The only portions of the World Trade Center towers that survived the attack on 9/11 were the basement slurry walls, part of the original shoring and foundation system. The National September 11 Memorial and Museum at the World Trade Center that is currently under construction will preserve a portion of that wall making it the largest exhibit the museum will offer. The wall section displayed will be 62-ft by 64-ft.
The existing slurry walls are being incorporated into the foundation system of the new facility but not without some improvements. The are adding some kind of foundations improvements to stabilize the toe of the walls, the New York Times article calls them caissons, but I don’t know if its a tangent or secant wall or something else. They are also lining them with additional concrete and reinforcement in front of the walls along with additional tiebacks to stabilize them. In the portion of the wall that will be displayed, a counterfort wall will be constructed behind it and new tiebacks will be installed on the front. Work for the counterfort wall will be done by hand in order to avoid the existing tieback cables. All of the existing tiebacks will be left intact. Check out the NY Times article for a great graphic showing the system. (Illustration by New York Times)