The mass of concrete beneath the old foundation is 126 feet 6 inches square, 13 feet 6 inches in depth, and extends 18 feet within the outer edge of the old foundation and 23 feet 3 inches beyond this line. In placing this work, 70 per cent of the area of the earth upon which the Monument was standing was removed.
The second part of the strengthening of the foundation consisted in constructing a continuous buttress beneath the shaft and extending out upon the concrete slab, so as to distribute the pressure over the foundation. In this operation sections of the rubble masonry were removed and replaced with concrete. As compared with the original bulk of the old foundation, 51 per cent of its contents was removed and 48 per cent of the area of the base of the shaft undermined. The new foundation rests on a bearing surface 126 feet 6 inches square, or 16,002 square feet, as compared with 6,400 square feet for the old foundation.
The entire work of underpinning was accomplished without causing the slightest crack or the least opening in any joint of that portion of the Monument already constructed, which, including the foundation, was 80 feet square at its base. The new foundation rests upon a bed of fine sand 2 feet in thickness, below which is a bed of bowlders and gravel. Borings were made in this deposit for a depth of 18 feet without passing through it. Thus, as completed, the new foundation covers two and a half times as much area and extends 13¹⁄₂ feet deeper than the old one, being 36 feet 10 inches in depth. The bottom of the work is only 2 feet above the level of high tide in the Potomac. No settlement has occurred to date.
THE WASHINGTON MONUMENT AS SEEN FROM THE MALL GROUNDS
Work was resumed on the shaft in 1880. The first 13 courses—26 feet—were faced with white marble from Massachusetts. The remainder is Maryland white marble similar to that used in the lower section. The new work was backed with dressed New England granite to the 452-foot elevation, above which the walls are entirely of marble, of through-and-through blocks, and from the 470-foot level, where the ribs of the pyramidion begin, the courses are secured to each other by mortise and tenon joints cut in the builds and beds of the stone. During the working season of 1880, 26 feet were added to the shaft; in 1881 there were added 74 feet; in 1882, 90 feet; in 1883, 70 feet; in 1884, 90 feet. These additions brought the walls of the shaft to a height of 500 feet on August 9, 1884. The pyramidion topping the shaft is supported on 12 marble ribs, which spring from the interior faces of the walls of the well, beginning at the 470-foot level. The covering slabs are 7 inches in thickness and are supported upon projections or spurs on the marble ribs. The pyramidion has a vertical height of 55 feet 5¹⁄₈ inches and consists of 262 separate stones. The weight of the pyramidion is 300 tons. The capstone, which weighs 3,300 pounds, was set in place on December 6, 1884. Over it is a small pyramid of pure aluminum 5.6 inches at its base, 8.9 inches high, and weighs 100 ounces, the largest piece of this metal ever cast in any country to that time. The following inscriptions appear on the four faces of the aluminum capstone:
JOINT COMMISSION AT SETTING OF CAPSTONE:
CHESTER A. ARTHUR, M. E. BELL, EDWARD CLARK, JOHN NEWTON,
ACT OF AUGUST 2, 1876.
CORNER STONE LAID ON BED OF FOUNDATION JULY 4, 1848.
FIRST STONE AT HEIGHT OF 152 FEET LAID AUGUST 7, 1880.
CAPSTONE SET DECEMBER 6, 1884.
CHIEF ENGINEER AND ARCHITECT, COL. THOMAS LINCOLN CASEY, CORPS OF ENGINEERS.
ASSISTANTS: GEORGE W. DAVIS, CAPTAIN FOURTEENTH INFANTRY;
BERNARD GREEN, CIVIL ENGINEER; MASTER MECHANIC, P. H. MCLAUGHLIN.
LAUS DEO.