Geotechnical Engineering Circular (GEC) No. 8
Design And Construction Of Continuous Flight Auger Piles
Chapter 4: Construction Techniques And Materials
This chapter provides details of the construction techniques, materials, and recommended practice for the construction of CFA piles for transportation projects. The guide specification included in Chapter 8 of this manual is a performance-based specification that allows the contractor to select the equipment, materials, and techniques to install a pile to provide the foundation capacity required for the job. This chapter is thus written with the performance-based specification in mind. Many types of equipment for installing CFA piles are presented, including some that are proprietary, which have been difficult to fit into the traditional design-bid-build project delivery method. Using a performance-based specification will allow for these systems to be considered more often because the contractor is bidding to provide the pile that meets the performance specifications at the least cost, regardless of pile type.
4.2 Construction Equipment
This chapter describes various types of drilling equipment used for the construction of CFA piles and DD piles, and provides details of tools, grouting/concrete equipment. Advantages and limitations of various types of drilling rigs are discussed, particularly with respect to torque capabilities.
4.2.1 Drilling Rigs
A typical crane-mounted drill rig for CFA piles is illustrated in Figures 4.1 and 4.2. The continuous-flight hollow-stem auger is driven by a hydraulic gearbox located at the top of the auger. The only downward force (referred to as "crowd" or "downcrowd" among contractors) that can be applied by such a system is via the total weight of the gearbox, augers above ground, and any soil on the auger flights. Typical crowd values are in the range between 13 and 45 kN (3,000 and 10,000 lbs) and typically are around 22 kN (5,000 lbs).
The pile leads, which are similar to those used by driven pile rigs, serve to provide a guide for the auger. The leads may hang freely from the crane boom or be fixed to the crane. The torque arm, or stabilizing arm, holds the leads at a point near the ground surface and absorbs torque from the drilling operation. A hydraulic spotter may be used to install batter piles.
The top of the auger is held into the leads via the attached gearbox. The swivel at the top of the auger provides for freedom of rotation of the auger without disconnecting the grout/concrete line, so that grout placement can begin immediately after completion of the drilling. The auger is hollow to act as a conduit for grout/concrete placement. Grout, rather than concrete, is more common with this type of rig, and is delivered from a piston pump through the grout hose, as shown in Figure 4.1.
Figure 4.1: Typical Crane-Mounted CFA Rig
Source: Deep Foundations Institute
In the current U.S. practice, torque capacities for crane-attached rigs range from 20 to 120 kN-m (15,000 to 90,000 ft-lbs); rigs in the range of 27 to 50 kN-m (20,000 to 36,000 ft-lbs) are most common for private commercial work. Auger diameters of up to 450 mm (18 in.) are most common with these rigs, although diameters of 600 mm (24 in.) are possible with crane-mounted rigs at the higher end of the range of torque.
For CFA piles used on transportation projects, a suggested minimum torque capacity of 40 kN-m (30,000 ft-lbs) should be required. This value may not be sufficient to avoid soil mining for some pile lengths and diameters and soil conditions. Under a performance-based project delivery method, the contractor will have the responsibility of selecting the appropriate rig torque capacity for the project requirements to ensure that piles are installed properly and without soil mining. The minimum torque capacity recommended above may be relaxed for light duty projects, including small soundwall piles or low headroom conditions.
Figure 4.2: Photo of Crane-Attached CFA System
A diagram of a special CFA pile rig adapted to function in low headroom conditions is provided in Figure 4.3. These special rigs avoid using a crane mast and utilize segmental auger sections to achieve the low headroom capability (Figure 4.4). The torque capacity and crowd for such rigs are limited to about 28 kN-m (21,000 ft-lbs) and 13 kN (3,000 lb), respectively. Because of these limitations, the low headroom equipment should only be used in the most favorable soil conditions described in Chapter 2, for which minimal risk of soil mining exists.
Figure 4.3: Low Headroom CFA Pile Rig
Source: Deep Foundations Institute
Figure 4.4: Low Headroom Rig with Segmental Augers
Hydraulic rigs are common in European practice and are readily available in the United States. These rigs typically have torque capacities in the range of 90 to 400 kN-m (66,000 to 300,000 ft-lbs) and can apply a crowd of up to 270 kN (60,000 lb). The rigs shown in Figures 4.5 and 4.6 are typical examples. In European practice with this type of equipment, pile diameters ranging from 450 mm to 1,200 mm (18 to 48 in.) are possible, with a range of 600 to 900 mm (24 to 36 in.) being most common. Lengths are typically less than 28 m (90 ft), although somewhat longer piles can be installed with the Kelly-bar extensions, as shown on the rig in Figure 4.6.
Figure 4.5: Hydraulic Rig Drilling on M25 Motorway in England
Source: Cementation Foundation Skanska
The hydraulic pressure used to drive the auger can readily be measured and provides an indication of applied torque or downward force. Thus, these rigs lend themselves readily to computer monitoring and control. Besides the more sophisticated built-in controls, the high torque and downward crowd forces offer advantages over conventional crane attachments in the ability to drill larger piles and control the tendency for soil mining. The fully hydraulic rigs are often used in the United States for installation of DD piles, because of the need for downcrowd and greater torque for installation of DD piles. Compared to crane-mounted rigs, the most significant disadvantages of the hydraulic rigs are the high cost of the equipment and the greater weight of the rig. The heavy rig weight can be a problem on some sites because of the need for a more stable working platform than a crane rig may require. Additional significant disadvantages include slower drilling rates and a lack of "reach", which requires that the entire rig be moved from pile to pile. These two issues can lead to lower production rates than with a crane rig.
