented inspection of Frank’s Island was done by an engineer after the failure of the original structure. His analysis indicated that the “solid blue clay strata” “weighed 95 lbs. to the cubic foot” according to the Fazio article.
With a general consensus that the soil of Frank’s Island would support a masonry structure, Latrobe worked on a design that would allow for the weight of the tower to be spread across a much-wider base. His tower and base designs used a series of “inverted arches” to hollow out, and thus lighten the structure as much as possible. Latrobe also specified a unique and integral design for the structure’s foundation using a cypress piling and planking system. One of Latrobe’s greatest concerns was that the U.S. Congress, through its policy of awarding bids to the lowest bidder, would end up hiring a contractor not qualified to execute the lighthouse’s construction in accordance with his design. Therefore, Latrobe drafted the bid with excruciating detail – possibly warding off any prospective bidders. This opened the door for Winslow Lewis to enter the picture. What is most important to note is that Latrobe specifically designed his lighthouse with the soil conditions of Frank’s Island foremost in his mind. One notices, with a simple glance at the drawing of his lighthouse, its uniquely wide base. Despite the failure of the original structure, if there is any evidence to indicate that its builders did not follow Latrobe’s design to the letter, one should at least contemplate the possibility that the Mississippi River Lighthouse would have been a success had it been constructed properly.
