Dear Colleagues,
Welcome back. I hope you all had an enjoyable Spring Break. While you were away,
construction of the new 2nd Street portal and exit ramp from I-395 began in earnest. The
vibrating pile driver arrived from Florida – a little late because of the snow – and began to sink
the steel H piles, also called soldier beams, into the earth along 2nd Street. Installing the piles is
the first step in creating a retaining wall to support the excavation that will follow once the
support system is completed. After the piles are driven into the earth, horizontal lagging boards
will be placed in between steel vertical piles to create a shoring wall that will hold back the earth
as the excavation proceeds downward. Tiebacks will also be placed through the piles at ten- to
twelve-foot intervals along the wall to help hold back the earth behind the lagging boards.
Tiebacks are essentially steel cables that are inserted into holes that were previously drilled
laterally into the soil behind the sheeting wall. The length of the tieback is based on loads and
soil characteristics. Grout is then forced into the holes to secure the cables. Finally, the tieback is
fastened to the piles or steel walers, horizontal steel beams that span multiple piles. Once the
grout has sufficiently cured, the tiebacks are tensioned to provide additional support for the
excavation walls.
The process of installing lagging boards is continuous and tiebacks are repeated every ten
to twelve feet down as its facing soil is removed until the proper subgrade is reached. The
combination of piles, lagging boards, and tiebacks creates a wall that holds back the immense
weight of the earth that surrounds the area to be excavated, preventing the walls from collapsing
on the workers and the machinery as the pit grows deeper. If you want to see a completed
support wall and hear a little more about soldier pile walls, visit
https://www.youtube.com/watch?v=Ugv-srxGajc to view an animated example of the process or
http://www.macroenterprisesltd.com/Soldier_Beams___Lagging.html for examples of a
completed retaining wall and its elements.
The history of piles and hammers is as fascinating as the technology that controls them
today. Driven piles or columns are the oldest type of deep foundation support for buildings,
bridges and other structures, especially those built on unsteady land or over water. Historically,
these columns have been composed of timber, concrete, steel, or a combination of those
materials. They have been used throughout the ages and across many different cultures.
“Archeologists have determined that driven piles have been used since Neolithic times. The lake
dwellers of what is now Switzerland, northern Italy, Austria, and eastern France constructed their
villages on driven pile-supported platforms as a defensive measure. The builders of an ancient
village discovered at Wangen, Switzerland utilized 50,000 piles in the construction of their
village. Primitive civilizations around the world have used driven piles and poles to support their
homes in water and in flood prone areas. Predecessors to the Aztecs living around lakes near
Mexico City built their sites … on pile supported mats topped with soil called “Chinampas” or
on areas contained by driven piles and filled with soil and stone. Ancient tribes in the Java Sea
area built stilted villages and ancient Chinese engineers built bridges on driven piles.”
The Romans used wooden piles to support bridges and structures over various soils and
above water in their far-flung empire. The most famous of the early Roman pile-supported
bridges was that built by Julius Caesar over the Rhine -- most likely between Andernach and
Neuwied, downstream from Koblenz -- during his campaign in Gaul.
Caesar's Rhine Bridge, by John Soane (1814)
Reconstruction of a Roman pile driver, used
to build the Rhine bridge
The oldest, however, was the Pons Sublicius, the first bridge across the Tiber. Built of wood on
wooden piles sometime before 500 B.C.E., it no longer exists and its exact location is uncertain.
Legend has it that the bridge’s support capacity proved critical in the early stirrings of the
Pons Sublicius
Roman republic. An army of Etruscans invading Rome stormed the bridge in 508 B.C.E.
According to some legends, Horatius Cocles, the bridge’s sole defender, knew that the combined
weight of the invading soldiers was more than the bridge’s support capacity. They charged, the
bridge collapsed toppling the invaders into the river, and the attack ended.