Figure 4.6: Soilmec Hydraulic CFA Rig with Kelly-Bar Extension
4.2.2 Augers and Drilling Tools
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 mining of the soil around the pile.
Figure 4.7: Augers for Different Soil Conditions: Auger for CFA Piles in Sandy Soil (Top) and Augers for DD Piles (Bottom)
Figure 4.8: Auger for Use in Clay, with Auger Cleaner Attachment
The base of the auger is usually a double start, with two cutting faces merging into a single flight auger a short distance above the tip. The double start cutting head helps keep vertical alignment better than a single cutting face, but can tend to pack with clay where the two flights merge. The cutting teeth on the base of the auger may utilize hardened points for drilling weak rock (Figures 4.9 and 4.10).
Figure 4.9: Cutter Heads for Hard Material (left) and Soil (right)
Figure 4.10: Hardened Cutting Head
The augers and tools in Figures 4.11 through 4.15 are for use with full or partial displacement piles in which all or a portion of the soil is displaced laterally rather than excavated. These systems have advantages in many circumstances over conventional CFA piles as described in previous chapters.
Virtually all of these displacement-drilling systems are proprietary in some form or another. Some types of DD piles are designed to allow placement of reinforcement inside the tool prior to concreting. Some have a sacrificial shoe that is left in place on the bearing surface and may help provide improved end-bearing capacity and result in less chances of a soft toe condition.
The common characteristics of DD piles include greater requirements for torque and downforce compared to conventional CFA tools that do not displace soil. The depth to which a pile can be installed is limited by the capability of the rig. The greater torque demand due to rig capabilities for DD piles can be more significant for limiting pile depth than for conventional CFA piles. At the same rig capacity, partial displacement systems can generally penetrate more deeply than full displacement systems because there is some opportunity to drill through dense soil layers. The level of soil removal for partial displacement systems can vary widely depending on the rig operator controlling the rate of penetration.
The DeWaal displacement pile, which is installed in the United States by the Morris-Shea Bridge Co. of Alabama, utilizes a short section of screw auger below the soil displacement bulge in the drill pipe (Figure 4.11). This pile type uses a sacrificial shoe, which is usually knocked out with a full-length center bar, and grout, which is placed by gravity through the center of the pipe.
Figure 4.11: DeWaal Drilled Displacement Pile
Source: Prof. W. Van Impe of Ghent University, Belgium
Another term used for DD piles is screw piles. The Omega pile, which is an European pile type, is an example of a screw pile. This system uses a short, tapered screw section leading into the displacement bulge (Figure 4.12). A small reinforcement cage is placed through the hollow auger prior to concrete placement. Omega piles are installed in the U.S. market by L.G. Barcus & Sons.
Figure 4.12: Omega Screw Pile
Source: Prof. W. Van Impe of Ghent University, Belgium
The Fundex screw pile is one of the oldest types of screw piles in use. American Pile Driving, Inc., of California, is the U.S. representative for this technology. Fundex piles are installed with an over-sized sacrificial shoe and a full-length cage that is placed in advance of grout/concrete placement. Concrete is placed by gravity through the hollow pipe. The pipe is oscillated as it is removed, and the oversized shoe on the bottom of the pipe is intended to provide a rough texture to the hole and thus to the surface of the pile.
Figure 4.13: Fundex Screw Pile
Source: Prof. W. Van Impe of Ghent University, Belgium
Berkel and Company Contractors, Inc. of Kansas, has developed their own proprietary tools for installing full and partial displacement piles. The full displacement pile in Figure 4.14a is similar to others in outward appearance, but uses pressure grouting to construct the pile with reinforcement placed after completion of grouting. The partial displacement pile in Figure 4.14b is intended to provide partial displacement of soil but still allowing some soil removal via the flights on the enlarged auger above the short screw section at the bottom. Partial displacement techniques are intended to allow pile penetration to depths beyond those possible with full displacement piles, because the removal of some soil can allow easier penetration. The full and partial displacement piles shown in Figure 4.15 are manufactured by Bauer Maschinen of Germany, and are sold to multiple contractors in the U.S. using Bauer equipment.
Figure 4.14: Drilled Displacement Piles
(a) Full Displacement Pile
(b) Partial Displacement Pile
Figure 4.15: Additional Drilled Displacement Piles
(a) Full Displacement Augers
(b) Partial Displacement Auger
Source: Bauer Maschinen
The double rotary system represents another class of augering equipment (Figures 4.16 through 4.18). These rigs include a full-length casing, which is advanced simultaneously with the auger, generally by rotating the casing in the opposite direction of the auger. This technique is especially useful for constructing secant pile walls using CFA piles, as the fully cased system provides stability for the hole, allows rapid drilling without the need for drilling fluid, and increases the verticality of the piles. The casing makes the system quite stiff, and the casing itself acts as a type of core barrel for cutting through hard materials, including the secondary concrete piles.
