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| FHWA > Engineering > Geotech > Design And Construction Of Continuous Flight Auger Piles > Chapter 7 |
Geotechnical Engineering Circular (GEC) No. 8
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| Soil Type | Rate of Penetration (Revolutions per Auger Pitch) |
|---|---|
| Clay soils | 2 to 3 |
| Cohesionless soils | 1.5 to 2 |
When penetrating mixed soil profiles, the higher rate of penetration (lower revolutions) should control. For example, in a mix of layers of cohesionless and clay soil, the use of the slower penetration rate appropriate for the clay (2 to 3 revolutions per pitch) could result in excessive flighting of the sand strata. For partial displacement drilled piles, the rate of penetration will affect how much relative displacement occurs, and this parameter has a significant effect on axial resistance. For drilled full displacement piles, the rate of penetration is usually dictated by the need to displace the soil.
In the manual control system that is widely used in commercial construction, the auger speed is predetermined by the gearbox setting, the depth of penetration is monitored by direct observation of the top of the auger in the leads, and the rate of penetration is observed using a stopwatch. These data should be documented in the inspector's notes. This approach is not sufficiently accurate for transportation projects and should not be used as the primary means of QA/QC for the drilling process. These manual observations should be made by the inspector during drilling as a check and/or backup to the automated systems.
The recommended system for transportation projects uses a depth encoder and revolution counter to monitor and display the rate of penetration graphically to the operator in units of revolutions per meter (or foot) of penetration (or meters (or feet) of penetration per revolution), and simultaneously records this information for plotting after the pile is complete. This system is most often used with hydraulic fixed mast drilling equipment, in which the operator has control of the crowd on the tool, the torque applied, and the speed of revolution (see Figure 7.1). The cab mounted display and monitoring parameters of the drilling system are required for drilled displacement piles and for CFA piles in non-cohesive soils. CFA piles may be installed without monitoring and control of the drilling phase only in soils that are demonstrated to be non-caving and not subject to flighting (similar to the contraction of drilled shafts in dry, open holes).
When crane-mounted drilling systems are used instead for CFA piles, the operator has no ability to apply crowd to the auger other than the dead weight of the system. A monitoring system typically used on one of these rigs uses a depth encoder and a clock to monitor the rate of penetration which produce a printed record of depth at various time increments to document the results. The speed of auger rotation is controlled via the gearbox and recorded. This system provides documentation of the operation and a simple visual control. This system does not provide the level of control that should be expected for most transportation projects, but may be acceptable in some cases of non-critical foundations such as soundwalls or other systems installed to shallow depths in favorable (non-caving) soil conditions.
Figure 7.1: Operator with Cab Mounted Display Used to Control Drilling
Control of the grouting/concreting phase of construction may be the most important aspect of QA/QC for CFA piles. The obvious objective is that adequate grout or concrete be delivered to the discharge point of the auger at the proper pressure to complete the pile. Poor grouting/concreting can result in a pile that cannot perform as intended in supporting the structure, including both geotechnical and structural failure.
For the operator to have control and documentation of the operation requires that the pressure and volume be monitored as a function of auger depth. In addition, it is desirable to monitor that the auger is extracted in a slow, continuous manner without excessive or reverse rotation. Upon reaching the required tip elevation, the contractor should establish a flow of concrete or grout with minimal lifting of the auger, typically 150 to 300 mm (6 to 12 in.). After the plug is blown, an initial charge of grout or concrete should be pumped before starting the auger lifting process to develop pressure in the grout or concrete at the bottom of the hole. Some of the initial volume of grout will probably push up the auger flights. The volume of grout/concrete delivered to the lowermost 0.9 to 1.8 m (3 to 6 ft) of pile length should be over-supplied by approximately twice the theoretical volume required to fill the pile for that length.
