Pier and Grade Beam Support - Excavation and Construction - Deep Foundations.

The previous sections have described shallow foundation excavation and construction, foundation construction in open and braced excavations, and groundwater control. The next two sections will discuss the excavation and construction of deep foundations.

A common type of deep foundation support is through the use of piers and grade beams. The typical steps in the construction of a foundation consisting of piers and grade beams are as follows:

1. Excavation of piers. Figures 16.9 to 16.11 show the excavation of the piers using a truck-mounted
auger drill rig. This type of equipment can quickly and economically excavate the piers to the
desired depth. In Figs. 16.9 to 16.11, a 30 in. (0.76 m) diameter auger is being used to excavate
the pier holes.

Truck-mounted auger drill rig used to excavate piers.
FIGURE 16.9 Truck-mounted auger drill rig used to excavate piers.





Close-up of auger being pushed into the soil.
FIGURE 16.10 Close-up of auger being pushed into the soil.




Close-up of auger being extracted from the ground with soil lodged within its groves.
FIGURE 16.11 Close-up of auger being extracted from the ground with soil lodged within its
groves.



2. Cleaning of the bottom of the excavation. Piers are often designed as end-bearing members. For example, there may be a loose or compressible upper soil zone with the piers excavated through this material and into competent material. The ideal situation is to have the groundwater table below the bottom of the piers. This will then allow for a visual inspection of the bottom of the pier excavation. Often an experienced driller will be able to clean out most of the bottom of the pier by quickly spinning the auger.

A light can then be lowered into the pier hole to observe the embedment conditions (i.e., see Fig. 16.12). A worker should not descend into the hole to clean out the bottom, but rather any loose material at the bottom of the pier should be pushed to one side and then scraped into a bucket lowered into the pier hole. If it is simply not possible to clean out the bottom of the pier, then the pier resistance could be based solely on skin friction in the bearing strata with the end-bearing resistance assumed to be equal to zero.

 A light has been lowered to the bottom of the pier in order to observe embed- ment conditions.
FIGURE 16.12 A light has been lowered to the bottom of the pier in order to observe embed-
ment conditions.
 


3. Steel cage and concrete. Once the bottom of the pier hole has been cleaned, a steel reinforcement cage is lowered into the pier hole. Small concrete blocks can be used to position the steel cage within the hole. Care should be used when inserting the steel cage so that soil is not knocked off of the sides of the hole. Once the steel cage is in place, the hole is filled with concrete. Figure 16.13 shows the completion of the pier with the steel reinforcement extending out of the top of the pier.

The pier hole has been filled with concrete. The steel reinforcement from the pier will be attached to the steel reinforcement in the grade beam.
FIGURE 16.13 The pier hole has been filled with concrete. The steel reinforcement from the
pier will be attached to the steel reinforcement in the grade beam.

4. Grade beam construction. The next step is to construct the grade beams that span between the piers. Figure 16.14 shows the excavation of a grade beam between two piers. Figure 16.15 shows the installation of steel for the grade beam. Similar to the piers, small concrete blocks are used to position the steel reinforcement within the grade beam. A visqueen moisture barrier is visible on the left side of Fig. 16.15. Figure 16.16 shows a pier located at the corner of the building. The steel reinforcement from the grade beams is attached to the steel reinforcement from the piers.



Once the steel reinforcement is in place, the final step is to place the concrete for the grade beams.

Figure 16.17 shows the finished grade beams. The steel reinforcement protruding out of the grade beams will be attached to the steel reinforcement in the floor slab.
Excavation for the grade beam that will span between the two piers.
FIGURE 16.14 Excavation for the grade beam that will span
between the two piers.





Steel reinforcement is being installed within the grade beam excavation.
FIGURE 16.15 Steel reinforcement is being installed within
the grade beam excavation.




Corner of the building where the steel reinforcement from the two grade beams has been attached to the steel reinforcement from the pier.
FIGURE 16.16 Corner of the building where the steel reinforcement from the two grade
beams has been attached to the steel reinforcement from the pier.


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.
FIGURE 16.17 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.

5. Floor slab. Before placement of the floor slab, a 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. 16.18. Although not shown in Fig. 16.18, small concrete blocks will be used to elevate the steel reinforcement off the subgrade and the steel will be attached to the steel from the grade beams. The final step is to place the concrete for the floor slab. Figure 16.19 shows the completed floor slab.

Positioning of the steel reinforcement for the floor slab.
FIGURE 16.18 Positioning of the steel reinforcement for the floor slab.




Concrete for the floor slab has been placed.
FIGURE 16.19 Concrete for the floor slab has been placed.

6. Columns. When designing the building, the steel columns that support the superstructure can be positioned directly over the center of the piers. For example, Fig. 16.20 shows the location where the bottom of a steel column is aligned with the top of a pier. A steel column having an attached base plate will be bolted to the concrete. Then the steel reinforcement from the pier (see Fig. 16.20) will be positioned around the bottom of the steel column. Once filled with concrete, the final product will be essentially a fixed-end column condition having a high lateral resistance.

Location where a steel column will be attached to the top of a pier.
FIGURE 16.20 Location where a steel column will be attached to the top of a pier.

The main advantage of this type of foundation is that there are no open joints or planes of weakness that can be exploited by soil 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 to the piers. In addition, the steel columns of the superstructure can be constructed so that they bear directly on top of the piers and have fixed end connections. This monolithic foundation and the solid connection between the steel columns and piers will enable the structure to resist soil movement or seismic shaking.

Usually the structural engineer will design this foundation system. 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 downdrag 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 will also need to inspect the foundation during construction in order to confirm the embedment conditions of the piers.

This type of foundation can also be used to resist the effects of expansive soil. However, when dealing with expansive soils, it is often preferred to use a raised concrete slab or raised wood floor to elevate the first floor off of the expansive subgrade. Loads from the raised floor are transferred to the grade beams and then to the piers. To mitigate uplift loads on the grade beams, a layer of easily deformable material, such as Styrofoam can be placed beneath the grade beams.

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