Figure 4.16: Double Rotary Cased CFA Piles
(a) Movable Rotary Drive (b) Fixed Twin Drive
The double rotary can have independent movable rotary drives (Figure 4.16a) or fixed twin drives (Figure 4.16b and 4.17). The movable drive system works more like a conventional drilled shaft in that the two drive motors can move independent of each other, allowing the auger to be removed from the casing while leaving a cased hole. The reinforcement and concrete is then placed into the cased hole as with a cased drilled shaft, and the rig reattaches to the casing and withdraws the casing. This system has the advantage of pre-placement of the reinforcement prior to concrete placement, like a drilled shaft. If the pile is terminated in a water-bearing zone, it is necessary to add water or drilling fluid to stabilize the base of the cased excavation and then place concrete using a tremie. Double rotary cased CFA drilling systems are manufactured and sold in the U.S. by Bauer, Delmag, and Soilmec.
Figure 4.17: Double Rotary Fixed Drive System
(a) Cutting Head (b) Concrete Placement (c) Cuttings Discharge
With the fixed twin drives (Figures 4.16b and 4.17), the two motors are not capable of independent operation. The auger and casing are advanced together and removed together while concrete is pumped through the center of the auger, similar to a conventional CFA pile. The control of the rate of rotation of the auger relative to the casing is important; the auger must rotate faster so that the proper amount of soil is flighted to the top, which allows the auger to advance without hanging up inside the casing, and to maintain soil on the augers to stabilize the base of the excavation. The flighted soil is discharged through a discharge chute located at the drive head (see Figure 4.16b). In some systems, (Figure 4.17b), the auger/casing system is then moved to a location for depositing spoils after concrete placement; then the auger is reversed and the soil is discharged from the bottom of the casing (Figure 4.17c). Reinforcement is then placed into the fluid concrete.
Some rigs are equipped with a Kelly-bar extension, which allows the auger to penetrate deeper than the casing and extend the hole beyond as a CFA pile (Figure 4.18). This system may be advantageous when drilling to form a wall through a granular material by extending the piles to depths beyond the capability of the casing. The Kelly-bar extension also allows the contractor installing the casing slightly behind the leading edge of the auger and thereby permits the cutting of relief prior to forcing the casing forward.
Figure 4.18: Double Rotary System with Kelly-Bar Extension
Source: Bauer Maschinen
4.2.3 Equipment for Concrete/Grout and Reinforcement Placement
The concrete or grout is normally obtained from a ready-mix plant and delivered to the site by trucks. For most CFA pile construction, the concrete or grout is pumped under pressure through pump lines to the top of the auger string and through the auger to provide positive pressure at the point of discharge at the base of the auger. Some types of DD piles are designed for placement of concrete into the top of the large diameter auger tool without pressure (see Figures 4.11 through 4.13). General reference in this section will be to the more common construction of CFA piles or partial DD piles. Most CFA piles in the United States are currently constructed using sand-cement grout, while most CFA piles in Europe are constructed using concrete of small aggregate. In general, the practices and equipment used in the United States and Europe are similar. The terms "grout" and "concrete" will be used interchangeably in this section, unless specific differences are referenced.
188.8.131.52 Auger Plug
The grout discharge point should be located at the bottom of the auger below the cutting teeth (e.g., Figures 2.7 and 4.10). In most cases, this grout discharge point is oriented away from the leading edge of the cutter head so that high ground pressures do not press against the plug during drilling. Some augers are equipped with a centered plug so that a single bar can be placed through the center of the auger string prior to concrete placement. Ordinarily, the pressure of the grout blows out the plug. A center plug is usually made of a steel shoe or plug of some other hard material. The normal off-centered plug is most often cork or plastic.
Problems with the plug (or "bung" as it sometimes is called by contractors) can occur if the plug does not come out or the plug comes out prematurely and the line fills with soil. In either case, pumping grout through the line is no longer possible and the pile must be abandoned and re-drilled. If the pile needs to be abandoned, the contractor must reverse the direction of rotation and remove the auger while leaving soil behind to keep the hole from collapsing. After correcting the problem (e.g., by clearing the discharge point), the pile is re-drilled. The pile can be re-drilled a short distance away from the first location, as long as the pile layout allows doing so. Alternatively, the pile can be re-drilled in the same location, although it is likely that some adverse effect on the subsequent pile performance will occur due to soil disturbance. Depending upon conditions, the re-drilled pile may be acceptable as is, may require to be deepened, or additional piles may be required to compensate.
Some contractors have successfully used an auger having no plug in the bit by pumping compressed air through the auger during the drilling process. The air pressure can be useful in some stiff clays or other difficult drilling conditions in helping prevent the soil from adhering to the auger and breaking up the soil as it is cut. This technique is used successfully in some parts of the country having stiff cohesive soils, such as Texas and northern Georgia. Soil conditions that are more susceptible to mining, such as relatively clean sands, may not be suitable for the use of compressed air during pile installation. One case of using compressed air to install 600-mm (24-in.) diameter piles in northeast Florida showed a significantly lower side-shear capacity for a test pile installed using compressed air when compared to a test pile installed without using compressed air. The contractor used compressed air during installation of a 16.8-m (55-ft) long, 600-mm (24-in.) diameter piles. Instrumented Statnamic™ load tests were performed on two piles installed less than 3 m (10 ft) apart, one installed with compressed air and one without. The unit side-shear resistance measured in the pile installed using compressed air was about half of that measured in the pile installed without compressed air. The soils consisted of relatively clean, poorly-graded fine sands, with the piles tipped into a loose to firm clayey sand layer. As with many techniques, the use of compressed air should take into consideration the potential impact on pile capacity. With a performance-based specification, a contractor could choose to use air, provided that a test pile is successfully load-tested and the pile meets the required performance.
