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Design and Construction of Continues Flight Auger Piles

Quiz Questions

1. CFA piles are a type of drilled foundation in which the pile is drilled to the final depth __________________ using a continuous flight auger (Fig. 2.1)
in one continuous process
in multiple drilling process
2. While the auger is drilled into the ground, the flights of the auger are filled with soil, providing ______ support and maintaining the stability of the hole (Fig 2.2a)
3. At the same time the auger is withdrawn from the hole, concrete or a sand/cement grout is placed by pumping the concrete/grout mix through the hollow center of the auger pipe to the base of the auger
4. Simultaneous pumping of the grout or concrete and withdrawing of the auger provides continuous support of the hole. Reinforcement for steel-reinforced CFA piles is placed into the hole filled with fluid concrete/grout immediately after withdrawal of the auger
5. CFA piles are typically installed with diameters ranging from
0.3 to 0.9 m (12 to 36 in.) and lengths of up to 30 m (100 ft),
1 to 3m and lengths of up to 60 m.
6. Because CFA piles are drilled and cast in place rather than being driven, as are driven piles, noise and vibration due to pile driving are minimized. CFA piles also eliminate splices and cutoffs. Soil heave due to driving can be eliminated when ___________ CFA piles are used.
7. A disadvantage of CFA piles compared to driven piles is that the available QA methods to verify the structural integrity and pile bearing capacity for CFA piles are less reliable than those for driven piles. Another disadvantage of CFA piles is that CFA piles generate soil spoils that require collection and disposal.
8. The key component of the CFA pile system, contributing to the speed and economy of these piles, is that the pile is drilled in one continuous operation using a continuous flight auger, thus ________ the time required to drill the hole.
9. If the auger turns too rapidly, with respect to the rate of penetration into the ground, then the continuous auger acts as a sort of "Archimedes pump" and conveys soil to the surface. As illustrated in Figure 2.5, this action can result in a reduction of the horizontal stress necessary to maintain stability of the hole. Consequently, lateral movement of soil towards the hole and material loss due to over-excavation can result in ground subsidence at the surface and reduced confinement of nearby installed piles.
10. Figure 2.7 shows:
The hole at the base of the auger for the concrete to pour out
The hole to suck-in the excessive underground water.
11. Upon achieving the design pile tip elevation, the auger is lifted a short distance [____________ ] and grout is pumped under pressure to expel the plug at the base of the internal pipe and commence the flow. The auger is then screwed back down to the original pile tip elevation to establish a small head of grout or concrete on the auger and to achieve a good bearing contact at the pile tip.
typically 150 to 300 mm (6 to 12 in.)
typically 600 to 1200 mm (24 to 48 in.)
12. The grout is pumped continuously under pressure [________________________] while the auger is lifted smoothly in one continuous operation.
typically up to 2 MPa (300 psi) measured at the top of the auger
typically up to 6 MPa (900 psi) measured at the top of the auger
13. Simultaneously, as the auger is lifted, the soil is removed from the flights at the ground surface so that soil cuttings are not lifted high into the air (potential safety hazard).

  • After the auger has cleared the ground surface and the grouting/concrete procedure is completed, any remaining soil cuttings are removed from the area at the top of the pile and the top of the pile is cleared of debris and contamination.
  • The reinforcement cage is lowered into the fluid grout/concrete to the required depth and tied off at the ground surface to maintain the proper reinforcement elevation.
14. It is essential that the grouting process begin immediately upon reaching the pile tip elevation; if there is any delay the auger may potentially become stuck and impossible to retrieve. To avoid "hanging" the auger (i.e., getting the auger stuck), some contractors may wish to maintain a slow steady rotation of the auger while waiting for delivery of grout; this rotation without penetration may lead to soil mining as described in the previous section and should be avoided. Another concern with excess rotation is degradation and subsequent reduction or loss of side friction capacity. The practice of maintaining rotation without penetration is not recommended. The best way to avoid such problems is to not start the drilling of a pile until an adequate amount of concrete/grout is available at the jobsite to complete the pile.
15. As seen in Figure 2.9, the top of the pile can be difficult to find among the surface disturbance.
Attention to detail in the final preparation of the surface is critical to ensure that structural integrity is maintained. Figure 2.10 shows a sequence of the final preparation of the pile surface and placement of the reinforcement. A dipping tool is typically used to remove any soil contamination near the top of the pile (top two photos of Figure 2.10) before placing reinforcement into the fluid grout (bottom two photos).
16. Reinforcement is placed into the fluid grout/concrete immediately after the auger is removed. In general, reinforcement lengths between 10 and 15 m (33 and 50 ft) are considered feasible, depending on _____________.
soil condition
weather and climate condition
17. A common practice in Europe is to utilize a small vibratory drive head to install the cage. The photo in Figure 2.12 is from a project in Munich, in which 1-m (3-ft) diameter piles were used to construct a wall through gravelly sand, and 18-m (60-ft) long cages were installed along the entire length of the pile to provide flexural strength. These cages were machine welded using weldable reinforcement and the piles were constructed using concrete. The use of a vibratory drive head could lead to problems with cages______________________, and could also produce segregation and bleeding if the concrete mix is not well proportioned. The system appeared to work very well on this project, as the concrete in the exposed piles appeared to be sound and free of segregation or voids.
that are not securely tied or welded
that are made of plastic
18. In conditions as Fig 3.2 it is better to use:
Driven piles or drilled shaft
Augured piles
19. CFA piles can be used to construct secant or tangent pile walls in a manner very similar to that of drilled shaft walls, which can be designed as cantilever or anchored walls. The most significant distinction relating to CFA piles, as opposed to other types of vertical elements, is the construction method for the vertical element. The CFA piles are intended to provide a reinforced vertical wall member having a similar function as that of a drilled shaft, slurry wall section, or sheet pile. In almost all such cases, the contractor provides designs of CFA piles for retaining structures as a design-build option. Figure 3.13 illustrates a typical CFA secant pile wall.
20. A variety of auger types may be used to drill the piles depending on the soil conditions encountered. Figures 4.7 through 4.10 illustrate some of the auger types that may be used for CFA piles. The pitch for CFA piles is, in general, smaller than that for DD piles (Figure 4.7). The augers for drilling in clay soils may tend to have a larger pitch to facilitate removal of the cuttings (Figure 4.8). Selecting the correct auger pitch is important because, for a given soil type, an excessively large pitch could result in ________ the soil around the pile.
mining of
pushing down
21. While some contractors and engineers have personal preferences for either grout or concrete, both have been used successfully in CFA pile applications. In general, the perceived advantages and disadvantages of grout relative to concrete may be summarized as follows.


