Prestressed Concrete Piles

Transcription

1 FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS FIGURE Excavation for the grade beam that will span between the two piers. Usually this foundation system is designed by the structural engineer. The geotechnical engineer provides various design parameters, such as the estimated depth of the bearing strata, the allowable end-bearing resistance, allowable skin friction in the bearing material, allowable passive resistance of the bearing material, and any anticipated down-drag loads that could be induced on the piers if the upper loose or compressible soil should settle under its own weight or during the anticipated earthquake. The geotechnical engineer also needs to inspect the foundation during construction in order to confirm the embedment conditions of the piers Prestressed Concrete Piles Introduction. Common types of prestressed concrete piles are shown in Fig Prestressed piles are typically produced at a manufacturing plant. The first step is to set up the form, which contains the prestressed strands that are surrounded by wire spirals. The

2 13.18 CHAPTER THIRTEEN FIGURE Steel reinforcement is being installed within the grade beam excavation. 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 Solid square concrete piles, such as shown in Fig , are the most commonly used type of prestressed piles. As shown in Fig , 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. A 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 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)

3 FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS FIGURE Corner of the building where the steel reinforcement from the two grade beams has been attached to the steel reinforcement from the pier. FIGURE The concrete for the grade beams has been placed. The steel reinforcement from the grade beams will be attached to the steel reinforcement in the floor slab.

4 13.20 CHAPTER THIRTEEN FIGURE Positioning of the steel reinforcement for the floor slab. FIGURE Concrete for the floor slab has been placed. Pile Driving. Large pile-driving equipment, such as shown in Fig , is required 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,

5 FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS FIGURE Location where a steel column will be attached to the top of a pier. FIGURE Typical prestressed concrete piles; dimensions in millimeters. (Reproduced from Bowles 1982 with permission of McGraw-Hill, Inc.) if the depth to adequate bearing material is believed to be 30 ft (9 m), then an indicator pile could be manufactured 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 about the depth of the bearing strata. Once the indicator piles have been driven, the remaining prestressed piles are manufactured with the lengths of the piles based on the depths to bearing strata as determined from the indicator piles.

6 13.22 CHAPTER THIRTEEN FIGURE Prestressed concrete piles stockpiled at the job site. 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 13.5). 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 1-ft increments so that the number of blows per foot can be easily counted. Table 13.6 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 1 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 13.6, 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 shows the completed installation of the prestressed concrete pile. The wood block shown on top of the concrete pile in Fig was used as a cushion 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 that it strikes an underground obstruction, such as a boulder or large piece of debris, which causes the pile to deflect laterally and break. For example, Fig 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. In Fig , 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 piles and low blow counts at the bearing strata. If a pile should break during installation, 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

7 FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS FIGURE Pile-driving equipment. A prestressed concrete pile is in the process of being hoisted into position. should be in line with the proposed grade beam location. The structural engineer will need to redesign the grade beam for its longer span. Pile Load Tests. The best method to evaluate the load capacity of a pile is to use a pile load test. A pile load test takes a considerable amount of time and effort to properly set up. Thus only one or two load tests are usually recommended for a particular site. The pile load tests should be located at the most critical area of the site, such as where the bearing strata are deepest or weakest. The first step is to install the piles. In Fig , the small arrows point to the prestressed concrete piles, which have been installed and are founded on the bearing strata. The next step is to install the anchor piles, which are used to hold the reaction frame in place and provide resistance to the load applied to the test piles. The most common type of pile load test is the simple compression load test (i.e., see Standard Test Method for Piles under Static Axial Compressive Load, ASTM D , 2000). A schematic setup for this test is shown in Fig and includes the test pile, anchor piles, test beam, hydraulic jack, load cell, and dial gauges. Figure shows an actual load test where the reaction frame has been installed on top of the anchor piles and the hydraulic loading jack is in place. A load cell is used to measure the force applied to the top of the pile. Dial gauges, such as shown in Fig , are used to record the vertical displacement of the piles during testing.

8 13.24 CHAPTER THIRTEEN TABLE 13.5 Example of Pile-Driving Information that Should Be Recorded for the Project Pile-Driving Record Date: March 7, 2001 Project name and number: Grossmont Healthcare, F.N Name of contractor: Foundation Pile Inc. Type of pile and date of casting: Precast concrete, cast 2/6/01 Pile location: See pile-driving records (Table 13.6) Sequence of driving in pile group: Not applicable Pile dimensions: 12 in by 12 in cross section, lengths vary Ground elevation: Varies Elevation of tip after driving: See total depth on the driving record Final tip and cutoff elevation of pile after driving pile group: Not applicable Records of redriving: No redriving Elevation of splices: No splices Type, make, model, and rated energy of hammer: D30 DELMAG Weight and stroke of hammer: Piston weight 6615 lb. Double-action hammer, maximum stroke 9 ft Type of pile-driving cap used: Wood blocks. Cushion material and thickness: Wood blocks approximately 1 ft thick Actual stroke and blow rate of hammer: Varies, but stroke did not exceed 9 ft Pile-driving start and finish times; and total driving time: See driving record (Table 13.6) Time, pile tip elevation, and reason for interruptions: No interruptions Record of number of blows per foot: See driving record (Table 13.6) Pile deviations from location and plumb: No deviations Record preboring, jetting, or special procedures used: No preboring, jetting, or special procedures Record of unusual occurrences during pile driving: None TABLE 13.6 Actual Blow Count Record Obtained during Driving of a Prestressed Concrete Pile Location: M 14.5 Start time: End time: 8:45 a.m. 8:58 a.m. Blow count record Blows per foot: 0 to 5 ft 1, 2, 3, 5, 9 5 to 10 ft 9, 9, 11, 10, 9 10 to 15 ft 7, 5, 4, 3, 2 15 to 20 ft 2, 2, 1, 1, 1 20 to 25 ft 1, 1, 1, 1, 1 25 to 30 ft 1, 1 for 2 ft, 1, 2 30 ft 8, 50 for 10 in Total depth 31.8 ft

9 FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS FIGURE A prestressed concrete pile has been successfully driven to the bearing strata. The wood block shown on top of the concrete pile was used as a cushion to protect the pile top from being crushed during the driving operation. FIGURE Lateral displacement of a prestressed concrete pile during the driving operations.