184.108.40.206 Pumping Equipment
The grout pumping equipment should be a positive displacement pump capable of developing pressures at the pump of up to 2.4 MPa (350 psi). The typical grout pump operates with reciprocating pistons, each delivering around 10 to 30 liters per stroke (0.4 to 1 ft3 per stroke). The size and capacity of the pump must be suitable for the size of the pile being constructed. Several examples of pumps are shown in Figure 4.19. Most commonly, the pump is located close to the piling rig with grout lines running to the rig and an operator manning the pump (Figures 4.19a and 4.19b). The grout line is typically around 63 to 100 mm (2.5 to 4 in.) in diameter and can extend 30 to 60 m (100 to 200 ft) from the pump. Some contractors have the pump mounted directly on the rig, which allows pumping to be controlled by the operator (Figure 4.19c). The pumping operation shown in Figure 4.19d includes a rotating drum for holding a full truck load of grout/concrete [approximately 8 m3 (10 yd3)] on-site so that a ready-mix concrete truck can discharge into the holding drum and return to the concrete plant.
It is important that the pump does not deliver an excessive grout volume with each stroke, which would cause the operator to have difficulty controlling the pile grouting operation. In general, a pump should deliver a volume per stroke that corresponds to around 100 mm (4 in.) of pile length or less. If the volume per stroke is too large in relation to the pile size, the operator cannot maintain a steady progress of pumping and cannot construct a uniform pile. If the volume per stroke is too small in relation to the pile size, the operation is slow and inefficient. A related problem could also be when there is a tendency to withdraw the auger too rapidly in relation to the grout volume supplied.
In order to verify the volume and pressure of grout delivered to the pile, it is necessary that instrumentation be provided to monitor the grouting operations. Two methods are available for real-time monitoring of grout/concrete volume during installation: stroke count and in-line flowmeter. The simplest of these two methods is to count the strokes from the pump, which can be automated by using the pressure sensor or a proximity switch. In this method, the cumulative volume is determined by multiplying the number of strokes by an estimated volume of grout/concrete delivered per stroke. Volume estimation by counting strokes suffers from the inaccuracy of assuming a constant volume per stroke, and possibly due to variations in the efficiency of the ball-valves sealing off against the seats. Sometimes, the pump strokes are inconsistent and the volume delivered per stroke can vary. The automated stroke counter can miss strokes or count erratic behavior as multiple strokes. The volume pumped per stroke can also vary with the pressure against which the pump is operating, and can vary with time for other reasons. With the introduction of modern sensors and monitoring equipment, stroke counting is now considered a poor quality control practice.
Figure 4.19: Typical Concrete/Grout Pumps
(a) Trucks Discharging into Pump (b) Close up View of Grout Pump
(c) Concrete Pump Mounted on Rig (d) Pump with Rotating Drum On-Site
The second and preferred method to monitor volume is to use an in-line magnetic flowmeter, shown in Figure 4.20. This device provides a more accurate and reliable indication of the actual volume delivered. Flowmeters work by placing a magnetic field around a tube such that the conductive medium moving through the tube induces a voltage in the medium. The voltage of the medium is proportional to the average flow velocity. The flowmeter thus makes a voltage measurement that is proportional to the average velocity of the grout flowing through it; this average velocity can be converted to volume using the known cross-sectional area of flow. The flowmeter is sensitive only to conductivity of the grout and is independent of density or viscosity. The interior of the tube is generally lined with a ceramic material for durability.
Figure 4.20: In-Line Flowmeter
A pressure sensor should be mounted in-line to provide a real-time monitor of the pressure being delivered to the auger and to ensure that positive grout pressure is maintained in the hole as it is being filled. The best place for this sensor would obviously be at the base of the auger, as shown in Figure 4.21. This instrument provides a measure of grout pressure inside the auger about 1 m (3 ft) above the tip. The system requires an interior mount and a cable extending up through the auger and through a slide ring body at the swivel atop the auger. At present, the "in-auger"pressure sensors have been difficult to maintain and therefore are not widely used.
A more common location for measuring pressure is in the line just above the swivel on top of the auger string. If the line is completely filled, the pressure at the auger tip should differ by the difference in head from the top to bottom, minus a small loss due to flow in the lines. Pressure measurements in the line farther away from the auger can be affected by losses between the measurement location and the auger, and thus it is preferred that the pressure measurements be made as near to the auger as possible.
The minimum pressure during all grouting operations should be displayed in real-time for operator control and inspector observation. This information can be used to immediately correct areas of the pile where the pressure has dropped due to grout contamination with soil or other problems. These readings should also be recorded for quality control documentation.
220.127.116.11 Finishing the Top of the Pile
After the grout placement is complete and the auger is withdrawn, the workers must finish the top of the pile prior to reinforcement placement. A recommended procedure is to place a small form or casing around the top of the pile to prevent fall-in from surrounding soil. Sheet metal ductwork or prefabricated column forms are often used for this purpose, as shown in Figure 4.22. While not all contractors use this technique in private contracts, the use of the top form to prevent fall-in is required in public projects. Besides the use of the top form, it is also necessary to scoop the grout or shovel out the contaminated uppermost portion of grout. The workman in Figure 4.22c is using a folding circular screen to remove soil contamination from the fluid grout.
