Engineering and Construction

Common types of prestressed concrete piles are shown in "FIGURE 5.5 Typical prestressed concrete piles; dimensions in millimeters."  Prestressed piles are typically produced at a manufacturing plant and the first step is to set-up the form, which contains the prestressed strands that are surrounded by wire spirals. The concrete is then placed within the form and allowed to cure. Once the concrete has reached an adequate strength, the tensioning force is released, which induces a compressive stress into the pile. The prestressed piles are then loaded onto trucks, transported to the site, and stockpiled such as shown in Fig. 16.21.

Solid square concrete piles, such as shown in Fig. 16.21, are the most commonly used type of prestressed piles. As shown in Fig. 16.21, the end of the pile that will be driven into the ground is flush, while at the opposite end; the strands protrude from the concrete. The main advantage of prestressed concrete piles is that they can be manufactured to meet site conditions. For example, the prestressed concrete piles shown in Fig. 16.21 were manufactured to meet the following specifications: 12 in. (0.3 m) square piles

Design load = 70 tons (620 kN) per pile
Required prestress = 700 psi (5 MPa)
28 day compressive stress = 6000 psi (40 MPa)
Maximum water-cement ratio = 0.38
Portland cement type V (i.e., high sulfate content in the soil)

FIGURE 16.21 Prestressed concrete piles stockpiled at the job site.

Large pile-driving equipment, such as shown in Fig. 16.22, is required in order to drive the piles into place. If the piles are to be used as end-bearing piles and the depth to the bearing strata is variable, then the first step is to drive indicator piles. An indicator pile is essentially a prestressed pile that is manufactured so that it is longer than deemed necessary. For example, if the depth to adequate bearing material is believed to be at a depth of 30 ft (9 m), then an indicator pile could be manufactured so that it is 35 ft (11 m) long. Usually about 10 to 20 percent of the piles will be indicator piles. The indicator piles are used to confirm embedment conditions and thus some indicator piles may be driven near the locations of prior borings, while other indicator piles are driven in areas where there is uncertainty as to the depth of the bearing strata. Once the indicator piles have been driven, the remainder of the prestressed piles are manufactured with the lengths of the piles based on the depths to bearing strata as determined from the indicator piles.

FIGURE 16.22 Pile driving equipment. A prestressed concrete pile is in the process of being hoisted into position.

It is always desirable for the geotechnical engineer to observe the driving conditions for the prestressed piles. Prior to driving the piles, basic pile driving information should be recorded (see Table 16.6). In addition, during the actual driving of the piles, the number of blows per foot of penetration should be recorded. The pile-driving contractor typically marks the pile in one-foot increments so that the number of blows per foot can be easily counted.





Table 16.7 presents actual data during the driving of a prestressed pile. At this site, soft and liquefiable soil was encountered at a depth of about 15 to 30 ft (4.6 to 9.2 m) below ground surface.



Although the blows per foot at this depth were reduced to about one per foot, the driving contractor actually allowed the hammer to free fall and thus the energy supplied to the top of the pile was significantly less than at the other depths. For the data in Table 16.7, the very high blow counts recorded at a depth of 31 ft (9.5 m) are due to the presence of hard bedrock that underlies the soft and loose soil. Figure 16.23 shows the completed installation of the prestressed concrete pile. The wood block shown on the top of the concrete pile in Fig. 16.23 was used as a cushion in order to protect the pile top from being crushed during the driving operation.

FIGURE 16.23 A prestressed concrete pile has been successfully driven to the bearing strata. The wood block shown on the top of the concrete pile was used as a cushion in order to protect the pile top from being crushed during the driving operation.

A major disadvantage of prestressed concrete piles is that they can break during the driving process. The most common reason for the breakage of a prestressed concrete pile is because it strikes an underground obstruction, such as a boulder or large piece of debris that causes the pile to deflect laterally and break. For example, Fig. 16.24 shows the lateral deflection of a prestressed concrete pile as it was driven into the ground. In some cases, the fact that the pile has broken will be obvious.

FIGURE 16.24 Lateral displacement of a prestressed concrete pile during the driving operations.

In Fig. 16.25, the prestressed concrete pile hit an underground obstruction, displaced laterally and then broke near ground surface. In other cases where the pile breaks well below ground surface, the telltale signs will be a continued lateral drifting of the pile and low blow counts at the bearing strata.


If a pile should break during installation, the standard procedure is to install another pile adjacent to the broken pile. Often the new pile will be offset a distance of 5 ft (1.5 m) from the broken pile. Grade beams are often used to tie together the piles and thus the location of the new pile should be in-line with the proposed grade beam location. The structural engineer will need to redesign the grade beam for its longer span.

After the piles have been successfully installed, the next step is to construct the remainder of the foundation, as follows:  Construct the Remainder of the Foundation - Piles.