Wooden piles also support tenth-century structures in Venice. As the Adriatic Sea
envelops the city today, the thousand-year-old piles continue to support their buildings. It is the
rising water more than the sinking buildings that may eventually destroy the city.
Early American buildings used wooden piles. In the 1890s, steel-reinforced concrete piles
were sometimes used, especially in Chicago; but H-shaped steel piles, like the ones BBC is using
on this project, have predominated in the construction industry since the twentieth century.
Until recently, the piles were driven into the ground using some form of a hammer. Early
engineers drove piles by hand or by stone drop-hammers falling from small wooden rigs. A
steam-driven steel hammer, stronger than the stone hammer and more powerful than hammers
dropped from wooden rigs, was developed in 1845 by a Scot named James Nasmyth. It came to
United States in the 1870s.
Power hammer invented by James Nasmyth in Germany in 1842, engraved by E.
Zimmer, from 'Le XIXeme siecle' by Hans Kraemer, c. 1900.
Piles are driven today by vibratory or impact-vibration hammers supported by crane
booms and driven by high-powered motors. They were originally developed by D. D. Barkan in
the former Soviet Union in the 1950s. The Russians licensed the technology to the Japanese, who
made significant modifications and exported the technology around the world. While the rest of
the world was using vibratory or impact-vibration hammers initially designed by Russian or
Japanese engineers, Americans developed their own style of hammers after 1969. They have
now been adopted across the globe.
Those of you who were here during the construction of the Hotung International Law
Building and the Sport and Fitness Center will remember the disruption to our academic rhythms
caused by pile-driving. That was because we drove our piles with a hammer rig that literally
pounded the soldier beams into the ground. The sound of the impact when the steel hammer hit
the head of the steel pile was explosive. Moreover, the vibration waves emanating from the
impact of the hammer and the pile head, as well as from the movement of the earth below as it
was displaced by the driven steel toe of the column, shook the windows and the floors. Since the
piles were driven right next to the Gewirz Student Center and because the installation began
early in the day, sleeping-in was difficult and students often left the building for most of the day.
I am happy to say that our experience will be very different during this project since BBC
is using a vibration driver, thus eliminating the harsh sound of metal impacting metal that one
normally associates with pile-driving. “Vibratory pile hammers contain a system of counter-
rotating eccentric weights, powered by hydraulic motors, and designed in such a way that
horizontal vibrations cancel out, while vertical vibrations are transmitted into the pile.” The pile
driving apparatus is raised as the crane lifts the attached pile above the ground. It is then fastened
to the pile by a clamp and a chain is run through the pile for added safety in case the clamp
should fail. Hydraulic fluid is typically supplied to the driver by a diesel engine powered pump
head through a set of long hoses. Vibratory hammers are available with several different
vibration rates, ranging from about 1200 vibrations per minute to about 2400 VPM; the vibration
rate chosen is influenced by soil conditions at the site and other factors such as power
requirements and purchase or rental price of the equipment.
Vibratory pile drivers are often used to mitigate noise when a shoring wall is very close
to a residence or office building. Such is the case in this project. The use of a vibratory hammer
instead of an impact hammer was specifically negotiated between Georgetown and the
Developers in order to minimize our inconvenience. In an earlier Construction Note, I sent a
video of the vibratory driver; but today you can stand along the 2nd Street fence and watch this
amazing machine drive piles 30 feet into the ground in 60 seconds with a minimum of noise and
vibration.