Figure 4.21: Sensor for Concrete Pressure at Auger Tip
Source: Bauer Maschinen
4.3 Construction Materials
The component materials of a CFA pile consist of grout/concrete and reinforcing steel.
4.3.1 Grout and Concrete
Both grout and concrete have been successfully used for the construction of CFA piles. Concrete used for CFA piles is very similar to the concrete used for wet-hole placement in drilled shafts. Grout used for CFA piles is similar to concrete except that the grout mix contains only sand, not coarse aggregate. While the grout used for pressure-grouting applications in some types of micropiles and other grouting applications is often a mixture of only cement and water; such thin, fluid grouts are not used for CFA piles. Both grout and concrete mixes typically contain a mixture of Portland cement, fly ash, water, aggregate (fine aggregate only for grout) and admixtures. Water reducers are typically added to concrete mixes, and fluidifiers have been developed to overcome problems associated with grout placement. Retarders are often added to grout or concrete mixes to increase grout flowability or extend the slump loss time of concrete. Regardless of whether grout or concrete is used, the mix must be made such that all solids remain in suspension without excessive bleed-water. Additionally, the mix must be capable of: (1) being pumped without difficulty; (2) penetrating and filling open voids in the adjacent soil; and (3) allowing for insertion of the steel reinforcement.
Figure 4.22: Completion of Pile Top Prior to Installation of Reinforcement
(a) Spoil Removal (b) Clearing Top of Pile and Form Placement
(c) and (d) Contaminated Grout Removal
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).
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.
DD piles may induce excess pore water pressures in the surrounding soil that could result in water intrusion into a newly constructed pile as the excess pore pressure dissipates. Grout may be more susceptible to this effect than concrete. Special fluidifiers are often added to grout mixes to counteract these effects, as will be described in Section 4.3.6.
A grout mix will have a slightly lower elastic modulus than a comparable concrete mix at the same compressive strength. While a lower elastic modulus may be of concern in structural applications where deflections control the design, it would typically have a relatively small effect on the load/settlement characteristics of CFA piles. The elastic shortening of a pile is proportional to the modulus of the grout/concrete pile. As CFA piles are relatively short, the load/settlement characteristics are predominantly controlled by the interaction between the pile and surrounding soil, regardless of whether grout or concrete is used.
Figure 4.23: Sand-Cement Grout Mixes
A study at the University of Houston (O'Neill et al., 1999) compared the chemical resistance of auger grout (i.e., grout steel in CFA piles) and conventional Portland cement concrete to solutions of acid, sodium sulfate (Na2SO4), and sodium chloride (NaCl). The researchers tested samples in chemical solutions over a period of two years and determined the following:
- The grout gained over 3% weight in a solution with 2% of sodium over 2 years. By contrast, the concrete gained about 1% in 500 days.
- The sulfate solutions produced a 2% weight loss in a period of 180 to 270 days in the auger grout. The concrete had a weight loss of 0.2 to 0.3% in 500 days, indicating a faster degradation of auger grout in a sulfate environment.
- Leaching of calcium in sulfates was about five times higher in auger grout than in concrete.
- Sulfates produced a slightly increased degradation in pulse velocity in auger grout compared to cement concrete.
- There was a notable decrease in compressive strength in auger grouts immersed in hydrochloric acid (pH = 2 to 4) or sulfate solutions. Sulfuric acid (H2SO4) at pH = 4 and 5 parts per million (ppm) of sulfate had minimal effect on the grout and concrete.
Based on this study, it can be expected that auger grouts will not perform as well as normal Portland cement concrete in aggressive soil environments that contain sulfates and acids.
The following sections describe the components of grout and concrete used for CFA piles.
Ordinary Type I or Type I/II Portland cement can normally be used in grout/concrete for CFA piles. The cement should meet the requirements of ASTM C 150 or AASHTO M85. Special sulfate resistant cements should be considered in environments where the sulfate content of the geo-material or groundwater is extremely high.
18.104.22.168 Pozzolanic Additives
Both grout and concrete mixes may contain pozzolanic additives. The most commonly used is fly ash (ASTM C 618-94 1995); however, finely ground silica fume and blast furnace slag (ASTM 989-94 1995) can also be used. The use of pozzolanic additives results in lower permeability of the hardened concrete and tends to retard the set time of the cement paste, thereby increasing the time that the grout/concrete remains workable. As a consequence of providing a more workable mix, the use of fly ash, silica fume, and/or slag will probably severely retard the early strength gain of the grout mix, typically until about 10 to 14 days of age. If these additives are to be used in the mix, the submitted mix design should include information on strength development vs. time so that the design engineer is informed of the delay in strength gain corresponding to the mix and make any adjustments, if necessary.
