Dear Colleagues,
Welcome back. I hope you had an enjoyable and productive summer and a good Labor
Day weekend. For those of you who are new to Georgetown, I have been sending periodic
updates about the I-395 Capitol Crossing construction project since it began. You might want to
read the earlier Notes in order to put this one into context.
http://www.law.georgetown.edu/campus-services/facilities/construction-info/index.cfm
The intense heat we endured through much of June and July did not make for a pleasant
summer for the BBC Construction team. Nonetheless, their progress has been remarkable. As
those of you who were here last year can see, the project has entered into a new phase of
construction. Although utility work continues on the west side of the project and beneath what
will be the new exit portal from the north-bound highway, utility work on 2nd street is complete.
What we are witnessing now, and will continue to witness for the next several months, is the
installation, below ground, of concrete caissons and a slurry wall that will support the columns
and steel beams that will eventually support the highway platform and the buildings upon it.
Work will proceed almost around the clock but the sounds emanating from the highway will
seldom be louder than the ambient sounds emanating from the highway itself. The warning beep
of equipment backing up may prove irksome to some of you but as an urban dweller, I hope that
sound will sound merge into the sounds of the night. Unfortunately, the warning beep is an
OSHA regulation that can’t be countermanded.
Unlike my previous Notes, this one is somewhat technical since the work we are
witnessing is based on some unfamiliar yet formidable scientific concepts. My wife read it and
told me only construction geeks like me would love it. I hope that is not the case; and maybe I
can develop some fellow travelers. In any case, to make this Note easier to fathom, I have
divided into two parts; part one is about caissons, and part two, to be distributed later this month,
will be about slurry walls.
The Basics
The strength of the unseen and the power of the unknown have delighted, intrigued, and
confused humans forever. Antoine de Saint-Exupéry once reminded us that “what is essential is
invisible to the eye.” Although he was talking about matters of the heart, the power of the
invisible resides in both the heart and the brain. What we cannot see, we cannot always
comprehend. The ancients believed in gods and totems, and the sometimes the stars and planets
to explain their mysteries. Science provides the magic for our time. Science can make things
look simple and can make the incomprehensible real. More often than not, the results of that
science are invisible to the eye.
So it is with caissons and slurry walls and the world that floats above them. In order to
engineer the proper bearing strength and friction metrics for the caissons and slurry walls,
engineers must first calculate the weight of the deck in terms of dead and live loads. The dead
load of this project, that is, the weight of the structure, façade, and other elements that are fixed
and permanent, is 318 million pounds. The live load, that is, the weight of all of the people,
vehicles, furniture, interior partitions, and other temporary, variable, and moving objects the
platform must support, is another 16,500,000 pounds. The engineers use these load calculations
to determine the number, depth, and diameter of the caissons and the depth, width, and length of
the slurry wall. When foundation elements are designed, a safety factor of 1.3 and 1.7 is applied
to the dead and live loads respectively. Thus the caissons and slurry wall are being built to
support a combined weight of almost 450 million pounds! One does not need to be an engineer
to realize the immensity of this project.
Below surface support for above ground structures is not new. Pre-literate man found that
pile or column foundations allowed them to build shelters high above the water or the land and
out of the reach of marauding animals and warring neighbors. Driven piles or columns are the
oldest type of deep foundation support for buildings and bridges. The Chinese have been using
piles or caissons for thousands of years. The Romans used wooden piles to support structures
over various soils and above water in their far-flung empire. In my March 17, 2015 Construction
Note, http://www.law.georgetown.edu/campus-services/facilities/construction-info/index.cfm I
described the supports used in the earliest Roman bridge, the Pons Publius, and Julius Caesar’s
bridge over the Rhine River. Wooden columns also support tenth-century structures in Venice.
As the water slowly but surely envelops that city today, the thousand-year-old piles continue to
support their loads. It is the rising water more than the sinking buildings that may eventually
doom the city of Venice.
Historically, these columns or caissons have been composed of timber, concrete, steel, or
a combination of those materials. The Capitol Crossing Project is using concrete caissons and a
subsurface concrete slurry wall to support its platform. Reinforced concrete caissons were
introduced in 1897 by François Hennebique, one of Europe’s premier bridge builders. A. A.
