Time for me to catch up on the story of the Swamp Angel of the Marsh Battery. At the end of last month, I wrote about the reconnaissance by Colonel Edward Serrell into the marshes behind Morris Island. He and his engineers probed and examined the mud in an effort to solve the problem which Brigadier-General Quincy Gillmore had given them – how to build a battery supporting one of the heavy Parrott rifles in the marsh.
During the first days of August, Serrell’s team conducted some experiments to determine the behavior and load bearing capacity of the marsh mud. After building a platform on the mud flats, the team piled on sandbags to reach a loading of 400 pounds per square foot. That stood for a day without settling. So they added more weight to reach 650 pounds per square foot. Other than leaning to one side, the platform sustained the weight.
The engineers continued piling on sandbags, pushing the experiment as only engineers will do. Serrell recorded these results:
After about another hour and a half, additional bags were piled upon the column, until a force of about 900 pounds to the square foot on the platform had been obtained, when the whole suddenly upset, throwing sand-bags all over… the platform, however, sank but about a foot at one corner, and the trial was considered merely as showing that the sustaining strength of the marsh was equivalent to over 600 pounds to the square foot, where the load is uniformly distributed. The ultimate sustaining strength was not ascertained.
The figure above shows a “ghost” of the test structure after it tipped over.
But Serrell was not simply setting up a 16,500 pound gun and its carriage on a platform in the marsh. He had to set it up for use. The recoil forces might push the gun and platform around the marsh. Or the vibrations caused by firing might damage the platform. And worse yet, if a platform that could sustain the gun on the marsh was designed, exactly how would the engineers build it under the eyes of the Confederates on James Island?
The final design was one of classic improvisation in the face of adversity. First, the engineers would put down a set of pilings to form a box within the mud. At first the work crews, detailed from the 7th New Hampshire Infantry, drove the pilings with a long lever anchored to a counter-weight.
But the crew found difficulty moving the counter-weight:
Instead of a counter-weight, a number of men took hold of the pole at either end, and having the plank secured by a sling, it was forced down into the mud until the point reached the hard bottom, when it was driven into the sand securely by heavy wooden mauls.
So with these pilings forming an open topped box in the march, a platform for the gun was built within. The platform consisted of a layer of grass, two layers of tarpaulin, 15 inches of sand, then three layers of 3-inch thick white pine planks. Only the top layer of the gun’s platform touched the pilings, and even at that only to cover the sump between the pilings and the platform. The pilings were not load bearing in that regard.
The engineers then built a parapet around three sides of the platform. They started with a layer of dried swamp grass, on top of which they placed two layers of heavy canvas. Then on top of that the crew laid two layers of yellow pine logs to form a “grillage.”
The layers crossed at right angles and were fixed with bolts and wire. The crew filled the gaps between the logs with sand and broken sand-bags. As with the gun platform, the parapet’s grillage did not rest upon the pilings. Upon the grillage, the crew piled sandbags… lots of sandbags. Most of the 13,000 bags used in the construction went into the parapet.
The result was a structure detailed in Serrell’s drawings:
So how did this keep the massive gun from sinking? Well as Serrell described:
The foundation of the parapet receives a much greater load to the square foot than the gun-deck, and the resultants of the forces are tending rather to elevate the whole structure on which the gun rests than to allow it to settle.
That is, if the battery should sink in the mud, the gun would be left standing on its own foundation, while the displacement would elevate the surface of the surrounding marsh, and tend, so far as it acted through or under the sheet piling which surrounds the gun platform, to elevate it also.
An illustration might help the translation from “engineer-speak.” The box formed by the piling confined a column of marsh mud. That column received pressure, thus keeping the gun platform elevated even with the weight of the gun, as the parapet pressed down on the grillage. Following the colors, the red arrows depict the pressure down from the parapet weight. The yellow arrows show how the pressure was dispersed. The green arrow is the pressure confined within the pilings, pushing the gun platform up:
Feel free to comment about fluid dynamics or other brainy topics. The bottom line is this work was a set of moving parts. The pilings also served to dampen the recoil force.
In addition to the 13,000 sand bags, Serrell recorded:
… 123 pieces, 15 to 18 inch diameter yellow pine timber, 45 to 55 feet long; 5,000 feet 1-inch boards; 8 [tarpaulins], 18 by 28 feet each; 9,156 feet 3-inch pine plank; 300 pounds 7-inch, and 300 pounds 4-inch, spikes and nails; 600 pounds round and square iron; 75 fathoms 3-inch rope.
All those materials were in addition to the gun, carriage, and ammunition. To move these materials, work crews built a road connecting the battery to the landing on St. Vincent’s Creek. In addition, a plank walk way extended from the left breaching batteries, across the marsh to Black Island and the Marsh Battery.
The walkway included a platform for a guard force, should the Confederates attempt to disrupt the construction. Other security measures included guard boats with howitzers and booms across the creek.
Serrell offered these particulars of the work on the Marsh Battery:
- Average pressure on the foundation – 513 pounds per square foot.
- Average pressure on the gun-deck platform – 123 pounds per square foot.
- Elevation of the gun from 35° to 37°.
- Greatest range to Charleston – 9,240 yards, or 5 ¼ mile.
- Labor expended – 91 man/days of engineer officer labor, 1,384 man/days of engineer soldiers, 7,390 man/days detailed infantry, 172 days’ work of four-horse teams, and 93 days’ work of boats.
On the night of August 17, Lieutenant Andrew Wadlie, 3rd New Hampshire Infantry and Lieutenant Charles Parsons, 1st New York Engineers, supervised the placement of an 8-inch Parrott rifle in the Marsh Battery. The gun they moved was this one:
Do we have pictures of this big Parrott in the battery?
We’ll get to those shortly!
(Citations from OR, Series I, Volume 28, Part I, Serial 46, pages 232-235.)