Fly ash is now widely available in most areas of the United States as a by-product of burning coals. ACI 232.2R-96 of the American Concrete Institute (ACI, 2006) provides an excellent overview of the use of fly ash in concrete. ASTM C 618 categorizes ash by chemical composition. Class C and Class F ashes are most commonly used in concrete and grout mixes. As a group, these ashes tend to show different performance characteristics. However, there are important differences in fly ash from different sources and the performance of a fly ash is not determined solely by its classification as either Class C or Class F. For instance, problems have been reported in some cases when power companies turn to scrubber systems to remove sulfur dioxide from stack gasses. This occurs when fly ashes are mixed with scrubber products and contain free lime and calcium sulfates or sulfites (see p. 95 in Mindess et al., 2003). The mix design for CFA piles should be developed specifically for a project site using locally available materials.
Water used for mixing the grout/concrete should be potable (free of organic contamination and deleterious material) and should have low chloride and sulfate contents.
All aggregate should meet the appropriate specifications. Some of the relevant ASTM specifications for aggregate are: ASTM C 33-93, Specification for Concrete Aggregate; ASTM C 87-90, Test for Effect of Organic Impurities in Fine Aggregate on Strength of Mortar; and ASTM C 227-90, ASTM C 289-94, ASTM C 295-90, ASTM C 586-92, all of which address tests that measure the alkali susceptibility of aggregates.
In general, rounded gravel is strongly preferred over crushed stone due to the benefits in terms of workability of the mix for pumping and placement. Aggregate gradation will depend upon the specific mix design requirements. Concrete mixes having extremely high workability will tend to require a greater ratio of fine to coarse aggregate to minimize the tendency for segregation and bleeding.
22.214.171.124 Fluidifiers for Grout and Water Reducing Admixtures for Concrete
Both low-range and high-range (i.e., superplasticizer) water reducing admixtures have routinely been used in concrete mix designs for drilled shafts. ASTM C 494 is a performance specification that classifies an admixture as water-reducing if it reduces the water requirements by 5%. Thus both low-range and high-range water reducers are specified by ASTM C 494. High-range water reducers may also be conveniently specified by requiring that the performance specification ASTM C 1017 also be met, as this specification requires that an increase in slump of 90 mm (3.5 in.) or greater be obtained. For low-range water reducers, admixture Type D (per ASTM C 494) is preferred for piles over admixture Type A to provide some retarding properties and reduce slump loss. Similarly, for high-range water reducers (superplasticizers), admixture Type G is often preferred over admixture Type F to reduce slump loss, but the newer polycarboxylate-based superplasticizers are designed to maintain a high slump for extended periods.
Low-range water reducers can be used to obtain water/cement ratios in the range of 0.40 to 0.45, and can consist of lignosulfonates, hydroxylated carboxylic acids, and similar compounds (see ASTM C 494). High-range water reducers can be used to obtain water/cement ratios of 0.3 or lower while maintaining a high slump (see ASTM C1017). Many of the older naphthalene-based superplasticizers had a tendency for rapid slump loss, could even result in a flash set, and thus were very risky to use for cast-in-place deep foundations. However, many of the modern superplasticizer products are polycarboxylate compounds that lose their effectiveness much slower and are very useful for drilled shafts and CFA piles. These products also act as a mild retarder. It is important to note that high-slump concrete mixes must be designed carefully to avoid problems of segregation and bleeding. Water reducers can be very effective at reducing the water/cement ratio for a given workability requirement and thus reducing the tendency for segregation and bleeding in the mix.
Grout fluidifiers have been developed for intrusion grout mixtures to offset the effects of bleeding, reduce the water/cement ratio while providing a desired consistency, and retard stiffening so that handling times may be extended. A grout fluidifier may be specified by meeting the requirements of ASTM C937. Grout fluidifiers typically contain a water reducing admixture, a suspending agent, aluminum powder, and a chemical buffer to assure timed reaction of the aluminum powder with the alkalies in the Portland cement.
Retarders [described in ASTM C494-92 (ASTM, 2006)] consist of lignosulfonic acids, hydroxycarboxylic acids, sugars, and phosphates. Many of these possess water-reducing capabilities and can be classified as water-reducing, set-retarding admixtures [Type D in ASTM C494 (ASTM, 2006)]. Retarding admixtures may be needed in the grout/concrete mix when it is placed during periods of high temperature [> 20° C (68° F)] to reduce the slump loss in the period during which the grout/concrete is placed. Some types of retarders slow down the rate of early hydration of cement, but hydration proceeds normally after the effect is overcome. Some inorganic retarders are more complex and can form coatings around the cement particles that severely reduce the rate of reaction. Thus, retarders can slow the rate of early strength development. The strength should approach that of unretarded concrete within eight days, unless an overdose has been used. Overdosing the mix with retarder can prevent set entirely.
126.96.36.199 Air Entraining Agents
Air entraining agents (ASTM C 260-94, 1995) can be used when deterioration of the grout/concrete by freeze-thaw action is possible. Entrained air will also improve workability and pumpability and reduce bleeding. However, it can produce a slightly more permeable grout/concrete and thus be more susceptible to deterioration due to a chemical attack (e.g., chlorides). When air is added, about 5% is needed to improve pumpability. Because air tends to be lost during the mixing, pumping, and placement processes, much of the entrained air is likely to be lost by diffusion by the time the grout/concrete begins to set.
188.8.131.52 Sampling and Testing
Representative samples of grout and concrete mixes must be obtained at the project site for QA/QC testing, as described in greater detail in Chapter 7. The three parameters most typically measured are temperature, workability, and strength. Workability is measured using slump testing for concrete and flow cone testing for grout. Strength testing is performed in a laboratory after curing samples from the field.
