Geotechnical Engineering Circular (GEC) No. 8
Design And Construction Of Continuous Flight Auger Piles
Chapter 2: Description Of Continuous Flight Auger Pile Types And Basic Mechanisms
This chapter provides a general overview of continuous flight auger (CFA) piles. CFA piles have also been referred to as auger-cast, augered-cast-in-place, auger-pressure grout, and screw piles. The term CFA is used to generally refer to these types of piles constructed according to the recommendations in this document. This overview includes: (1) a description of the main components of a CFA pile; (2) typical drilling and grouting equipment used; and (3) a description of the sequence of construction.
A comparison of CFA piles with common deep foundations is presented to provide context for readers who are more familiar with driven piles and drilled shafts. Considerations are presented for: (1) initial hole drilling; (2) potential soil caving or mining; and (3) subsequent grout or concrete placement, including reinforcement placement. Additionally, basic information regarding the effects of soil type (i.e., clay, sand, or mixed) on load transfer will also be presented.
CFA piles are a type of drilled foundation in which the pile is drilled to the final depth in one continuous process using a continuous flight auger (Figure 2.1). While the auger is drilled into the ground, the flights of the auger are filled with soil, providing lateral support and maintaining the stability of the hole (Figure 2.2a). At the same time the auger is withdrawn from the hole, concrete or a sand/cement grout is placed by pumping the concrete/grout mix through the hollow center of the auger pipe to the base of the auger (Figure 2.2b). 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 (Figure 2.2c).
Figure 2.1: CFA Pile Rig
Source: Cementation Foundation Skanska
Figure 2.2: Schematic of CFA Pile Construction
Source: Bauer Maschinen
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), although longer piles have occasionally been used. In the United States, the practice has typically tended toward smaller piles having diameters of 0.3 to 0.6 m (12 to 24 in.) primarily because less powerful rigs have typically been used for commercial practice with these piles in the United States. European practice tends toward larger diameters [up to 1.5 m (60 in.)]. In recent years, the trend in the United States has been toward increased use of CFA piles in the 0.6 to 0.9 m (24 to 36 in.) diameter range.
The reinforcement is often confined to the upper 10 to 15 m (33 to 50 ft) of the pile for ease of installation and also due to the fact that in many cases, relatively low bending stresses are transferred below these depths. In some cases, full-length reinforcement is used, as is most common with drilled shaft foundations.
CFA piles can be constructed as single piles (similar to drilled shafts), for example, for soundwall or light pole foundations. For bridges or other large structural foundations, CFA piles are most commonly installed as part of a pile group in a manner similar to that of driven pile foundations as illustrated in Figures 2.3 and 2.4. Similarly to driven piles, the top of a group of CFA piles is terminated with a cap (Figure 2.4). Typical minimum center-to-center spacing is 3 to 5 pile diameters.
Figure 2.3: Schematic of Typical Drilled Shaft vs. CFA Foundation
CFA piles differ from conventional drilled shafts or bored piles, and exhibit both advantages and disadvantages over conventional drilled shafts. The main difference is that the use of casing or slurry to temporarily support the hole is avoided. Drilling the hole in one continuous process is faster than drilling a shaft excavation, an operation that requires lowering the drilling bit multiple times to complete the excavation. In contrast, the torque requirement to install the continuous auger is high compared with a conventional drilled shaft of similar diameter; therefore, the diameter and length of CFA piles are generally less than drilled shafts. The use of a continuous auger for installation also limits CFA piles to soil or very weak rock profiles, while drilled shafts are often socketed into rock or other very hard bearing materials.
Figure 2.4: Group of CFA Piles with Form for Pile Cap
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 non-displacement CFA piles are used. 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. Handling of spoils can be a significant issue when the soils are contaminated or if limited room is available on the site for the handling of material. Certain types of CFA piles that do not generate spoils will be discussed later in this document.
The remainder of this chapter provides an overview of the construction process of CFA piles.
2.2 Construction Sequence
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 reducing the time required to drill the hole. While advancing the auger to the required depth, it is essential that the auger flights be filled with soil so that the stability of the hole is maintained. 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. The top of Figure 2.5 represents an auger having balanced auger rotation and penetration rates, so that the flights are filled from the digging edge at the base of the auger with no lateral "feed". The bottom of Figure 2.5 illustrates an auger having an excessively slow penetration rate and an insufficient base feed to keep the auger flights full; as a result, the auger feeds from the side with attendant decompression of the ground.