Raymond introduced concrete piles in the U.S. in 1901 and his Raymond Concrete Pile
Company became the foremost pile driving organization in the world during the twentiethcentury. The McKinney Drilling Company, a bridge construction business founded in 1937,
helped pioneer and develop the mechanized drilling machines used throughout the industry
today. McKinney is now owned by the Keller Group which also owns the Case Foundation
Company, the company that is performing the work on the Capitol Crossing project.
BBC Construction intends to install one hundred and forty-eight caissons between E
Street and Massachusetts Avenue. Eighty-two will be placed along the 2nd Street side of the
platform and other some others will be placed in the highway itself. Additional caissons will be
set on the west side of the highway, but a slurry wall rather than caissons will be the primary
form of support along the west edge of the platform. Once these essential elements are
completed, they will disappear from sight forever.
Caisson Construction
For the past few months, we have been watching some of those middle-of-the-highway
shafts being drilled and then filled with concrete. Approximately fifty caissons have been
completed to date. Most of them are either six-feet or eight-feet in diameter. Some of the eightfoot diameter caissons will reach a level of one hundred and thirty feet below the highway
surface. The shallowest will be seventy-five feet deep. In late July, those of us here for the
summer felt a vibration not unlike a small earth-quake as a sixty-foot metal casing that lines the
caisson shaft was vibrated into the ground. That casing hit a layer of soil that was very difficult
to penetrate. It caused a prolonged period of vibration and a greater than expected transmission
of waves into the surrounding soil. Most of the remaining casings are shorter and will be rotated
rather than vibrated into the shaft so we can expect fewer incidents of below-ground temblors.
Creating a caisson is tricky business. I attribute most of the knowledge I have gained to
conversations with David Salzer, Senior Vice-President of Balfour Beatty and Manager of this
project. Dave, a man of many talents, also teaches engineering classes at the University of
Virginia. He explains that loads placed on soil generally settle to some extent. Structural
problems can arise if a building settles more than 1 inch. Differential settlement can cause even
more damage than simple settlement. Differential settlement occurs when parts of the building
settle at different rates, causing cracks in foundations and walls and deterioration of the
building’s structural integrity. In some rare instances, soils may swell instead of settle, pushing a
building upwards and causing similar structural problems. Caissons and other foundation
systems work in tandem with the friction generated by the soil surrounding the caisson, to
support the structure and minimize settlement, thus preventing the structure from moving
vertically.
Drilling caissons requires multiple separate tasks that are performed by several different
pieces of equipment. The first five minutes of the YouTube video below gives you a close up
view of a drilling project that uses pieces of equipment that are similar to those BBC is using on
this project. The I-395 shafts, however, are much deeper than those in the video. And unlike the
crew member in the video, no one on this project will have to descend into the shaft.
https://www.youtube.com/watch?v=awW_uoRUJ8o
If you are really interested and have more time, you can view the entire process at
https://www.youtube.com/watch?v=gSpSw2jw2wc
Caissons are constructed by augering a cylindrical shaft and filling it with slurry, and
then lowering reinforcing steel and concrete into the shaft. The concrete forces the slurry to rise
and exit the shaft. The process is simple enough to describe, but not so easy to accomplish. The
pavement is first drilled using a core bit, a circular piece of drilling equipment that has teeth at its
center and around its edges. It has the same diameter as the shaft that will be drilled later.
Core bits
Bucket auger
An auger and a steel support casing
Steel support casing
and reinforced steel cage
Given the ultimate depth of the shaft, the earth must be stabilized as the drilling occurs. It is
stabilized first by a temporary steel casing that is spun or vibrated into the shaft. Once placed, a
spiral or bucket auger with a cutting edge slowly removes the soil down to a depth of twenty
feet. At that point, the crew begins to pump slurry into the shaft as excavation continues to the
desired depth.
Slurry is a fluid composed of small particles of polymer mixed with water that can
support the side walls of an excavation. The plant that produces the slurry is composed of those
eight blue tanks arranged in rows on 2nd Street between E & F Streets. Each holds 21,000
gallons of slurry. One tank is used for mixing fresh slurry while the other seven tanks are used to
store fresh slurry, regenerate and store used slurry, and store waste slurry for disposal. The tanks
are connected through a series of hoses, pipes, valves and manifolds, that direct the flow of
slurry from the plant to the shaft through a series of flexible pipes that run down the side of the
highway retaining wall, under the exit lanes, and then across the surface of the work-site.