During the lifting process, the operator must control the lift speed of the auger so that the proper volume of concrete is delivered under sufficient pressure. The auger should be pulled smoothly at a steady speed while grout/concrete is continuously pumped under pressure. Some contractors may slowly rotate the auger in the direction of drilling, while some may pull without rotation. To monitor and control this operation, it is important to observe and document the following: (1) position of the auger tip, (2) lifting speed, (3) volume of grout/concrete that is delivered, and (4) pressure with which the grout/concrete is delivered. In the event that the operator pulls the auger too quickly and the grout/concrete pressure drops below allowable levels, a common practice is to immediately re-drill down 1.5-m (5-ft) below the point where the pressure drop occurred and rebuild the pile from that point up. The operator should be able to observe the pressure drop within seconds and allow the re-drilling and grouting to take place almost immediately.
The manual method of monitoring and documenting the grouting/concrete operation involves the following:
The only means of documenting the operation using the above technique is by the inspector manually recording the observations. The rig operator depends on estimating volumes and manually observing the auger withdrawal, and on signals from the pump operator that the pressure and volume delivered are consistent.
In general, the simple manual observation and control system described above is not considered to provide sufficient control for transportation projects. These manual observations can be made by the inspector during grouting/concreting as a check and/or backup to the automated systems. They may be sufficient for non-critical foundations such as soundwalls or other shallow foundations in favorable soil conditions.
The system recommended for transportation projects includes automated monitoring of the auger position; volume of grout/concrete that is delivered; pressure with which it is delivered; and rotation and lifting speed of the auger. Such system should provide the following:
There are several methods for providing each of the above measurements, and a variety of different in-cab display systems. Some contractors use electronic monitoring of pressure pulses along with a calibration of volume per pump stroke to determine volume. With the pumps most commonly used, this system is inferior to an in-line flow meter because of possible missed strokes, variable volume per stroke, and other inconsistencies. The pressure in the line can be monitored at a range of locations. The best location is at the tip of the auger inside the auger itself (see Figure 4.20). Although such a system exists, it is not widely available and requires augers equipped with cabling, sensor cutouts, and a means of transmitting the signal through the swivel. The location of the sensor near the swivel at the top of the line is the next best position. The location of pressure sensors in the line near the pump is least effective because of the potential for losses between the measurement point and the auger.
Hydraulic rigs are typically equipped with pressure sensors in the hydraulic lines (see Figure 7.4), which provide feedback and documentation of the torque and crowd force used during drilling. These parameters can be very useful to monitor rig performance and drilling resistance in the soil, particularly for drilled displacement piles. It is quite possible that future research could develop correlations between such drilling parameters and axial resistance of the completed pile.
Figure 7.2: Depth Encoder Mounted on Crane Boom
Figure 7.3: In-line Flowmeter
Figure 7.4: Pressure Sensors on Hydraulics to Monitor Rig Forces
Figure 7.5 shows a display panel mounted outside of the cab of the rig for observation by the inspector. This allows the inspector to make periodic checks of the data being recorded during pile installation. An example of the documentation of a production pile is illustrated in Figure 7.6. Other systems may present the information differently, but similar information should be presented. The top of the data sheet provides project and pile information, and start and finish times. The leftmost column indicates, in a graphical way, the volume of concrete delivered as a function of depth, having a line indicating the target volume. The pile had an over consumption of concrete of 17% above the theoretical volume, which is comparable to a target value of 15% (15 to 20% is typical for CFA piles). Graphical representation of concrete pressures, forces in the rig (measured hydraulic pressures in psi), and rates of lifting and drilling are also provided. Note that a harder layer appears to have been penetrated at depths of around 46 to 54 ft, as indicated by the higher torque and thrust used in attempting to maintain the rotation and drill rate here. At this location, the rotation and drill rates drop slightly.
Figure 7.5: Display Panel for Observation by Inspector
Figure 7.6: Example Data Sheet from Project
Source: Jean Lutz S.A.