Figure 2.5: Effect of Over-Excavation using CFA Piles
After Fleming (1995)
As the auger cuts the soil at the base of the tool, material is loaded onto the flights of the auger. The volume of soil through which the auger has penetrated will tend to "bulk" and take up a larger volume after cutting than the in-situ volume. Some of the bulk volume is also due to the volume of the auger itself, including the hollow center tube. Thus, it is necessary that some soil is conveyed up the auger during drilling. To maintain a stable hole at all times, it is necessary to move only enough soil to offset the auger volume and material bulking without exceeding this volume. Controlling the rate of penetration helps to avoid lateral decompression of the ground inside the hole, the loosening of the in-situ soil around the hole, and ground subsidence adjacent to the pile.
The proper rate of penetration may be difficult to maintain if the rig does not have adequate torque and down-force to rotate the auger. When a soil profile being drilled has mixed soil conditions (e.g., weak and strong layers), difficulties may arise. For example, if a rig having a low torque-capacity is used for drilling through a mostly-hard profile, difficulties can arise when drilling through a weak, embedded stratum to penetrate the strong soil. If the rig cannot penetrate the strong soil stratum at the proper rate, the augers can "mine" the overlying weak soil to the surface and cause subsidence.
One solution to properly balance soil removal and penetration rate is to use auger tools that actually displace soil laterally during drilling. In this construction technique, these types of piles are more commonly described as Drilled Displacement (DD) piles. DD piles include a variety of patented systems, which typically consist of a center pipe within the auger, an auger of larger diameter than that of conventional CFA equipment (Figure 2.6), and some type of bulge or plug within the auger string to force the soil laterally as it passes (not shown in Figure 2.6). The advantage of this system is that soil mining is avoided. In addition, the soil around the pile tends to be densified and the lateral stresses at the pile/soil wall are increased, thus leading to soil improvement and increased pile capacity for a given length. The main disadvantage is that the demand for torque and down-force from the rig is greater and this creates a limitation on the ability to install piles to great depths, as well as in very firm to dense soils.
Figure 2.6: Displacement Pile
For some soil conditions, the concern for soil mining and the need to establish a good penetration rate is not as critical. CFA piles have been successfully installed in many geologic formations without any consideration of the rate of penetration or soil mining. Where soils are stable due to cohesion, cementation, and/or apparent cohesion due to low groundwater levels, and pile lengths are relatively short, it may be feasible to neglect some of the considerations of drilling rate and soil mining. For example, residual soil, weak limestone formations, and cemented sands are soil types that favor easy construction. In such instances, the continuous auger is essentially used to construct a small open-hole, drilled shaft, or bored pile. However, such practice should be allowed only after the completion of successful test installations and after load tests have confirmed that satisfactory results are obtained and that with no adverse effects from ground subsidence will take place.
When the drilling stage is complete and the auger has penetrated to the required depth, the grouting stage must begin immediately. Grout or concrete is pumped under pressure through a hose to the top of the rig and delivered to the base of the auger via the hollow center of the auger stem. The generic term grout will be mostly used in the remainder of this document; however, it is understood that grout or concrete can be used in this process. Figure 2.7 shows the hole at the base of the auger stem. Figure 2.8 shows grout being delivered to the project site by truck to a pump located near the drill rig.
Figure 2.7: Hole at Base of Auger for Concrete
Figure 2.8: Grout Delivered to Pump
220.127.116.11 General Sequence
The general grouting sequence is as follows:
- Upon achieving the design pile tip elevation, the auger is lifted a short distance [typically 150 to 300 mm (6 to 12 in.)] 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.
- The grout is pumped continuously under pressure [typically up to 2 MPa (300 psi) measured at the top of the auger] while the auger is lifted smoothly in one continuous operation.
- 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.
18.104.22.168 Start of Grouting
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.
After reaching the pile tip elevation, the operator typically must lift the auger about 150 mm (6 in.) and pump grout/concrete under pressure to expel the plug used as a stopper in the bottom of the hollow auger. This operation is typically called "clearing the bung" or "blowing the plug" among contractors. Occasionally, some contractors lift the auger up to about 300 mm (12 in.), although a distance limited to 150 mm (6 in.) is preferable. Lifting of the auger prior to blowing the plug must be limited to 150 mm (6 in.) because a greater lift-up distance does not favor the development of good end-bearing in the pile. If the lift-up distance is excessive, the stress relief in the hole walls below the auger may be large, the bearing surface may be disturbed, and this may result in mixing of grout with loose soil at the pile toe. Prior to starting withdrawal, the auger is re-penetrated to the original pile tip elevation while maintaining pressure on the grout.