Typical strength requirements for CFA piles are 27.6 to 34.5 MPa (4,000 to 5,000 psi). Strength testing of concrete utilizes conventional 150-mm (6-in.) diameter cylinders. For the sand-cement grout often used with CFA piles, some engineers use small cylinders 50 or 75 mm (2 or 3 in.) in diameter, but most use 50-mm (2-in.) cubes. There is not a consensus at present on which method is preferred, but the cubes are easier to prepare and transport. The compressive strength of properly prepared and tested cubes are slightly higher than that of cylinders with a height to diameter ratio of 2, so the strength requirement from tests on cubes is typically 10% higher than that of cylinders.
The ideal location and time to obtain samples for testing would be at the point of discharge into the soil and after the mix has been pumped through the lines and the auger, as the properties (particularly workability) can be altered by pumping extended distances, especially in hot weather. However, this location is generally not possible, therefore, samples are typically obtained from the discharge location into the pump hopper. Workability and temperature should be checked on every truck as a means of verifying consistency of the mix. Because the grout/concrete must be placed immediately when the auger achieves the tip elevation, the sampling and inspection must be expeditious.
Slump ranges for concrete for CFA piles should typically be 200 mm ± 25 mm (8 in. ± 1 in.), similar to that used for drilled shafts constructed using the wet method. Workability of grout is tested using a flow cone instead of the conventional slump test used for concrete. Standards ASTM C939 and U. S. Army Corps of Engineers CRD-C 611-94 provide specifications for flow cone testing in which fluid consistency is described according to an efflux time per standard volume. Because the grout mixes used for CFA piles are typically too thick to flow effectively from the standard 12 mm (0.5 in.) outlet specified in these standards, it is common practice to modify the above specs to provide a 19 mm (0.75 in.) opening. This modification can be made by taking out the removable orifice that extends out the bottom of the Corps of Engineers device to leave a 19 mm (0.75 in.) opening or to cut the flow cone specified in the ASTM standard to modify the outlet diameter. Grouts suitable for CFA pile construction typically have a fluid consistency represented by an efflux time of 10 to 25 seconds, when tested in accordance with the modifications described above.
Most standard mix designs will maintain workability for a period of up to 2 hours without any additional retarding admixtures (other than the typical grout fluidifier), if agitated continuously in the ready-mix truck. Flow cone or slump tests should be performed on site at the time of placement to ensure grout/concrete workability over time. If a project has an unusual concern for a lengthy time for rebar cage placement or a great depth, additional retarding admixtures may be used to extend the slump or flow life. Flow cone or slump tests at the time corresponding to rebar cage placement may be used to evaluate the workability associated with the mix at this critical time.
Grout or concrete should not be placed when its temperature falls below 4°C (40° F) or exceeds 38°C (100° F), unless approved procedures for cold or hot weather grouting are followed.
4.3.2 Reinforcing Steel
184.108.40.206 Reinforcing Steel Materials
Reinforcing bars for CFA piles typically consist of ASTM A615 Grade 60 steel, the same as those used for drilled shaft construction. Occasionally, CFA piles may be reinforced with high-strength threaded bars meeting ASTM A722 (1,035 or 1,100 MPa [150 or 160 ksi]). The high-strength bars are normally used where large tensile loads are to be supported. Transverse steel may consist of either circular ties or spirals. Steel pipe may be used in cases where large bending stresses may occur, such as in a wall. Steel pipe used in CFA piles is steel ASTM A572, Grade 50 having a nominal minimal yield strength of 350 MPa (50 ksi) or ASTM A252, Grade 2, nominal minimal yield strength of 420 MPa (60 ksi or Grade 60). In the case of steel pipes, the CFA pile is really designed as a concrete- or grout-filled steel element rather than a reinforced concrete member; guidelines suitable for micropile design would be appropriate in this case.
220.127.116.11 Reinforcing Cage/Section
Reinforcing cages should be fabricated so that lifting and handling does not cause permanent distortion or racking. For this reason, it is important that wire ties be used on all longitudinal bars at every tie or spiral. Welding is only permitted if weldable reinforcement is specified (see Figure 4.24). The use of weldable reinforcement is rare in U.S. practice, but can assist in handling the cage with a minimum of distortion. Spliced steel cages and/or coupled threaded bars are often necessary to install reinforcement in low headroom applications.
Where bending stresses are potentially high as in the case for a pile wall or slope stabilization scheme, it is possible to construct CFA piles using structural steel sections for reinforcement. Figure 4.25 illustrates the use of a steel pipe section within a CFA pile to provide flexural strength in a tangent pile wall application.
Reinforcement cages are normally specified with 75 mm (3 in.) clear cover to the outside of the pile. Plastic or cementitious spacers should be placed at intervals of no more than about 3 m (10 ft) along the cage to provide cover. Spacers made of steel should not be permitted as they may greatly accelerate corrosion of the reinforcing steel, particularly above the groundwater table. Centering guides made of steel, such as a wire "basket" or "football" tied at the base of single-rod reinforcement may be used. A reinforcing cage may be tied together at the bottom to create a "point" to facilitate installation into the pile. If CFA piles are used on a batter, special provisions may be necessary to maintain cover. For the project illustrated in Figure 4.26, a continuous PVC pipe was used on the bottom side of the cage to maintain cover and act as a "runner" to slide the reinforcement cage into position within the grouted pile.