Inspection of the installation of reinforcement and completion of the pile top are not subject to automated monitoring and depend wholly on the observation of the inspector. It is particularly important that the inspector note the point at which grout/concrete appears at the surface relative to the embedment of the tip of the auger. If grout/concrete has pushed far up the auger flights from the tip (more than about 3 m [10 ft]), it may be a sign that the auger has not remained charged with soil. The point at which grout/concrete first appears should be noted and should be relatively consistent from pile to pile. When grout/concrete appears at the surface, it will be no longer possible for the operator to maintain excess positive pressure at the tip because the grout/concrete is now vented to the surface. Therefore, it is particularly important that the volume of flow be consistent to ensure that the auger is not pulled too fast from this point on.
When the auger is removed, it is possible for some soil to spill into the top of the pile and contaminate the grout or concrete. The inspector should observe that the contractor dips out any contamination and finishes the pile with good quality grout or concrete, as shown in Figures 7.7 and 7.8. A small surface casing is normally required to stabilize the top of the hole.
Figure 7.7: Dipping Grout to Remove Contamination
Installation of reinforcement should proceed immediately after the pile top is prepared. Reinforcement should be clean and free of rust or contamination, of the size and dimensions indicated on the plans, and equipped with appropriate centering devices. These are normally plastic or sometimes made of mortar or grout. Centering devices should not be made of metal because of potential corrosion and contact with the rebar cage. Welding of the cage is permitted only if weldable reinforcing steel is used; however, this reinforcement is not common in the United States at present.
Reinforcement should be lowered into the fluid grout or concrete by gravity or, if necessary, with an additional gentle push as shown in Figure 7.9. Reinforcement should not be driven, hammered, or vibrated unless specifically permitted by the contract documents; vibration is normally permitted only for fully welded cages. If the cage cannot be placed to the full required depth, the actual installation depth should be recorded and the engineer notified. After installation, the cage should be supported at the ground surface for a sufficient amount of time (typically a few hours, depending on the setting of the grout/concrete mix) to avoid it settling into the pile. The cage is often kept in place by using wire to tie it to timber supports.
Figure 7.8: Cleaning the Top of a CFA Concrete Pile
Difficulties in placing the reinforcement may occur if the grout or concrete does not maintain sufficient workability for the duration of time required for placement. In addition, sandy soil profiles can promote rapid dewatering of the grout or concrete in the pile such that reinforcement placement is difficult even with a properly retarded mix. In such cases, anti-washout additives or viscosity modifying admixtures may be helpful in reducing water loss from the grout/concrete. Installation of rebar to depths in excess of 18 m (60 ft) is possible under favorable circumstances, although significant bending stresses rarely occur at such depths for foundation piles.
Most often, piles are connected to a pile cap, with the base of the cap lying below existing construction grade. This below-grade cutoff is typically constructed by excavating for the pile cap, chipping the top of the hardened pile down to the required elevation, and cutting the rebar, as necessary. If the shallow soils are cohesive and the cutoff elevation is within a few feet of the surface, it may be possible to dip the grout/concrete down to the desired depth. In the latter case, a surface casing must be used to maintain a stable hole above the cutoff elevation and prevent surficial soils from sloughing into the fluid grout or concrete and contaminating the top of the pile.
Figure 7.9: Placement of Reinforcing Cage with Plastic Spacers
Sampling and testing of the grout/concrete are important parts of QA/QC. The samples may be obtained for testing directly by the inspector or by the contractor under the direct supervision of inspectors. The general approach to QA/QC for the grout/concrete is that the mix design and the quality of the mix is the contractor's responsibility and the inspector obtains samples for testing to verify that the requirements for the project are met. Strength tests are the control parameter of most concern for design, while workability is measured in the field to ensure that the construction goes smoothly and that the mix characteristics are consistent.
