Beneath ground zero: the World Trade Center collapsed with a force equal to 600 tons of dynamite. Did the 16-acre, 70-foot-deep foundation survive the implosion? Engineers investigate.

When construction of the World Trade enter (WTC) commenced in 1967--two 110-story towers surging 1,250-feet skyward--it was the tallest building project ever attempted in the world to date. (The record was broken in 1974 with the construction of the Sears Tower in Chicago.) But not only was the WTC legendary in height; it dominated in depth as well: The towers were planted in what engineers call a mammoth "bathtub," a 16-acre, 70-foot-deep hole lined with a 3,000-foot-long waterproof wall.

The tub, which took an entire year to build, was designed to hold back tons of black oozy mud or silt from the towers' seven basement floors--housing a shopping center, parking garage, and commuter train station. To build the tub wall, construction workers first erected a 3-foot-wide trench and filled it with slurry, a mixture of water and bentonite, a type of sponge-like clay. The slurry cocktail hardened to create an outward force against the pressure of surrounding dirt and landfill, which would otherwise collapse into the excavation site. To further reinforce the trench, engineers dropped a steel cage into the slurry and pumped in concrete.

Once standing, the tub wall was supported by 1,500 tiebacks, long steel rods anchored to a sturdy rock layer called mica schist, or bedrock, 70 feet below sea level. When the basement construction was completed, the tiebacks were cut since the newly installed bottom floors provided equal outward support.

Now with the WTC destroyed, only the tons of debris beneath Ground Zero support the tub wall. And engineers worry that once the wreckage is removed, the tub could collapse inward, causing destabilization of nearby buildings or massive flooding from the adjacent Hudson River. "You can't just go digging next to those walls," stresses engineer George Tamaro. Instead, construction workers may first have to reinstall the tiebacks, a task that will demand many months of digging and reconstruction.

The diagram (left) illustrates how the bathtub fortress is designed to protect the WTC foundation seven stories below Ground Zero. May it stand.

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