10 13.26 CHAPTER THIRTEEN FIGURE This prestressed concrete pile struck an underground obstruction, displaced laterally, and broke near ground surface. The arrow points to the location of the breakage. The pile is often subjected to a vertical load that is at least 2 times the design value. In most cases, the objective is not to break the pile or load the pile until a bearing capacity failure occurs, but rather to confirm that the design end-bearing parameters used for the design of the piles are adequate. The advantage of this type of approach is that the piles that are load-tested can be left in place and used as part of the foundation. Figure presents the actual load test data for the pile load test shown in Figs and For this project, the prestressed concrete piles were founded on solid bedrock, and thus the data in Fig show very little compression of the pile. In fact, the recorded displacement of the pile was almost entirely due to elastic compression of the pile itself, instead of deformation of the bearing strata. Pile Cap, Grade Beams, and Floor Slab. After the piles have been successfully installed, the next step is to construct the remainder of the foundation: 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 Grade beam excavation: The next step is to excavate the ground for the grade beams that span between the piles. Figure shows the excavation of a grade beam between two piles. For the foundation shown in Fig , there is only one pile per cap; thus the pile caps are relatively small 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 , the pile located at the bottom of the picture is the same broken pile shown in Fig 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 ). As previously mentioned, replacement piles

11 FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS FIGURE Pile load test. The small arrows point to the prestressed concrete piles, which will be subjected to a load test. The large arrow points to one of the six anchor piles. FIGURE Schematic setup for applying vertical load to the test pile using a hydraulic jack acting against an anchored reaction frame. (Reproduced from ASTM D , 2000, with permission from the American Society for Testing and Materials.) should be installed in line with the grade beam. As shown in Fig , 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.

12 13.28 CHAPTER THIRTEEN FIGURE in place. Pile load tests. The reaction frame has been set up, and the hydraulic jack and load cell are FIGURE Pile load tests. This photograph shows one of the dial gauges that are used to record the vertical displacement of the top of the pile during testing.

13 FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS FIGURE Pile load test data. This plot shows the actual data recorded from the pile load test shown in Figs and The vertical deformation is the average displacement recorded by the dial gauges. The axial load is determined from a load cell. 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 has shown in Figs and 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. 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 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 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. Fig presents an overview of the grade beam with the steel reinforcement in place and the grade beam ready for the placement of concrete. 4. Floor slab: Prior to placement of the floor slab, the visqueen moisture barrier and a gravel capillary break should be installed. Then the steel reinforcement for the floor slab is laid out, such as shown in Fig Although not shown, the final step is to place the concrete for the floor slab. 5. Columns: When the building is designed, 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, a main advantage of the prestressed pile foundation is that there are no open joints or planes of weakness that can be exploited by

14 13.30 CHAPTER THIRTEEN FIGURE 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. the 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 the seismic shaking. Usually this foundation system is designed by the structural engineer. The geotechnical engineer provides various design parameters, such as the estimated depth of the bearing strata, the allowable end-bearing resistance, allowable skin friction in the bearing material, allowable passive resistance of the bearing material, and any anticipated down-drag loads that could be induced on the piles if the upper loose or compressible soil should settle under its own weight or during the anticipated earthquake. The geotechnical engineer should also perform pile load tests and inspect the foundation during construction in order to confirm the design recommendations. Design Considerations. There are several important earthquake design considerations for using piles, as follows:

15 FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS FIGURE The prestressed concrete pile at the bottom of the picture is the same pile shown in Figure 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. 1. Connection between pile and cap: It is important to have an adequate connection between the top of the pile and the pile cap. As shown in Fig , this can be accomplished by connecting the strands from the prestressed pile to the steel reinforcement in the pile cap and grade beam. Without this reinforced connection, the pile will be susceptible to separation at the pile cap during the earthquake. For example, Figs and show two examples where the tops of the piles separated from the pile cap during the Kobe earthquake. 2. Down-drag loads due to soil liquefaction: The pile-supported structure may remain relatively stationary, but the ground around the piles may settle as the pore pressures dissipate in the liquefied soil. The settlement of the ground relative to the pile will induce down-drag loads onto the pile. The piles should have an adequate capacity to resist the down-drag loads. The relative movement between the relatively stationary structure and the settling soil can also damage utilities. To mitigate damage to utilities, flexible connections can be provided at the location where the utilities enter the building. 3. Passive resistance for liquefiable soil: A common assumption is to assume that the liquefied soil will be unable to provide any lateral resistance. If a level-ground site contains an upper layer of nonliquefiable soil that is of sufficient thickness to prevent ground

16 13.32 CHAPTER THIRTEEN FIGURE The excavation for the grade beams is complete, and the tops of the prestressed piles are trimmed so that they are relatively flush. fissuring and sand boils, then this layer may provide passive resistance for the piles, caps, and grade beams. 4. Liquefaction of sloping ground: For liquefaction of sloping ground, there will often be lateral spreading of the ground, which could shear off the piles. One mitigation measure consists of the installation of compaction piles (see Sec ), in order to create a zone of nonliquefiable soil around and beneath the foundation FOUNDATIONS FOR SINGLE-FAMILY HOUSES In southern California, the type of foundation for single-family houses often consists of either a raised wood floor foundation or a concrete slab-on-grade.