Wednesday, July 9, 2014

Above is a photograph of the original foundations of the Washington Monument.

The Washington National Monument Society (WNMS) was founded in 1833 as a private association to fund and construct a monument to the memory of George Washington, the nation's first president. American artists were offered the opportunity in 1836 to submit designs for the monument. Robert Mills won the competition by submitting a plan for a 500-foot tall obelisk surrounded by a circular colonnade surmounted by equestrian statuary. Congress authorized the WNMS to construct the monument on public land.

Deciding where to put the monument was the problem. WNMS leaders wanted to follow the L'Enfant Plan for the city, which imposed a geometric pattern inspired by the road network of the city of Paris. The most obvious place to put the monument was in alighment with the White House and United States Capitol. Sure, it could have gone on North Capitol Street or it could have gone on Washington Circle, or it could have gone at Eastern Market. But the best place was on the National Mall.

The District of Columbia looked a heck of a lot different in 1836. Where Constitution Avenue is today a large stream, Tiber Creek, ran westward. Its mouth widened into an estuary, and the shoreline of the Potomac River pretty much followed the northeast shore of today's Tidal Basin and the northeast shore of the Washington Channel. The area often flooded. But to everyone's great satisfaction, right at the point formed by the southern bank of Tiber Creek and the Potomac River, there was a somewhat high hill. It was the perfect spot of the Washington Monument. Okay, sure, it wasn't exactly in line with the White House or Capitol. But it was pretty close.

The soil here is what is called "Pleistocene terrace". A terrace is a step-like landform that borders a shoreline or river floodplain. The terrace was laid down during the Pleistocene, a geologic era that began about 2.588 million years ago and ended just 11,700 years ago. Below the terrace was bedrock. There should have been Cretaceous soil beneath the Pleistoscene terrace, soil laid down some 145 to 66 million years ago. There wasn't, and geologists suspected that underground streams washed away the Cretaceous soil -- leaving the Pleistocene soil on top behind. The Pleistocene soil consisted of clay mixed with slight to moderate amounts of Bentonite (a highly water-absorbent clay made up of montmorillonite); sand; gravel; and the occasional boulder. The clay was moderately plastic, which meant it deformed under moderate pressure. The bedrock below all this was schist, sediment which had been welded together into hard rock. This was technically Wissachickon schist, which meant that it had leaves of quartzite throughout it. The schist was also decomposed, which meant it had been exposed to weather once upon a time and easily crumbled. The schist is from the Ediacaran geologic era, about 635 to 540 million years ago.

This created a problem for the monument builders. The soil was deep enough that they couldn't go down to bedrock. The bedrock itself was so crumbly, it might not have been a good idea to use it, either. But maybe they didn't have to? They were going to build a very heavy limestone obelisk. If they buried the foundations deep enough, perhaps the pressure of the Pleistocene terrace soil and the moderate deformation of the clays would keep the monument stable.

That's exactly what architect Robert Mills believed. Construction supervisor William Doughterty dug down 7.7 feet (2.34 meters) and then constructed a square, stepped pyramid out of blue gneiss (an extremely hard metamorphic rock). The foundation was 80 feet (24.38 meters) wide, and its eight steps made it 23.34 feet (7.1 meters) high. At its top, each side was just 58.5 feet (17.8 meters) on a side, while the monument's shaft was just 55 feet (16.8 meters) wide. The wider foundation helped spread the weight of the monument outward, so that it wouldn't pierce the soil like a needle and subside and destabilize. Each block weighed six to eight tons, and were mortared together with a mixture of cement, lime, and sand. The foundation rested on a loam (soil) composed of equal parts of sand and clay, with small boulders interspersed throughout the loam. The water table was 12.5 feet (3.8 meters) below the foundation.

The cornerstone for the Washington Monument was laid by President James Polk on July 4, 1848. The shaft itself was constructed of marble blocks mined near Baltimore. Each block was tested to ensure it could take the weight of the structure atop it.

And the the Society ran out of money.

It was 1854, and the Washington Monument was 150 feet (45.72 meters) high.

The country was deep in the convulsions that would culminate, six years later, in the Civil War. The Society had never raised much money, and it wasn't connected to any existing body such as a church, association, political party, or movement. The sectarianism affecting the country deeply eroded the Society's fund-raising capacity. Furthermore, an incident occurred in 1854 which undermined the WNMS even further. The Society had long for years accepted "memorial stones" for the memorial (provided they came with a hefty contribution). In 1853, Pope Pius IX sent a beautifully carved stone (originally from the Temple of Concord in Rome). Since 1845, a movement known as the Know-Nothings had arisen, angry and worried about the massive influx of German and Irish Catholic immigrants to the country. The Know-Nothings were a secret society of middle-class Protestant white males, and were instructed to reply to any inquiry about their activities with the words "I know nothing." The Know-Nothings were particularly upset that Piux IX had repressed the liberal republican Revolutions of 1848 in Italy, and a widely-held conspiracy theory said that the Pius was sending hordes of Catholic immigrants to the United States as a "fifth column" to take over the country.

