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World Trade Center

What is the Bathtub problem that the newspapers are talking about?

 The foundation of the Twin Towers rested directly on the rock at a depth of more than 70' below the ground surface. A slurry wall (or diaphragm wall), 3' thick and about 80' deep, was constructed to support the soils surrounding the basements. The wall, which was held by the tiebacks/anchors, also created a watertight environment for construction and throughout the service life of the Center. With the completion of the basements, the wall was supported by the basement slabs and thus the tiebacks were released to avoid leakage of water. With the collapse of the 110-floor buildings, the slurry wall could now be destabilized because of the collapse of the basement slabs, which once acted as the support for the slurry wall. The failure or large deformation of the slurry wall may destabilize surrounding buildings and may lead to flooding of the underground space. The water may flow to NJ through the PATH tube that crosses the Hudson River.

It is reported that more than half of this wall has failed (see ENR link below).

Typical cross-section of diaphragm wall (from Civil Engineering, 1969)

Other geotechnical problems: Subway Tunnel collapsed
The Rebirth of Ground Zero by Newsday

Lower Manhattan Development Corporation

World Trade Center Building Performance Study

Forum on the Technical Implications of the World Trade Center Collapses
  Columbia News

PBS (letters)
Read also my Thought after the World Trade Center Forum

Update: February 7, 2002

General Readings check with Engineering News Record

Technical Papers

Other links
NYC Emergency Information
Federal Emergency Management Agency
My last visit to World Trade center
  far view    close view
Questions? e-mail Hoe Ling at

A Thought after the World Trade Center Forum - The importance of accurately documenting soil test results
I am a geotechnical engineer, and thus I am interested more in the issues of foundation and retaining structures (such as the bathtub). I have been trying to know about the soil properties the first day I heard about the bathtub problem, and apparently what we know are the very simple parameters related to early days design, such as the N-value. Now we have more reliable and powerful tools for analysis and design that also require additional soil information. But usually we cannot trace back those information. I raised this issue during the Forum to George Tamaro and Raymond Sandiford. I would assume that there were no records available now on the soil test results of those days. I am wanting to revisit the analysis for the slurry wall of the bathtub since it is such a historic and unforgettable site. If it is indeed the first site in US to use the slurry wall technique,  I would have expected more verifications of analysis against this site in the literature!

Why the availability of the soil test data would be of significance in the case of World Trade Center disaster. When the accessibility to the site is still restricted, the designers can start simulating the behavior of the wall numerically with the appropriate soil models and properties. Then they can have some thoughts about the excavation, etc., perhaps related to the rescue operation, before one can actually access the site. They can also simulate the wall considering all possible scenarios of debris removal. This issue could be very important for any future related disaster.

Incidentally I heard that Petronas Tower recorded a settlement half of the predicted value. I have tried all efforts in trying to get the soil test results for Petronas site. I afraid the evaluation of soil properties was not reasonably done (or interpreted through methodologies of old days) and then using also the old methods of analysis and design. As a consequece, the design was less economical (though it can be more conservative). To me, it would be great if we can find an answer to the doubts; we should be able to answer why. I think I have my answer there, but I need the real number to prove it.

Nevertheless, owners are always afraid of giving us the information since they are afraid of the unnecessary voice. Until today, I have nothing from Malaysia about the soil properties of Petronas Tower. However, my hunt for the soil test results of World Trade Center and Petronas Tower continues....

Hoe Ling (Dec 25, 2001, modified on Feb 7, 2002)

Real Life is far more complicated
Researchers are used to the terms, such as "uncertainties", "it requires additional studies", "our prelimanry studies showed that ....". My experience told that the media and policy makers do not want this kind of answer but a straight "yes" or "no". For example, they want to hear you telling them if the wall will collapse or not before you have a chance to look into the basic details. It is not that we don't know the answer, but the problem is far more complicated than just giving an answer as yes or no. The engineers are also in a position to make the scenario yes or no and we can make it. If we would have known that the terrorists will come with the airplanes and fire, the designers could have designed the twin towers differently. In any case, I should say that the policy makers are not willing or ready to accept our kind of answers.

Hoe Ling (Feb 7, 2002)