Engineering and Construction

Prestressed Concrete Piles - Excavation and Construction.

After the piles have been successfully installed, the next step is to construct the remainder of the foundation, as follows:

1. Cut-off top of piles. Especially for the indicator piles, the portion of the pile extending above ground surface may be much longer than needed. In this case, the pile can be cut-off or the concrete chipped-off by using a jackhammer, such as shown in Fig. 16.26.

2. Grade beam excavation. The next step is to excavate the ground for the grade beams that span between the piles. Figure 16.27 shows the excavation of a grade beam between two piles. For the foundation shown in Fig. 16.27, there is only one pile per cap, thus the pile caps are relatively small as compared to the size of the grade beams.

Those prestressed piles that broke during installation should also be incorporated into the foundation. For example, in Fig. 16.27, the pile located at the bottom of the picture is the same broken pile shown in Fig. 16.25. The replacement pile, which was successfully installed to the bearing strata, is located at a distance of 5 ft (1.5 m) from the broken pile (i.e., the pile near the center of Fig. 16.27). As previously mentioned, replacement piles should be installed in-line with the grade beam. As shown in Fig. 16.27, both the broken pile and the replacement pile will be attached to the grade beam; however, the broken pile will be assumed to have no support capacity.

FIGURE 16.26 Prestressed concrete piles have been installed and the excavations for the pile caps and grade beams are complete. The strands at the top of the pile will be connected to the steel reinforcement in the pile cap and grade beam.

FIGURE 16.27 The prestressed concrete pile at the bottom of the pic- ture is the same pile shown in Fig. 16.25. The pile near the center of the photograph is the replacement pile. The broken pile and the replacement pile will be attached to the grade beam.

Once the grade beams have been excavated, the next step is to trim the top of the prestressed piles such that they are relatively flush, such as shown in Figs. 16.28 and 16.29. The strands at the top of the pile are not cut off because they will be tied to the steel reinforcement in the grade beam in order to make a solid connection at the top of the pile.

FIGURE 16.28 The excavation for the grade beams is complete and the top of the prestressed piles are trimmed so that they are relatively flush.

FIGURE 16.29 Close-up view of one of the presstressed piles showing a trimmed top surface with the strands extending out of the top of the pile.

3. Installation of steel in grade beams. After the pile caps and grade beams have been excavated, the next step is to install the steel reinforcement. Figure 16.30 shows a close-up view of the top of a prestressed concrete pile with the steel reinforcement from the grade beam positioned on top of the pile. Note in Fig. 16.30 that the strands from the prestressed pile are attached to the reinforcement steel in the grade beams. This will provide for a solid connection between the pile and the grade beam. Figure 16.31 presents an overview of the grade beam with the steel reinforcement in place and the grade beam ready for the placement of concrete.

FIGURE 16.30 Close-up view of the top of a prestressed pile with the steel reinforcement from the grade beam positioned on top of the pile. The strands from the pile are attached to the steel reinforcement in the grade beam.

FIGURE 16.31 Overview of the steel reinforcement positioned within the grade beam excavation.

4. Floor slab. Before placement of the floor slab, the visqueen moisture barrier and a gravel cap-
illary break should be installed. Then the steel reinforcement for the floor slab is laid-out and the
final step is to place the concrete for the floor slab.

5. Columns. When designing the building, the steel columns that support the superstructure can be positioned directly over the center of the pile caps.

Similar to the pier and grade beam foundation, the main advantage of the prestressed pile foundation is that there are no open joints or planes of weakness that can be exploited by soils movement or seismic shaking. The strength of the foundation is due to its monolithic construction, with the floor slab attached and supported by the grade beams, which are in turn anchored by the pile caps and the prestressed piles. In addition, the steel columns of the superstructure can be constructed so that they bear directly on top of the pile caps and have fixed-end connections. This monolithic foundation and the solid connection between the steel columns and piles will enable the structure to resist soils move- ment and seismic shaking.

Usually the structural engineer will design this foundation system. The geotechnical engineer provides various design parameters, such as the estimated depth to the bearing strata, the allowable endbearing resistance, allowable skin friction in the bearing material, allowable passive resistance of the bearing material, and any anticipated downdrag loads that could be induced on the piles if the upper loose or compressible soil should settle under its own weight or during an anticipated earthquake. The geotechnical engineer should also perform pile load tests and inspect the foundation during construction in order to confirm the design recommendations.