Top/Down Construction - Use and Construction Sequence
DeepEX software can be used for the design of top/down excavations. This section examines the use of top/down construction method in deep excavation design, as well as, the construction sequence.
Figure: Top/Down Excavation in DeepEX.
Use of Top/Down construction method
Top/down excavation models can be designed in minutes with DeepEX software. 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. 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).
Top/Down construction sequence
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). Then the top floor slab is constructed with at least on construction (glory) hole left open to allow removal of spoil material.
Figure: Typical Top/Down Sequence.
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.
Top/Down Excavation - Diaphragm Walls with Concrete Slabs