For concrete, 150 mm (6 in.) diameter by 300-mm (12-in.) high cylinders (ASTM C 31. [ASTM, 2006]) should be made from samples of the mix from the field in the same manner as for per most other cast-in-place concrete construction including drilled shaft construction. Samples should be cured and tested according to ASTM C39 (ASTM, 2006) or the agency's normal procedures. Concrete compressive strength requirements for CFA piles are typically 24 to 31 MPa (3,000 to 4,500 psi) and will be specified according to the project requirements. Typical specifications require a set of at least six samples for each 40 m3 (about 50 yd3) of concrete placed, but no less than one set per day or per batch of concrete, if batch plant operations are started and stopped more than once per day.
For grout, 50 mm (2 in.) cubes are most often used for strength testing, (see Figure 7.10) per ASTM C109 (ASTM, 2006). These are small and easy to handle and transport, and are considered adequate for testing grout without coarse aggregate in the mix. If the grout mix has pea gravel as aggregate, the mix should be considered concrete and thereby tested using cylinders as outlined above. Because grout cubes are small, it is easy for small misalignment in the testing apparatus or uneven surfaces to result in incorrect dimensions and thereby unrepresentative low measured strengths. For this reason and also because the samples are small, it is prudent to make extra samples during field operations so that any discrepancies can be re-evaluated. Some engineers prefer to use 75 mm (3 in.) diameter by 150 mm (6 in.) high or 50 mm (2 in.) diameter by 100 mm (4 in.) high cylinders. In such cases, careful attention is necessary to the relationship between maximum aggregate size and the height-to-diameter ratio of the sample. If the samples are cast using a method or sample different than that used for the mix design, a relationship between the compressive strengths obtained by the methods will be required.
Compressive strength requirements for CFA piles constructed with grout are similar to that for piles constructed with concrete, as noted above. However, it should be noted that the compressive strength of properly tested cubes are slightly higher than that of cylinders with a height-to-diameter ratio of two, therefore, the strength requirement from tests on cubes are typically 10% greater than that of cylinders.
Workability and consistency of concrete are monitored by performing slump tests on samples of the mix at the site. Slump measurements (ASTM C 143, [ASTM, 2006]) should be made on each truck on the project to ensure that consistent mixes are delivered. A slump of approximately 200 ± 25 mm (8 ± 1 in.) is typical for CFA piles, as is for drilled shaft placement in wet hole conditions. The relationship of slump loss over time should be established as part of the mix design and included in the approved installation plan. In general, a mix should be developed such that it maintains slump (or flow for grout) for a period of at least two hours for routine projects. The workability as a function of time is highly temperature-dependent and adjustments to the mix may be needed in warm weather. The contractor should place concrete quickly to avoid a decrease in workability over time as the cementitious material hydrates.
The addition of water at the project site should only be permitted through the approved installation plan or with prior approval by the engineer, and only to the extent that the water-cementitious material ratio does not exceed the ratio of the approved design mix. If the slump of the mix as delivered is not suitable, adjustments should be made at the plant unless the project is specifically planned for water to be held back and added at the site. In any case, it is critical that the mix have adequate workability; sometimes it may be necessary for the contractor to adjust the mix with water at the site rather than complete a pile with inadequate workability in the mix. Such practice should be a rare exception and corrections must be made to the operation. If water is added at the site, the inspector should have samples made and/or tests performed after the water has been added and the mix ready for placement.
Figure 7.10:Cubes for Grout Testing
Similar to the case of concrete, the workability and consistency of grout must be monitored by performing flow cone tests on samples of the mix at the jobsite. Flow cone measurements should be made on each truck on the job, to ensure that consistent mixes are delivered. As with concrete, water should not be added at the jobsite unless specifically allowed in the project specifications. The preferred practice is that water should not be added at the project site without approval from the project engineer. If the workability of the mix is not suitable as delivered, adjustments should be made at the plant. Nevertheless, it is critical that the mix have adequate workability. Sometimes it may be necessary to adjust the mix with water at the site rather than complete a pile with inadequate workability in the mix. Such practice should be a rare exception and corrections to the grout mix must be made to the operation. Sometimes, grout additives are added at the project site. If so, the specific manufacturer's recommendations must be followed.