22.214.171.124 Withdrawal of Auger
Grout should be pumped to develop pressure at the start of the grouting operation. The pressure developed should be monitored to ensure that an adequate value is maintained. The grouting pressure typically depends on the equipment being used, but commonly, the applied grouting pressure is in the range of 1.0 to 1.7 MPa (150 to 250 psi) as measured at the top of the auger. As a minimum, the pressure must be in excess of the overburden pressure at the discharge point at the tip of the auger after accounting for elevation head differences between the measurement point and the auger tip. The grouting pressure must be maintained as the auger is slowly and smoothly withdrawn. This pressure replaces the soil-filled auger as the lateral support mechanism in the hole. When the grout pressure is applied, the grout also pushes up the auger flights and presses the soil against the auger.
If over-rotation has been applied during drilling, it could be difficult to maintain the grout pressure during withdrawal. Over-rotation refers to the excess rotation of the auger relative to the depth penetrated for each turn. During over-rotation, the auger does not have a sufficient feed of soil from the cutting edge to maintain the flights full of soil and to prevent soil from loading the auger from the side. Over-rotation of the augers during drilling tends to clear the auger of soil and permits the concrete or grout to flow up the auger flights rather than remaining below the base of the auger under pressure. If grout flows up the auger flights for a large distance, it will be vented to the surface while the auger is still in the ground and at that point it will no longer be possible to maintain pressure in excess of surrounding overburden stress.
As the auger is slowly and steadily withdrawn, an adequate and controlled volume of grout/concrete must be delivered at the same time to replace the volume of soil and auger being removed. An overrun in the grout replacement volume of about 15 to 20% above the theoretical volume of the pile should be required. The necessary volume of grout must be delivered continuously as the auger is removed, and this volume should be measured and monitored to ensure that an adequate volume of concrete/grout is delivered. If the auger is pulled too fast in relation to the ability of the pump to deliver volume, the soil will tend to collapse inward and form a neck in the pile. Continuous monitoring of the volume is required to avoid the possibility that the rig operator could pull too fast for a short segment and then slow down for the volume to "catch up". This discontinuous withdrawal could result in the pile being constructed as a series of necks and bulges rather than the uniform structural section that is desired.
During grouting, the auger should be pulled with either no rotation or slow continuous rotation in the direction of drilling. A static pull with no rotation can help maintain a static condition at the base of the auger against which the grout pressure acts. Some contractors prefer to slowly rotate the auger during withdrawal to minimize the risk of having the auger flight getting stuck. In addition, some augers have an off-center discharge plug at the base and slow rotation may help avoid concentrating the distribution of the grout pressure to an off-center location within the hole. If rotation is used, it must be very slow so that the auger does not tend to conduct the soil on the auger flights to the surface ahead of the auger. When the grout reaches the surface, the grout pressure is vented, and the high pressure under the auger can no longer be maintained. At this point, it is important that the proper volume of grout be continuously delivered per increment of length as the auger is removed, and the grout that is on the augers should not be allowed to flow back into the hole. If grout and soil become mixed on the auger during this process, the soil and contaminated grout could fall into the top of the pile and be difficult to remove.
After withdrawal of the auger and removal of spoil, it is necessary that the top of the pile be cleared of debris and any soil contamination be removed. The use of a small form is recommended to provide definition of the top of the pile prior to placement of the reinforcement.
Figure 2.9: Grout at Surface after Auger Withdrawal
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).
Figure 2.10: Finishing Pile and Reinforcement Placement
126.96.36.199 Grout vs. Concrete
In the current U.S. practice, the majority of work is completed using a sand/cement grout mixture. In European practice, concrete is used almost exclusively. In the United States. contractors are starting to use concrete more frequently. Successful projects have been completed using both materials. Some of the advantages of each material are discussed below. Details of these materials are provided in Chapter 4.
Grout has been used since the early days of CFA pile construction in the United States because of the fluidity and ease of installation of reinforcement. The typical range of compressive strength of grout in transportation projects is 28 to 35 MPa (4,000 to 5,000 pounds per square inch [psi]). Grouts used with CFA piles are usually rich in cement, having 8 to 11 sacks per 0.76 cubic meter (1 cubic yard). Aggregate is generally limited to sand with the gradation of concrete sand. A Fluidifier is often used as a pumping aid, to act as a retardant, and to help control shrinkage because the mix is so rich in cement. Fly ash and slag are often substituted for a portion of the cement. Typical percentages for fly ash and slag are around 12% to 15%, for a combined replacement of cement of 24% to 30%. When only fly ash is used to partly replace the cement, its content can range commonly between 25 and 70 lbs per cubic yard (pcy) of concrete, with 40 lbs per cubic yard being a typical quantity.
