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 Indonesia. 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).
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 (Pasar Baru Bandung, Project)
Figure 2: Menara Merdeka Project. Left: Looking up at a glory hole, Top right: Llowest 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
The Top-Down Construction in Indonesia
The top down construction in Indonesia was introduced for the first time in 1994. The project was the Bank Indonesia C-Building Projects (12 stories, and 3 basement), and PT. (Persero) Wijaya Karya was the main contractor.
In, 1995 another project was Menara Merdeka Building (30 stories, 3 basement) also built by top-down construction. The main contractor was joint operation of PT. (Persero) Pembangunan Perumahan and PT. (Persero) Wijaya Karya.
The developing of this method was introduced by Ir. Ign. Chen, and called the new concept as Up & Down Construction. The modification was the dismissing of diagprahm wall as the element of the basic top-down construction method. The first project that using the Up & Down Construction Company was Bandung Electronic Centre (BEC), 6 stories plus 2 basement, in 2002, and the main contractor was PT. (Persero) Pembangunan Perumahan.
The largest project in Indonesia that use the top-down construction was Pasar Baru Bandung Project , 2003. The huge building was consisted 120,000 sq.m, 10 stories plus 3 basement, and the time line for complishing the project was only 8 months. The main contractor was PT. (Persero) Pembangunan Perumahan.
The Project Manager of Pasar Baru Bandung, decided to deploy the Up & Down Construction with several modifications to take that challenge. The Pasar Baru Bandung was acomplished precisely at the time, and regarded as the fastest commercial building project at that time.
There were 6 top down construction projects until the year of 2005 in Indonesia:
1. Bank Indonesia C Building Projects - PT. (Persero) Wijaya Karya - Ir. Abbas Sudrajat
2. Menara Merdeka Building Project - PT. (Persero) Pembangunan Perumahan &
PT. (Persero) Wijaya Karya - Ir. H. Sugeng Haryono, MM (Project Manager)
3. Puri Mustika Hotel Building Project - PT. (Persero) Pembangunan Perumahan -
Ir. Andi Reman (Project Manager)
4. Bandung Electronic Centre (BEC) Building Project - PT. (Persero) Pembangunan
Perumahan - Ir. Anton Satyo Hendriatmo (Project Manager)
5. Pasar Baru Bandung Building Project - PT. (Persero) Pembangunan Perumahan -
Ir. Anton Satyo Hendriatmo (Project Manager)
6. St. Baromeus Hospital Bandung Project - PT. (Persero) Pembangunan Perumahan-Ir. Nurlistyo Hadi (Project Manager)