The Building

9 - 1 1 R e s e a r c h

an attempt to uncover the truth about September 11th 2001

mirror of “NERDCITIES/GUARDIAN” site : disclaimer

M.I.T., Rotch Visual Collections

Visual Communications in Building Technology Project

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3.1 THE FOUNDATION SYSTEM

All buildings interface with the ground through their foundation systems. The type of foundation to be used and how it is to be constructed are crucial decisions in any building process. These decisions must respond to the site conditions, the needs of the developer, the concepts of the architect, and construction limitations. Yet they must be made with a minimum of information as to what really lies below the surface. In the case of 60 State Street, the geotechnical consulting firm of Haley and Aldrich (H&A) had the responsibility of site evaluation, establishment of design criteria and the identification of potential foundation systems.

The Old State House, Faneuil Hall, the Sanborn Building, and both old and new office buildings surround the Sixty State Street site. Boston's old waterfront had originally come to the edge of the site and ground water problems were expected. Subsurface exploration indicated a stratum of dense glacial till, capable of carrying the 60 State Street tower in direct bearing, 40 to 50 feet below grade. Working with these conditions, H&A prepared their recommendations for CCF and SOM. This information, combined with the developer's desire to develop below grade parking facilities, produced the decision to use a mat foundation.

With the identification of the mat foundation, it was necessary to devise a means of retaining the steep sides of the required excavation. Site conditions placed serious constraints on this choice. Damage to adjacent structures had to be avoided. This meant providing a rigid retaining structure, minimizing the movement of ground water, and avoiding vibration from construction procedures. These criteria, along with a desire to avoid the noise associated with other construction procedures, led to the choice of a bentonite slurry trench wall for the retaining structure. The slurry trench procedure had the additional advantage of providing not just a retaining wall, but also a final basement wall. The subcontractor for this slurry wall was the Franki Foundation Company.

The slurry prevented the collapse of the sides of the trench. When the trench reached into firm glacial till, a steel reinforcing cage was dropped into place. Concrete was then tremied in, displacing the slurry, which was collected, filtered, and reused. After the trenches were poured and the wall panels had set up, the panels in between were dug and poured in the same manner, resulting in a continuous permanent retaining wall.

Figure 3.6 CONSTRUCTION OF A SLURRY WALL

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1. Drawing of retaining walls & tie diagrams.
2. Corner of trench and foundation hole.

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3. Bentonite slurry tanks.
4. Slurry tanks in place.

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5. Clam shell digger.
6. Foundation piers.

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7. Beginning to assemble wall reinforcement.
8. Slurry wall rebar cage on ground.

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9. Lifting slurry wall rebar cage.
10. Lifting slurry wall rebar cage.
11. Positioning slurry wall rebar cage.

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12. Lowering slurry wall rebar cage.
13. Guiding concrete shoot for pouring of foundation piers.

Upon completion of the slurry wall, excavation was begun (see Figure 3.7) . Initially the slurry wall acts as a simple cantilever, retaining the earth on the outside of the excavation. However, as the excavation progressed downward, it becomes necessary to support the slurry wall. This support is supplied by 300 post-tensioned soil tie-backs. The installation of these tie-backs requires the drilling of a downward slanted hole, sometimes in excess of 100 feet in length, into the firm glacial till. Once the hole has been drilled, post-tensioning cables are placed in the hole along with a grouting tube. The cables are then grouted firmly into the ground and then post-tensioned, providing the necessary support for the slurry wall. Upon completion of the excavation, the post-tensioned slurry wall supported an unencumbered site that reached from 40 to 50 feet below the surrounding grade. The post-tensioned tie-backs are considered only temporary construction supports. Upon completion of the substructure, the below grade floors take over the support function of the wall as indicated in Figure 3.7.

Figure 3.7 SUPPORT SYSTEMS FOR SLURRY WALL

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1. Foundation wall schematic--retaining wall and tie diagram.
2. Excavation near foundation walls.

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3. Drilling in and installing tiebacks.
4. Detail of tieback installation.

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5. Slurry wall tieback--post-tensioning of cables.
6. Completed foundation wall with tiebacks.

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7. View from above of foundation site.

Upon the completion of the excavation, the construction of the foundation is begun (see Figure 3.8). Any overexcavated portions of the site were filled with lean concrete and then a working mat is created over the entire site with additional lean concrete. Upon this working mat 430 tons of No. l0 reinforcing bars, required for the 6-foot thick reinforced-concrete mat, were placed. These bars were placed in two layers: one at the bottom, and one at the top of the mat. In addition, where interior gravity columns, with their high loads, come into the mat, it is necessary to have additional shear reinforcement (see Figure 3.8, 3). On November l, 1975, 6000 cubic yards of concrete were placed to create the foundation mat. This continuous foundation pour took over 30 hours.

Figure 3.8 CONSTRUCTION OF FOUNDATION

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1. Foundation mat reinforcement.
2. Inspection of rebars.
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3. Shear head reinforcement beneath building column.
4. General view of reinforced foundation mat.

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5. Foundation mat pouring continues.
6. Maneuvering of concrete shoot for pouring of foundation mat.

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7. General view of complete foundation mat.

The foundation mat serves only the tower portion of the structure. In addition, there is the plaza level of the structure, also containing three basement levels. In this area, because of the complex soil profiles, it was necessary to use a combination of spread footings and caissons for the foundation. The caissons were selected over the pile foundation, to satisfy the same noise control requirements that helped lead to the slurry trench solution.

All through the construction process the firm of Haley & Aldrich continually monitored a series of gauges and meters placed around the construction site to indicate signs of potential trouble in the retaining wall system. While the slurry trench method is usually considered more time-consuming and costly than other traditional methods like sheet-piling or soldier-piling methods of retention, in the case of Sixty State Street it has proved to be a worthwhile investment for the owner and the City.

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