The partial replacement of cement with fly ash and slag tends to produce mixes with higher workability and pumpability, reduced bleeding, and reduced shrinkage. Fly ash and/or slag tend to slightly retard the early strength gain of the grout mix relative to an equivalent mix using only Portland cement, although long-term strength (e.g., 56 days) may be comparable. If fly ash and/or slag are used in the mix, the submittal of the grout mix design should include information on strength development vs. time.
Grout is so fluid that the workability is typically measured using a flow cone (as described in Chapter 7) in lieu of the slump measurement that is typical for concrete mixes. With the use of grout aids (which provide some retarding effect) and the relatively rapid construction of these piles (casting is normally completed in a matter of minutes), the loss of workability during the placement operations is not normally a significant consideration and retarding admixtures are not commonly used. However, the mix must remain fluid during rebar placement.
Workability during time after placement of the grout is important in CFA pile construction because of the need to place reinforcement within the pile soon after completion. It is considered good practice to start drilling only after the concrete or grout has arrived on the project site and delivery of the full volume needed for completion of the pile without interruption is assured. The characteristics of the soil at the site play a significant role in the workability of the grout during rebar placement. Rebar may be easy to install for up to 30 minutes after grout casting for piles in saturated clays; on the other hand, dry sandy soils may tend to dewater the grout very quickly. If sandy soils are producing rapid dewatering of the grout, conventional measurements of concrete setting time alone may not provide a reliable indication of the ability to place the reinforcing cage. Retarding admixtures or anti-washout agents [such as viscosity-modifying admixtures (VMA)] may be needed for piles with long rebar cages constructed in sandy soils. The use of test pile installations, together with a willingness to adjust the mix characteristics based on observations, are important components of achieving constructability.
The concrete used by most European and some U.S. contractors generally uses a pea-gravel size aggregate with around 42% sand used in the mix. Concrete is cited as being less costly than grout, less prone to overrun volume, and is considered to be more stable in the hole when constructing piles through soft ground. Concrete slump for CFA construction is similar to that of wet-hole drilled shaft construction, with target slump values in the 200 mm ± 25 mm (8 in. ± 1 in.) range. Considerations for rebar placement are similar to those for sand/cement grout cited above.
188.8.131.52 Reinforcement (Rebar)
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 conditions. However, longer reinforcement has been used. Contractors often install a single, full-length bar (i.e., it extends the entire depth of the pile) into the center of the pile (Figure 2.11) and a cage of partial-length bars around the perimeter of the pile. The full-length bar provides continuity and acts as a guide for the cage. A full-length bar is also used for tension resistance. In this case, a wire "football" is installed on the end of the bar to anchor the bar in the grout. A minimum cover of 75 mm (3 in.) is commonly adopted for most applications. Difficulties in placing the cage can arise if the concrete starts to set and loses workability. Soil conditions can also have an effect on the reinforcement placement, as a free draining sand or dry soil can tend to dewater the concrete rapidly and lead to increased difficulty in placing the cage.
Figure 2.11: Placement of a Single Full-Length Bar
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 that are not securely tied or welded, 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.
Figure 2.12: Vibratory Drive Head Used to Install Rebar Cage
It is worth noting the differences in grout placement for a CFA pile in contrast with that of a drilled shaft foundation. Concrete used for a drilled shaft is placed through a tremie pipe into a fluid-filled hole or via a drop chute into a dry open hole. In each case, the inspector and contractor have some means of observing the location of the concrete relative to the surface and/or tremie. Additionally, the reinforcement cage is pre-positioned and held in place, often with some access tubes for subsequent non-destructive testing to verify structural integrity. The concrete must maintain workability to pass through the cage and fill the hole. The placement of grout in CFA piles can only be monitored remotely and indirectly by measurement of the volume delivered through the auger at any given time and the pressure at which it was pumped. The grout must maintain workability so that the cage can pass through the grout. Quality control issues are present with both systems and difficulties can arise with either system. The CFA pile system is particularly dependent upon operator control during grout placement and auger withdrawal.
This chapter provides a general overview of CFA piles, including the general construction sequence used and potential limitations and/or difficulties using the technique. CFA piles have some significant advantages in terms of speed and cost effectiveness if used in favorable circumstances, and can clearly pose difficulties in terms of quality control if careful construction practices are not followed. The following chapter will describe potential applications of this technology in transportation related projects and will outline favorable and unfavorable project and geotechnical conditions for CFA piles.