Figure 4.24: Machine-Welded Reinforcement Cage on Project Site in Germany
Figure 4.25: Use of Steel Pipe to Reinforce a CFA Pile for a Wall
Figure 4.26: Installation of Reinforcement Cage into Battered CFA Pile
Where a single bar is used for tensile reinforcement, centralizers are used at spacing of no more than 3 m (10 ft). Some rigs are equipped to install a center bar through the hollow auger prior to placement of grout/concrete. These bars cannot be used with a centralizer because the centralizer cannot fit through the auger stem while attached to the bar. Splices may sometimes be used with CFA piles, but it is better to avoid the use of splices. Splices are common for piles supporting tensile forces or for piles reinforced with a single full-length center bar. Mechanical splices are preferred in such cases, and high-strength threaded bars are convenient for this purpose.
4.4 Summary Of Recommended Practices
This chapter describes equipment, techniques, and materials used to construct CFA piles. A wide variety of techniques and equipment have been used to construct these piles. Several parameters are summarized below that are key components of good quality construction. The specifications section of this document (Chapter 8) provides detailed guidelines.
- 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.
- Drilling. In order to avoid excessive flighting and to construct piles of consistent quality and axial capacity, target penetration rates must be established and maintained during drilling of CFA piles. It is essential that this parameter be controlled by the rig operator and monitored for verification. Automated monitoring systems must be used to provide direct feedback to the operator and verification of performance. Details of monitoring systems will be described in Chapter 7. It is essential that the installation method used for construction of production piles be consistent with that used for construction of load test (control) piles.
- Cementitious Materials. Either grout or concrete may be used for construction of CFA piles. Each has relative advantages under different circumstances. In general it is recommended that: (1) the specifications for grout/concrete materials be performance-based verified using strength tests on either cubes or cylinders; and (2) testing for workability and mix temperature be routinely performed on each truck as a means of monitoring consistency. Mix proportions and characteristics should be established based on test piles or control piles and maintained at a consistent quality throughout the project. Workability of concrete is monitored using slump tests. Workability of grout is monitored using flow cone tests with a modified opening enlarged to 19 mm (0.75 in.). Workability of the mix must be maintained for the entire duration of pile construction, including rebar installation into the pile. Slump or flow cone tests should be performed at times corresponding to rebar cage installation.
- Placement of Grout or Concrete. Placement of grout or concrete through the auger is a critical part of the operation and must be monitored using automated systems to ensure that adequate volumes are pumped at a positive pressure at all times as auger withdrawal is in progress. Slow, steady pulling of the auger at a rate appropriate for the delivery from the pump is essential. Some contractors prefer to use a static pull of the auger and some prefer a very slow rotation in the direction of drilling. It appears that both methods can be used successfully. The auger should never be allowed to turn in place without either drilling or pumping taking place. The systems utilizing automated monitoring of volume and pressure delivered to the pile as a function of auger tip elevation are the most effective to obtain consistent quality and verification. In-line flow meters are the preferred means of monitoring volume of grout/concrete over stroke counters.
- Completion of the Pile Top. It is essential that the contractor continue to deliver the appropriate volume of grout/concrete to the pile when the auger is close to the surface and significant positive pressure can no longer be maintained. The completion of the pile top requires manual work to remove any debris or contaminated grout/concrete near the top of the pile before reinforcement is placed into the fluid grout/concrete. The use of a small form at the pile top extending above grade is recommended to maintain a sound surface. If below-grade cutoff is required, it is necessary to complete the pile to grade and then chip or cut the top down later. It is necessary to flush the grout/concrete to the surface of the working platform to remove any questionable or contaminated material.
- Reinforcement. Installation of reinforcement requires that the grout/concrete mix retain adequate workability for the time necessary to install the cage after removal of the auger and clearing the top of the pile. The mix requirements with respect to this aspect of the work can vary with differing soil conditions, particularly with respect to the tendency of dry sandy soils to rapidly dewater the pile. The mix should be developed to demonstrate that workability is maintained within the slump or flow cone guidelines for the entire duration of time required for drilling and grouting the pile and placing the rebar cage. In addition, other measures such as anti-washout admixtures may be required if soil conditions cause excessive dewatering of the mix after casting that results in rebar installation difficulties. Designers should include reinforcement cages that use: (1) fewer heavy bars instead of many smaller bars; (2) are no longer than the minimum necessary to provide structural capacity and anchorage; and (3) allow the cage installation proceed with minimum difficulty. The contractor should tie the cage to permit handling without permanent distortion.
- Installation Plan. The contractor should submit an installation plan including details of the equipment and methods proposed for the project. Many aspects of the construction work are performance-oriented with respect to the contractor's equipment requirements and methodology. The installation details and monitoring of the installation are key components of verifying that the performance requirements are met. Contractors should be held accountable for developing an installation plan that will achieve the required objective.
- Test Piles and Test Installations. The recommended means of verifying that the installation plan will achieve the project requirements is using a carefully monitored test pile program. The program should consist of pre-production static load tests, production static and/or rapid and/or dynamic load tests, and post-installation integrity tests in sufficient quantities to provide the data necessary to demonstrate that the installed piles meet the load and deflection criteria established in the project plans with an appropriate factor of safety. It is imperative that the demonstrated installation procedure be followed for all production pile installations.