Fort Delaware - Pentagonal Fort

Pentagonal Fort

Plan showing position of barbette guns in 1858. Second tier casemate inside Fort Delaware.

The present Fort Delaware was erected mainly between 1848 and 1859, becoming the largest third-system fortification completed in the United States at the time. Although major construction was wrapped up before the Civil War, the post engineer did not declare the fort finished until 1868. Construction of the counterscarp wall, slate pavement, and "hanging shutters and doors" were still ongoing. The fort was designed by Army chief engineer Joseph G. Totten, and construction was supervised by Major John Sanders. Engineers in supporting roles included Captain George B. McClellan, Major John Newton, Lieutenant William Price Craighill, Lieutenant Montgomery C. Meigs and civilian engineer Edwin Muhlenbrach.

Foundation
Construction of a deep foundation was necessary since the island's soils were of "a compressible mud often forty feet deep; the level where sand was finally reached." In 1849, Sander's crews began using steam-powered drivers to place long piles (similar to modern telephone poles) in excavated areas to provide adequate support. In 1850, pile driving was complete; crews had driven 4,911 piles, reusing 1,095 piles from the Delafield fort. Because of the star fort's failed foundation, Totten and Sanders decided to evaluate the weight resistance before moving forward with construction. The engineers proposed a singular test consisting of 30 blows from a 8-foot height using an 800 pound weight. A total of 5,754 piles were tested and 2,955 failed more than one fifth of an inch. Roughly 1,700 piles were subsequently spliced and re-driven an additional 10 to 20 feet. The pile driving was finally complete in 1851 and the wooden grillage was the next layer constructed.

Stonework
The fort is primarily composed of gneiss, granite, brick and cement. Initially, the stone for the scarp wall was gneiss imported from Port Deposit, Md. In 1852, Major Sanders reported the gneiss was too hard for his stone masons and cutters to shape, slowing the progress of construction. After that, purchase records show granite was bought from John Leiper's quarry in nearby Delaware County, Pa. Leiper's granite was also used throughout for various items such as the steps for the circular stairways. Stone from Vinal Haven, Maine was used for the stairways' large platforms. A huge storm damaged the island's dyke and seawall in 1854. Due to the threat high-tide waters, efforts were directed toward repairing the dyke and adjacent seawall, slowing progress on the stonework.

Brickwork
More than two million bricks were purchased from Wilmington, Del. and Philadelphia, Pa. for the scarp wall's interior. These bricks were used in construction of underground cisterns, casemates, powder magazines, soldier barracks, officer quarters, bread ovens and the fort's breast high wall. Masonry arches and vaults were used throughout the entire fort to equally distribute weight and to provide stability. Poured concrete was used as a layer above the vaults to offer counter resistance and to "create a strong floor system."

Major Sanders Dies
On July 29, 1858, Major Sanders died on the island due to "complications of carbunculous boils;" Lt. William Craighill replaced him until Maj. John Newton was able to assume the role of supervising engineer. In 1859, in official correspondence, Major Newton reported that the fort would be ready to armament and garrison by 1860. In 1861, before the war started, Capt. Augustus A. Gibson took command of the fort and a small garrison of only 20 regular army soldiers. Construction languished until the end of the Civil War; redirected focus was centered on mounting cannon inside the fort. In 1861, 20 columbiad guns were received and work to mount these guns quickly began. By 1862, another 17 guns were delivered here. By 1866, approximately 156 guns, were mounted in total, filling the fort's casemates and ramparts to capacity.