There are some interesting statistics that define this project. This driving rig being used
on 2 Street is actually a small one in comparison to those used on other jobs. Nonetheless, it
cost $2 million. Its amazing capacity to quickly sink piles is such that BBC has collapsed the
original completion schedule for this phase of the work. BBC anticipated that it would take three
weeks, one week per block, to install the 186 piles along 2nd Street between Massachusetts
Avenue and E Street. In three days last week, the rig drove piles almost the entire distance
between Massachusetts Avenue and G Street and began to drive them in front of McDonough
Hall. If all goes well, I suspect the 186 piles will be installed in less than two weeks.
nd
In addition to the soldier beams that will be driven along 2nd Street, another 265 will be
driven along 3rd Street. They all vary in length. Pile lengths range from thirty to thirty-five feet
along 2nd Street, but those along 3rd Street will generally range between sixty-five and seventy
feet. The longest is ninety-two feet. The total linear feet of steel used for beams and walers is
currently around 21,313 feet -- more than four miles of steel. The total weight of steel is
currently calculated to be 1,347,910 lbs. The lagging boards are three inches thick and twelve
inches wide and made from oak. BBC will set 128,000 feet of timber between the piles. If we
laid them on the ground end to end they would reach all the way to Annapolis.
Others have waxed more eloquently than I about pile driving. William Dean Howells, in
his novel The Rise of Sylas Lapham, wrote
Nothing gave Lapham so much satisfaction in the whole construction of his
house as the pile-driving. When this began, early in the summer, he took Mrs.
Lapham every day in his buggy and drove round to look at it; stopping the mare
in front of the lot, and watching the operation with even keener interest than the
little loafing Irish boys who superintended it in force. It pleased him to hear the
portable engine chuckle out a hundred thin whiffs of steam in carrying the big
iron weight to the top of the framework above the pile, then seem to hesitate,
and cough once or twice in pressing the weight against the detaching apparatus.
There was a moment in which the weight had the effect of poising before it fell;
then it dropped with a mighty whack on the iron-bound head of the pile, and
drove it a foot into the earth.
‘By gracious!’ he would say, "there ain't anything like that in THIS
world for BUSINESS, Persis!
I am not expecting any major disruptions to life in Gewirz or McDonough Hall as this
phase of the project progresses. Driving will occur between 7:00 a.m. and 3:00 p.m. and should
be completed relatively quickly. During the installation last week, the staff in the Williams
Library experienced some noise and some vibration. For the most part, both were within
acceptable city regulations and were only occasionally above the ambient sound of the traffic
coming off the highway. Sounds inside our buildings were muffled. Indeed, the sound of our
HVAC system in many rooms at the Law Center overwhelms the sound of the equipment.
Depending on where you sit, you may feel a slight vibration as the pile sinks because the piles in
front of McDonough and Gewirz are a little closer to those buildings than they were to Williams.
Last week, I sat in offices and study carrels on each floor of Williams along the west wall and
found the vibration to be barely noticeable, except upon the windows, in most cases. I did the
same in McDonough today. You may have a slightly different experience but neither the noise
nor the vibration seemed to affect the students who were studying in the library.
There is one caveat to my statement. As I have mentioned before, BBC has been finding
mysterious things under the ground around the Law Center. New things had been installed
without proper documentation, and old things were not removed and over time were forgotten.
There once was an eight story apartment house on the lot that existed across from the Williams
Library. When the vibrator began to sink the pile, it hit what appeared to be foundation walls
from that building. That caused a very loud noise and an unexpected level of vibration. Given
where they are drilling this week, I don’t expect a similar problem; but then mysteries arise only
when they actually appear.
Unless you practice construction or real estate law, you will probably never have a
chance to view a project as massive as this so close up. I encourage you to take a few minutes in
the next two weeks to step outside and watch the work. This combination of human labor,
machinery, and technology shows us the immense capacity of the human imagination. May it
always be used for good.
Sources
Mlyniec, CONSTRUCTION NOTES, TRANSFORMING A CAMPUS IN WASHINGTON, D.C. (2006)
http://www.piledrivers.org/files/uploads/272d1300-0887-4849-bdb0-4044b9ff8859.pdf
http://en.wikipedia.org/wiki/Deep_foundation
http://www.foundationrepairservices.com/service/soldier-piles-and-lagging/
https://www.youtube.com/watch?v=hrlBu1d5PBM