On March 5, 1854, a group of Know-Nothings broke into the materials shed on the grounds of the Monument and made off with the "Pope's stone". It was never recovered. The revelation that the Society had accepted a "tyrant's stone" angered many Know-Nothings. The stone's destruction also angered the far larger number of Roman Catholics in the country. Contributions from both sides fell to nothing. The WNMS asked Congress in the summer of 1854 for a $200,000 contribution to finish the memorial. Congress seemed on the verge of doing so when, in February 1855, the Know-Nothings attempted to take over the WNMS. A clerk of the Society, F.W. Eckloff, issued a notice in the newspapers calling for a public meeting to elect the Society's board of directors. The problem is that Eckloff had no such authority to issue any such notice, and no elections were due to happen until 1856. Nonetheless, a large crowd of Know-Nothings showed up and elected a new board.

The "old Board" and the "new Board" fought publicly over who actually controlled the memorial. Crowds of Know-Nothings seized the memorial grounds and worksheds, replaced Dougherty, and attempted to build the monument themselves using leftover existing stones. About four feet were added to the memorial before work stopped. The Know-Nothings were unable to raise any funds, either, due to the dispute over which board represented the WNMS, and it wasn't until 1858 that the Know-Nothings stopped trying to control the WNMS.

Congress expressed interest in federalizing the project in 1859, now that the internal problems were over. Lieutenant Joseph C. Ives of the U.S. Army Corps of Engineers inspected the monument, and said he could find no evidence of settling. He believed the foundations and shaft to be well-constructed, and the foundations adequate for the work. Congress, however, did not act.

The Society still didn't have any funds, however. Construction stopped during the Civil War. After the conflict ended, the Society remained crippled and penniless.

Attempts were made in the early 1870s to have the monument torn down, but Congress refused. In 1873, when Congress again expressed interest in taking over the project, 1st Lieutenant William Louis Marshall was asked to inspect the memorial. Marshall was hampered by not having Ives' original data and tests results (just the conclusionary report), but overall agreed that no settling had occurred and the construction was sound. Marshall warned, however, that the roughly shaped stones which made up the foundation were not sufficiently uniform in texture and strength. He believed that dressed stone offered more resistance to compression and would distribute the weight more uniformly over the bed of the foundation.

Congress asked Marshall for a second inspection in 1874. This time, even though Marshall reaffirmed his prior report, he suggested that the monument's height be shortened to 400 feet from 600 feet. He also though the thickness of the walls should be reduced from 11.46 to 7.3 feet, that the cap be made of cast iron sheets rather than arches of stone, and that the upper 200 to 250 feet of the shaft be constructed of brick. This would dramatically reduce the weight of the memorial, and help reduce the pressure on the foundations.

The Corps of Engineers transmitted Marshall's report to the Board of Engineers for Fortifications for a peer review. The board questioned Marshal's conclusions. First, they pointed out that some settling had, indeed, occurred. Second, they noted that 4.8 tons per square foot were already pressing on the foundations. Raising the shaft to 400 or more feet would add another 1.8 tons per square foot, and that this was too much pressure for the soil to bear.

Finally, on August 2, 1876, President Ulysses S. Grant approved legislation which allowed the federal government to seize control over the memorial. A new Joint Commission on the Construction of the Washington National Monument was created to construct the memorial, although the Society could still advise on construction and solicit funds. The act also required that the federal government inspect the monument's foundation.

The Corps of Engineers appointed a three-man board to examine the foundations once more. The board concluded, as the Board for Fortifications had before it, that the soil was already loaded to the limit of safety. Redesigning the memorial simply would not help, even if Marshall's alterations were adopted. The problem was that the foundation was not sufficiently wide enough to redistribute the weight of the shaft. Already, the memorial had subsided by 8 or 9 inches, there was evidence that the shaft was out of plumb, and the foundation itself was not level. The new board feared that if the structure was allowed to rise any further, not only would the shaft be unsafe, the imperfections would become discernible to the naked eye. Almost as bad, the new board discovered that the marble blocks at the base were too close together. They had become so compressed that the mortar between them had come loose. With no buffer, the stones smashed (slowly) against one another. As a result, many of the marble blocks had cracked broken in two horizontally, while others had shed chips from their surfaces and where they joined with neighboring blocks -- sure signs that the blocks were breaking up.

