World Trade Center Geotechnical Issues

World Trade Center Geotechnical Issues world trade centre is an example of early diaphragm project (also referred as the slurry wall) in the US. The term “diaphragm walls” refers to the final condition when the slurry is replaced by tremied concrete that acts as a structural system either for temporary excavation support or as part of the permanent structure.The slurry wall, a 3-foot-thick (91 centimetres), below-ground, concrete structure surrounding the World Trade Centre, designed to keep its basement levels from being flooded by the Hudson River (Dunlap, 2005). This slurry wall plays a major role during the collapse, if the wall had been collapsed the Hudson River water will flood the subway and could cause a major damage.

How the WTC Slurry Wall Worked

During the site clearance of world trade centre four levels of rock anchors where used at the section adjacent to subway tunnel and six levels of rock anchors where placed in most of the other location. The rock anchors are very important part of site clearance (Gutberle, 1994). They provide the temporary support to the walls during site clearance. During the clearance the slurry level was keyed into bedrock by 2.4 m and after the first bracing level was installed the wall was cantilevered up to 15 cm. Once all the anchors were in place the wall was further pushed in soil by 6 cm using heavy machineries. This cantilevering was only installed in the section adjacent to the subway to provide the temporary support, where the depth of the first anchor level was about 10 m below surface.

Details of WTC bathhub

The most difficult challenge before the site clearance and reconstruction work was the removal of rubble. They have to complete this task without any further damage to the slurry wall. If the wall is damaged the Hudson river water will enter into the subway which could have destabilized nearby buildings, damaged underground facilities and flooded the subway tunnels. The task was very risky, costly and time consuming in nature.There was much speculation regarding the safe removal of debris. It took a while for the engineers to identify the damage to the retaining wall. The waterproof wall spanned 1 m wide and 21 m tall and wrapped more than 900 m around the 16-acre site which formed the footprint for the WTC.

World Trade Center Geotechnical Issues

In the start of the site clearing operation there is a significant doubt in the mind of engineers that whether the wall will hold or not while clearing the debris. A group of engineers performed some analysis and come up with an opinion.  According to them, it was possible to shore up the wall by steel tiebacks, quite similar to the process used during construction (Hussein, 2010). The steel tiebacks were made of reinforced steel, and were drilled through the wall at a slight downward angle and anchored to bedrock outside the wall. This provided a downward push to the wall and transferred the strain from the wall to the tiebacks.
Furthermore, water from fire hoses, sewers and fresh-water lines rushed into the Port Authority Trans-Hudson commuter train tunnel directly beneath the WTC. The water rushed in through the tunnel to the New Jersey station until the tunnel was clogged, for which pumps were installed. In addition, the two subway tunnels underneath the WTC had also collapsed due to puncturing by steel and concrete.

Conclusion

Engineers tried to anchor the sections of a huge concrete basing underneath the collapsed towers in order to make way and prevent the flooding of the PATH train system.  A 40 to 70 m deep cavern, known as the “bathtub” contained the foundation and the basement levels of the WTC, and its main purpose was to keep the water of the Hudson River from flooding the basement. When the towers collapsed, it caused the walls of the basement levels to collapse as well. Had the bathtub collapsed, the water would have kept rushing in and flooded the basement. To prevent this, cables and braces were used to support the collapsing thick walls of the basement level. There was also the possibility of the flooding water to reach the tubes of the PATH train system, which connected that Hudson River and the New Jersey train station. This water would have eventually reached through the PATH tubes and ultimately to the New York subway tunnels.
In addition, well points had to be used to deal with the water. Piles had to be driven twice as deep in order to sheet the trench, thereby allowing the engineers to eliminate most of the cross bracing. The adjacent building on Greenwich Street had to be investigated to make sure that its stability was uncompromised. Order Now.