Tuesday, August 25, 2015

Who gives a darn about some stupid slurry wall? The folks responsible for keeping the Hudson River out of the World Trade Center, for one


The original design of the WTC "bathtub" (click to enlarge)

A slurry wall is a technique used to build reinforced concrete walls in areas of soft earth close to open water or with a high ground water table.[1] This technique is typically used to build diaphragm (water-blocking) walls surrounding tunnels and open cuts, and to lay foundations.
-- from the Wikipedia article on "Slurry wall" (links onsite)

by Ken

Ay-ay-ay, you're probably thinking. We aren't really going to be yammering on about a subject as tedious-sounding as a slurry wall, are we?

You bet we are. A slurry wall may not sound very interesting or important, but it becomes both interesting and important if you happen to have, say, a large river and a nearby giant building comples you would ideally like to keep separate.
[Feel free to skip, except for the (really short) last graf]

A trench is excavated to create a form for each wall, then filled with slurry; it is kept full of slurry at all times. The slurry prevents the trench from collapsing by providing outward pressure which balances the inward hydraulic forces and prevents water flow into the trench. Reinforcement is then lowered in and the trench is filled with concrete, which displaces the slurry.

Guide walls before excavation
Slurry walls are typically constructed by starting with a set of guide walls, typically 1 metre (3.3 ft) deep and 0.5 metre (1.6 ft) thick. The guide walls are constructed on the ground surface to outline the desired slurry trench(es) and guide excavation. Excavation is done using a special clamshell-shaped digger or a hydromill trench cutter. The excavator digs down to design depth, or bedrock, for the first cut. The excavator is then lifted and moved along the trench guide walls to continue the trench with successive cuts as needed. The trench is kept filled with slurry (usually a mixture of bentonite and water) at all times to prevent collapse.

Once a particular length is reached, a reinforcing cage is lowered into the slurry-filled pit and the pit is filled with concrete from the bottom up using tremie pipes. The concrete displaces the bentonite slurry, which is pumped out and recycled.

Slurry walls are built to enclose the desired area, blocking water and softened earth from flowing into it. On completion of concreting, digging within the now concrete wall-enclosed area can proceed. To prevent the concrete wall from collapsing into the newly open area, temporary supports such as tiebacks are installed. When completed, the structure built within the walled-off area supports the wall, so that tiebacks and/or other temporary bracing may be removed.

Slurry wall construction was used to construct the "bathtub" that surrounded most of the World Trade Center site.[2] Slurry walls were also used heavily in Boston's Big Dig tunnel project.


Does this perhaps ring a bell? It's what was built to keep the nearby Hudson River out of the site. And it's far from past-tense-only. The WTC slurry wall was carefully rebuilt, at a cost of tens of millions of dollars. The only thing is that just possibly it may not have been carefully enough rebuilt.

In the last two weeks, DNAinfo New York's Murray Weiss reports ("There's a Leak in Rebuilt World Trade Center's Hudson River Wall: Sources"), "Workers began to hear the sound of rushing water behind the walls of lower concourses of the complex." (This is, for the record, "according to sources.") And officials at the site began to worry about the possibility that seepage is occurring through -- you guessed it -- the slurry wall. Murray Weiss reports:
The discovery prompted the Port Authority to quietly call in engineering and construction experts to try to identify its cause, sources said.

Crews were also tasked with dismantling sections of walls and other previous construction along lower subterranean concourses to try to get to the running water and trace its origin. The work is expected to be extremely costly, sources say.

Sources say officials are concerned that the leak may be coming from a stretch of the 3,200-foot-long slurry wall that is hidden by other walls that house unopened commercial offices, retail shopping stores and underground warehouse space that are expected to be operational by next summer.

They fear that the slurry wall may not have been properly insulated, allowing water to seep through it, sources said.
Uh-oh, this doesn't sound good, does it?
The Port Authority spent tens of millions of dollars since 9/11 repairing the slurry wall after the Twin Towers collapsed.

When it was built in the 1960s, the slurry wall was hailed as an engineering feat withholding the massive pressure of the Hudson River and giving construction crews the ability to open a massive 16-acre hole from which the original World Trade Center rose.

The slurry wall is 4 feet thick and roughly 100 feet deep. Although a stretch along Liberty Street shifted more than 10 inches on 9/11, it managed to keep the Hudson River from breaking through and drowning the smoldering Ground Zero site with water.

The wall’s emotional significance was immortalized when a portion was left exposed inside the National September 11 Memorial Museum.


This seems to be what the owner of the site, the Port Authority of New York and New Jersey (there's a name we've heard a lot in recent years, almost never in happy contexts), would like us to think.
Asked about the running water and potential slurry wall issue, a Port Authority spokeswoman said engineers had no reports of running water or any known potential issues involving the site’s slurry wall.

She even emailed photos of sections of the slurry wall that are visible along the PATH train tunnels to demonstrate that they are dry and intact.
Murray isn't reassured, though. According to those famous sources of his, "the sections of slurry wall that are of concern to the officials are hidden from view by the subterranean concourses." The sources also say that "depending on the severity of the problem, it could further delay the opening of the remaining concourse commercial space."

That's in addition to the work involved being, as previously noted, "extremely costly."

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