ASTM C 939 (ASTM, 2006) or U.S. Army Corps of Engineers CRD 611-94 (USACE, 1994) provide specifications for flow cone testing in which fluid consistency is described according to an efflux time per standard volume (time for a specific volume to flow out of the cone). As 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 specifications and use a 19-mm (0.75-in.) opening. This modification can be made by: (a) removing the removable orifice that extends out the bottom of the Corps of Engineers device to leave a 19 mm (0.75 in.) opening; or (b) cutting the flow cone specified in the ASTM standard to modify the outlet diameter. Grouts that are 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. Grouts that are suitable for CFA pile construction should maintain fluid consistency within this range for a period of at least two hours, but in no case less than the time required to complete a pile and place reinforcement.
Post-construction integrity tests are used to supplement the installation monitoring to establish that a contractor's procedures are producing acceptable piles. There are several types of integrity tests that are useful for CFA or drilled displacement piles, most using technologies already in use on transportation projects for drilled shaft foundations. Several references are available that describe a wide variety of integrity test methods in greater detail. Two of these references are O'Neill and Reese (1999) and (DFI (2005).
Integrity test methods require careful interpretation, which should be performed by experienced personnel. However, integrity testing personnel cannot always determine whether an anomalous reading is a defect within the pile; therefore, the final decision on acceptability of the pile must be made by the design engineer based on the site specific soil conditions, construction records, the post-installation integrity testing report, and analysis of the possible effect on foundation performance.
As discussed previously, the most reliable means of achieving consistent QA/QC is automated monitoring and control during construction, with documentation of the installation via these measurements. The use of post-construction integrity testing is best utilized to verify that the installation parameters used for control (i.e., penetration speed, grout or concrete pressures and volumes during auger withdrawal) are appropriate for the site-specific project conditions. Integrity tests can also be used to further evaluate piles that did not meet drilling or grouting criteria. Coring of the piles can be used to supplement or to provide a visual check of suspected defects detected by integrity testing.
The necessary frequency of post-construction integrity testing is left to the judgment of the owner and can vary from project to project. A frequency of 10% to 20% of production piles subjected to integrity testing is typical. In addition, all preproduction and verification test piles should be tested. When agencies have little experience with CFA piles, particularly difficult project conditions exist, or project or site conditions give reason to expect problems with pile integrity, integrity testing of more than 20% of production piles may be required. A typical reasonable approach for load-bearing piles is to subject the first 10 to 15 piles to be constructed on a project to integrity tests to establish that the contractor's construction practice at the site is adequate. Thereafter, the frequency of such tests can be set to meet the specified frequency criteria, can be reduced, or even perhaps eliminate further integrity tests if the construction records for the remaining production piles are similar to those of the initial piles that were subjected to integrity tests.
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.
This test is performed by striking the top of the pile with a small instrumented hammer (Figure 7.11, left). A sonic, compressive wave travels down the length of the pile and is reflected by an anomaly in the pile, or the pile tip if the pile is free of defects, and travels back to the top where it is picked up by a receiver on top of the pile. The reflections are used to indicate major changes in cross sectional dimensions or material properties. Wave propagation through the pile is affected by the pile impedance, which is defined mathematically as EA/C, where E is the elastic modulus of the pile, A is the area of the cross section, and C is the wave propagation velocity, which is related to the elastic modulus and mass density of the pile.
Impedance changes occur where there is a change in cross-sectional area of the pile. A bulge (increase in cross-sectional area) or a neck (reduction in cross-sectional area) can be detected by an increase or decrease, respectively, in impedance of the signal. Changes in impedance also indicate where a change in grout/concrete density occurs, indicating a possible defect in the grout/concrete. Other types of processing are sometimes used to interpret the measurements of reflections including impulse response and impedance logging. Figure 7.11 (right) provides a simplified illustration of the sonic echo test. The displacement record shown in the figure indicates the reflection off the base of a pile of the length, L, embedded in sound rock. The reflection occurring at a time shorter than 2L/C (i.e., first upward spike of record) suggests an impedance change in the pile above the pile toe.