The Society was appalled by the board's report. They declared that the board had spent only an hour or two at the site investigating the memorial, had used the wrong benchmark stone for calculating the subsidence, falsely believed the memorial sat on loam and sand, and was trying to have the memorial torn down so that a more ornate design could be built.

On May 31, 1877, President Rutherford B. Hayes asked the Board of Engineers to meet with the Society to reconcile their differences. Major General Andrew P. Humphreys, Chief of Engineers, reconvened the board, which admitted it used the wrong stone as a benchmark. But the board didn't back off of any of its other criticisms. The board realized it had made a tactical error, however, in not offering any solutions to the problems it had identified. After six weeks, the board issued a new report in which it advised broadening and deepening the foundations of the memorial. The Joint Commission approved the report and submitted its own conclusions to Congress in October 1877, concluding "it must be assumed that the foundation is insufficient to sustain the weight of the completed structure." Congress agreed. It passed a joint resolution on June 14, 1878, in which it appropriated $36,000 to improve the memorial foundation. Days later, the Joint Commission appointed Lieutenant Colonel Thomas Lincoln Casey of the Army Corps of Engineers to supervise the project. Captain George W. Davis was named his assistant. The Joint Commission formally tasked Casey on July 1 with preparing a plan for foundations that would support a 525-foot-high monument.

Many engineers would have found the job impossible. Only one or two of the people who had been involved in the Washington Monument's first stages of construction were alive, and all of them were quite old. Almost none of the memorial's records or construction drawings had survived, most being lost during the 1850s during the fight with the Know-Nothings. All that Casey had to work with were the Corps records, a couple of articles in technical journals, and the memories of one or two old men.

Nonetheless, in under a month Lt. Col. Casey solved the foundation problem.

Casey proposed digging down around the existing foundation to a depth of about 36.8 feet (11.23 meters), right to the water table. This would put him 13.5 feet (4.11 meters) below the existing foundation. He would then dig 18 feet (5.5 meters) under the existing foundation on all sides, and extend outward from the existing foundations by 5 feet (1.5 meters). This would create a square ring below the existing foundations that would help to spread the weight of the massive obelisk overhead and reduce the pressure on any given square foot of soil beneath. To lock the square ring to the pyramidal base, he proposed three concrete buttresses on each side of the monument. These would actually extend up and over the existing foundation, and even cover a portion of the shaft (to help reinforce it). Through the hollow center of the square ring, Casey proposed a cross of either concrete or masonry. Tunneling below the already-massive memorial would be very risky. Casey's plan called for four-foot-wide (1.3 meters), 13.5 foot-high (4.11 meters) tunnels to be dug. The concrete would be poured in flat sheets, a dowel stone set into each sheet to connect it to the one above. Once the concrete got close to the existing foundations, rubble would be shoved into the remaining space.

On June 27, 1879, Congress granted authorized an additional $64,000 to carry out Casey's plan.

Excavation of the area below the existing base began immediately. Four derricks carried earth to the surface, while two concrete mixers run by steam engines poured concrete. Cuts were made in the surrounding hill, and tracks laid in each. Hoisting equipment used slings to bring up some earth, which was placed in a tub. The tub was secured to a car, and the car ran down the rails to carry the earth away. The same system was also used to deliver concrete to the site.

Casey demanded only the highest quality materials for the work. He selected J.B. White and Brothers, a New York firm, to supply the Portland cement for the project. Unfortunately, White and Brothers repeatedly failed to meet delivery schedules, which forced Casey to lay off workers and stretch out his construction timeline.

Excavation began on February 10, 1879. More than 10,334 cubic yards (7,900 cubic meters) of earth was removed, both downward until the water table was reached as well as under the existing foundation. It was replaced with 7,003 cubic yards (5,354 cubic meters) of concrete. The concrete consisted of one part Portland cement, two parts sand, three parts pebbles, and four parts broken stone. It was allowed to set for 7.5 months before the shaft above began to be constructed. The pouring of cement ended on November 1, and would have been a lot sooner if White and Brothers had made its deliveries on time.

The second step in the construction of the new foundations involved removing slots in the old foundation (one per side), and then pouring the buttress as well as the buttreses on each corner. Workmen took 348 cubic yards (266 cubic meters) of stone from the old foundation. Casey also decided that, while he still needed to cut into the existing foundation, a solid buttress (rather than three separate ones) would be a better way of not only locking the old foundation to the new but reinforcing the foundation and helping spread the memorial's weight. Pouring of the buttresses began in September 1879, and was completed on May 28, 1880. Casey wanted this concrete to be stronger than the concrete square ring, so it consisted of one part Portland cement, one and one-half parts sand, two and one-quarter parts pebbles, and three parts broken stone. The buttresses used 520 cubic yards (398 cubic meters) of cement.