Surprisingly, only 3.5 bags of slurry are mixed into the 21,000-gallon mixing tank. Because the
slurry fluid is denser than the earth, it can hold back the earth and any water that might otherwise
seep into the shaft. Thus, it stabilizes the shaft until the concrete is pumped in and the slurry is
replaced. Overall, 170,000 gallons of slurry will be used on this project.
Drilling continues until the shaft reaches its ultimate depth. When the shaft is completely
excavated, a reinforced-steel cage is lowered into the slurry-filled shaft to increase the strength
of the caisson. Concrete is then poured into the shaft using a tremie system that uses gravity or a
pump to send the concrete down to the bottom of the shaft. A tremie system uses a large metal
hopper connected to a series of pipes to distribute freshly mixed concrete into the shaft below the
slurry. The foot of the pipe is kept immersed in the concrete at the bottom of the shaft, while the
upper level of the tremie pipe is kept above the surface level. As the concrete rises and fills the
shaft, the tremie pipe is raised, but with the bottom end still below the concrete level. The
concrete forces the slurry out of the shaft while the concrete, because of its greater density,
remains. At an appropriate time, the thirty-foot metal casing is also removed from the shaft.
Tremie hopper
Tremie pipes being connected to one another
You can watch the operation of a tremie pour and the elimination of the slurry on this video.
https://www.youtube.com/watch?v=8W4Z3PPtBI8
The used slurry is pumped back to the slurry plant where it is cleaned and then used again
for another caisson. The concrete will then cure until it is strong enough to support the columns
that will be placed on top of the caisson. Steel rods are placed into the concrete at the top of the
caisson shaft. They will ultimately attach to the column that will be placed upon it. During the
remainder of this phase of the project, some of the caissons will be covered over because cars
will have to drive over them until the time for setting the columns occurs. At that point, the rods
will be re-excavated to permit the columns to be attached.
Connecting rods for the column
Coda
Despite the rigors of a construction site, there is poetry and dance to the operation. Stand
for a while and watch; you will see that there is a rhythm, a flow, almost a melody emanating
from the work. Fluids turn solid; steel melds with concrete; strength is in the unseen, reaching
down one hundred and thirty feet to support weights that defy comprehension. Men and
machines, orchestrated to achieve a wondrous though ultimately invisible power that will remain
long after the men who built it and we who watch it depart this earth. As Saint-Exupéry
reminded, “what is essential is invisible.”
The great Chicago poet, Carl Sandburg, captured this rugged beauty, drama, and mystery
in a series of poems written about urban life in the 20th century. In his poem, Skyscraper, he
writes;
***
Hour by hour the caissons reach down to the rock of the
earth and hold the building to a turning planet.
Hour by hour the girders play as ribs and reach out and
hold together the stone walls and floors.
Hour by hour the hand of the mason and the stuff of the
mortar clinch the pieces and parts to the shape an
architect voted.
Hour by hour the sun and the rain, the air and the rust,
and the press of time running into centuries, play
on the building inside and out and use it.
Men who sunk the pilings and mixed the mortar are laid
in graves where the wind whistles a wild song
without words
Souls of them all are here
***
Sources
Beatrice Diehl, L’2015, provided research assistance and photos for this Note.
Antoine de Saint Exupéry, THE LITTLE PRINCE, chapter 21
Carl Sandburg, Skyscraper, http://carl-sandburg.com/skyscraper.htm
Builder’s Engineers, Deep foundations: Pier Excavations,
http://www.abuildersengineer.com/2012/10/deep-foundations-pier-foundation.html
The Constructor, Civil engineering Home: Caisson (Pier) Foundations,
http://theconstructor.org/geotechnical/caisson-pier-foundation/503/
Pile Buck Home, Chap. 1, Introduction,
https://www.pilebuckinternational.com/chapter-1-introduction-3/
Mark Careyva, I‐395 Capitol Crossing Platform Bridges & Garage, Case Foundation Company,
Polymer Slurry Plant Setup & Mixing Process
Reliable Geoexpertise, Deep Excavations,
http://www.deepexcavation.com/en/retaining-systems
Wikipedia, Caissons (Engineering), https://en.wikipedia.org/wiki/Caisson_%28engineering%29
Videos
https://www.youtube.com/watch?v=awW_uoRUJ8o
https://www.youtube.com/watch?v=gSpSw2jw2wc
https://www.youtube.com/watch?v=8W4Z3PPtBI8