Top/down or up/down construction methods are another method for constructing deep excavations. In this case the basement floors are constructed as the excavation progresses. The top/down method has been used for deep excavation projects where tieback installation was not feasible and soil movements had to be minimized. Figures 1 through 2 show construction photographs from two top/down excavations in Boston. The general top/down construction sequence is shown in Figure 3.. The Post Office Square Garage in Boston (7-levels deep) is one of the best-instrumented and documented top/down projects in the US (Whittle, et al., Whitman et al., 1991).
The sequence construction begins with retaining wall installation and then load-bearing elements that will carry the future super-structure. The basement columns (typically steel beams) are constructed before any excavation takes place and rest on the load bearing elements. These load bearing elements are typically concrete barrettes constructed under slurry (or caissons). The top few feet of a barrette with a steel beam can be seen in Figure 2. Then the top floor slab is constructed with at least on construction (glory) hole left open to allow removal of spoil material (Figs. 2, 3).
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The excavation starting at the glory hole begins once the top floor has gained sufficient strength. Soil under the top basement floor is excavated around the basement columns to slightly lower than the first basement floor elevation in order to allow for the installation of the forms for the first level basement slab. Glory holes are left open within each newly formed basement floor slab and the procedure is repeated. Each floor rests on the basement columns that were constructed earlier (Fig. 2).
Figure 1:Top/Down Excavation (Beth Israel Deaconess Hospital, Boston)
Figure 2: Millenium Place excavation. Left: Looking up at a glory hole, Top right: Lowest most level note LBE on the left and the barrette, Bottom right: close up view the same barrette (LBE) and steel beam.
Figure 3: Top/down basic construction.
a) Slurry wall and basement column construction
b) Ground floor construction and pouring
c) Excavation and floor construction under and above the ground floor
d) Excavation & lowest basement floor completed
Top down excavation for hospital in Boston, Massachusetts analyzed with DeepEX software!
The Beth Israel Hospital, in the longwood area of Boston Massachusetts, USA has a 65ft (22m) deep excavation constructed with the top down method. Permanent slurry walls were used to support the excavation. Wall displacements were measured with inclinometers and results were benchmarked with DeepEX.