  • Grout mixes are sometimes preferred for easier insertion of steel reinforcement into the pile;
  • Grout mixes tend to be more fluid and have greater workability; and
  • Grout mixes tend to be easier to pump, and many contractors, who have historically used grout mixes, have grout pumps and equipment that may not be suitable for use with concrete.


  • Grout will generally have a higher unit cost than concrete;
  • Grout will tend to have a slightly lower elastic modulus than concrete; and
  • Grout will tend to be less stable within the hole when drilling through extremely soft soils (such as organic clays or silt).
22. Grout mixes will tend to be more susceptible to small variations in water content which could lead to segregation or excessive bleed water. In general, any mix (concrete or grout) having extremely high workability requires greater attentiveness to quality control both at the batch plant and at the project site.
23. Drilling Rigs. The rig must have adequate torque capacity to install the pile without excessive flighting of the soil during drilling. While specs may include a minimum torque provision, it seems most prudent to set as a performance requirement that the contractor provide a rig capable of doing the project. The torque and power of the rig will directly affect the depth to which piles can be installed and the resulting axial capacity that can be achieved.
24. Sampling and testing of the grout/concrete are important parts of QA/QC.
25. In 7.5.2 Integrity Testing by Surface Methods; the most commonly available, economical, and easily applied type of integrity test is the sonic echo test. The advantage of the method is that a test can be performed rapidly, inexpensively, and without any internal instrumentation or tubes in the pile. In general, the sonic echo test is the recommended method for routine testing of CFA piles of 760 mm (30 in.) diameter or less.
26. 7.5.3 Integrity Testing using Downhole Techniques
The most reliable of the post-installation integrity tests for identifying anomalies within cast-inplace deep foundations are those that use down-tube instruments, such as the cross-hole sonic logging (CSL) test, single-hole sonic logging (SSL) test, and the backscatter gamma test.

However, due to the difficulty and expense of downhole methods on routine projects, these methods are recommended for use on piles where bending moments are unusually high and/or piles larger than 760 mm (30 in.) in diameter are used.
27. The CFA pile inspector must be prepared and should have experience and knowledge of CFA pile construction techniques. Prior to construction the inspector should have and review the following:

  • Project plans and specifications;
  • Geotechnical report and/or other available subsurface information (often provided on the plans);
  • Contractor’s approved installation plan;
  • Details of load test program or pre-production test pile installations;
  • Details of required automated monitoring system and control parameters;
  • Details of grout or concrete mix design and sampling and testing requirements; and
  • Reinforcing details and methods for pile top finishing and cutoff levels.
28. The ________ is responsible for furnishing all design, materials, products, accessories, tools, equipment, services, transportation, labor and supervision, and manufacturing techniques required for design, installation and testing of CFA piles and pile cap connections for this project.
CFA pile Contractor
facility and land owner
29. The CFA Pile Contractor shall design and install CFA piles, including selection of the CFA pile type, diameter, length, pile cap connection, and installation means and methods that will provide the load capacities indicated on the project plans, without damage to existing nearby structures.

A minimum diameter will be specified and a minimum length may be specified. The CFA pile load capacities shall be verified by load testing as required, and the pile integrity will be verified by pile integrity tests as required. All piles must meet the test acceptance criteria specified herein.

30. Muck, organics, soft clay, or other unsuitable materials encountered within 5 ft (1.5 m) of the ground surface, such material shall be removed or otherwise treated to prevent problems with pile top construction. Excavation of unsuitable surface material and backfilling shall be completed to the Engineer’s satisfaction, or as required in the contract documents, prior to the construction of CFA piles. Should more than 5 ft (1.5 m) of unsuitable surface material be encountered, the CFA Pile Contractor shall advise the Engineer immediately and proceed with work as directed by the Engineer. Should the CFA Pile Contractor suspect that any soils that are excavated are contaminated by hydrocarbons, refuse, or other environmentally hazardous material, he or she shall contact the Engineer immediately and proceed with work as directed by the Engineer.