Casey never did tunnel beneath the monument and build his cross-walls. He realized that the square ring of concrete would prevent any lateral displacement of the soil beneath the monument, which was the most worrisome aspect of the old foundations. Since the cross-walls wouldn't add significantly to the spreading of the weight, they really weren't necessary. So this most dangerous aspect of the plan was abandoned.

Remember how, initially, a big colonnade was supposed to be constructed around the Washington Memorial? The memorial's early builders left the foundation exposed because they knew there would be construction around the foundation that would cover it up.

But by 1876, this colonnade had been abandoned. Indeed, Casey needed to build up a hill of soil around the foundations (old and new) to help stabilize the monument even further. On June 7, 1890, the foundations began to be covered. It took five weeks for crews to fill in the cuts and build up the embankment, which was 30 feet (9.1 meters) wide and 17 feet (5.2 meters) above the original grade at the site. In December, Casey reused all the blue gneiss stone harvested from the old foundation and placed it around the new embankment. By the end of 1891, the gneiss rubble had been covered with soil, making the embankment in the shape of an ellipse with a 155 foot (47.2 meters) wide minor diameter and a 220 foot (67 meters) wide major diameter. Minor re-grading occurred in 1972, 1977, and 2003.

The bottom of the new foundation is just two feet (0.6 meters) above the average high tide of the Potomac River. Water still permeates the soil above the water table, however, and water is usually about six inches (15.25 centimeters) over the foundation. Casey's excavations and borings taken over the next century revealed that a bed of fine sand about two feet (0.6 meters) thick lies beneath the new foundation. Below that is a bed of boulders and gravel.

A 10-foot-wide sidewalk was built around the base of the memorial beginning in June 1887, and was completed the next November. The terrace sloped to meet the natural surface in all directions. (Today, a riprap wall cuts into the terrace where the sidewalk is.)

The Washington Monument was completed on December 6, 1884, and dedicated on February 21, 1885. It was opened to the public in April of 1886, and was officially declared open on October 9, 1888.

The memorial presses downward with a force of 136,616,395 pounds (or 607.7 meganewtons) By comparison, the San Jacinto Monument weighs 70,365,199 pounds (313 MN), the Tower of Pisa weighs 31,922,870 pounds (142 MN), and the Eiffel Tower weighs 21,132,040 pounds (94 MN).

Since 1880, groundwater levels in Washington, D.C., have dropped significantly due to human use. Groundwater today is now 12 feet (3.65 meters) below the new foundations. Since water helps the soil resist the downward pressure of the monument, many geologists conclude that the monument is probably sinking faster today than it did prior to the 1940s.

One interesting side note:: Tiber Creek had gradually been narrowed and its banks reinforced with riprap between the 1840s and 1870s. In 1871, the District of Columbia began a project to enclose the stream in a masonry tunnel, and use it to help flush sewage into the Potomac River. Two skating ponds, one 6.5 acres (26,304 square meters) and the other 17 acres (68,796 square meters), were created on the north side of the National Mall. The smaller of these adjoined the Washington Monument grounds, and was called Babcock Lake. In 1878, both lakes were used by the Department of Agriculture to cultivate German carp, tench (or doctor fish), goldfish, and golden ide (or orfe). In the winter, the public used it for ice skating.

In December 1884, as construction on the Washington Monument shaft proceeded, Casey noted that the monument would.......well, wobble. The two northern corners of the structure settled, followed by the southern corners. Each time the shaft was added to, the monument wobbled.

Casey believed that Babcock Lake, which was just 250 feet (76.2 meters) from the monument's foundation, was weaking the soil. Worse, the pond was drained once a year. When the pond was empty, numerous large freshwater springs appeared along the lake's south shore. Casey worried that these springs would wash away the sand underpinning the sand layer below the memorial's foundations.

Casey wanted to fill in Babcock Lake -- a project which would require a whopping 83,000 cubic yards (63,458 cubic meters) of earth. Congress was reluctant to destroy a public convenience like Babcock Lake, but the Joint Commission was adamant that the monument was being destabilized. Congress finally relented. Special legislation had to be passed to permit such a vast quantity of earth to pass through the city. An entire hill in southwest Washington was levelled for the project. Babcock Lake began to be filled in March 1887.

Below are images of the construction of the new foundation between 1879 and 1880.

No comments:

Post a Comment