Figure 7.11: Sonic Echo Testing Concept
Although sonic echo testing is an economical and rather simple test, there are some important limitations to consider. As the sound wave travels along the pile, it loses energy and the strength of the reflected signal can become very weak. This means that for very long piles (i.e., depth-to-diameter ratio of greater than 30), the tip of the pile and anomalies or defects occurring at great depths will likely go undetected. Due to the nature of the design of CFA piles, the integrity of the upper 6 m (20 ft) is most critical for structural capacity, particularly for shear and bending moment. As sonic echo testing is more reliable at shallower depths, this limitation is not as significant as for long drilled shafts. This makes testing using sonic echo quite useful for rapidly evaluating a large number of piles. The hypothetical example shown in Figure 7.12 illustrates this concept. The long pile (A) has a weak reflection from the toe that may not be detectable. The short pile (B) has a strong reflection from the toe that is readily detected. Pile C illustrates a long pile containing a defect at a shallow depth. Although the reflection from the toe may be difficult to detect (as for pile A) as would a deep defect, the shallow reflection is readily detected.
Figure 7.12: Sonic Echo Testing of Long Piles
Another important limitation of sonic echo testing is that the wave energy is not likely to detect anomalies or defects unless these are large compared to the wave length generated by the impact. Some research indicates that defects that are shorter than 0.25 of the wave length are generally not detected. A typical hammer for sonic echo tests generates a wave length of approximately 1.6 m (5.3 ft), which means that defects or anomalies less than 0.4 m (15- to 16-in.) thick will go undetected. For most CFA pile diameters, this threshold of detection should be appropriate.
The most reliable of the post-installation integrity tests for identifying anomalies within cast-in-place 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.
CSL is performed using a source in one tube and a receiver in another to provide a measure of the wave speed of the material between the tubes. A strong signal measurement with an arrival time consistent with the wave speed of good grout/concrete is indicative of sound grout/concrete between the tubes. The SSL test (shown in Figure 7.13) utilizes a source and receiver on the same probe and is intended to sample the wave speed of the material surrounding the tube. The numerous dark lines shown on the time record on the right side of Figure 7.13 represent arrivals of signal energy plotted on a vertical scale of depth vs. time on the horizontal axis. The anomalous lack of dark lines at the 5 to 6 m depth interval represents a delayed arrival time and weak signal between these depths, which may be indicative of a defect in the pile.
The backscatter gamma test (see Figure 7.14), more commonly referred to as gamma-gamma logging, uses a small radioactive source on one end of the probe to emit gamma photons and a gamma ray detector on the other end. The photon count per unit of time can be calibrated to the grout/concrete density within a radius of about 100 mm (4 in.) around the tube.
To be effective, the access tubes for CSL or backscatter gamma testing should be distributed evenly along the circumference around the reinforcing cage with a spacing of about 0.3 m (1 ft). Tubes should be placed inside the cage to avoid damage during installation. It is recommended that tubes used for CSL tests consist of Schedule 40 steel, because such tubes will remain bonded to the grout or concrete. Polyvinyl chloride (PVC) tubes do not ordinarily remain bonded to the grout or concrete beyond a few days after initial set, and debonding will render the CSL tests ineffective. PVC tubes must be used for backscatter gamma testing because the steel tubes block the gamma photons from penetrating into the surrounding concrete or grout.
These downhole tests all require that the foundation contractor attach appropriate access tubing to the reinforcing steel prior to placing the steel in the grout column. While these tests are frequent with drilled shafts, downhole tests are more difficult to install in CFA piles (because the tubes must be pushed into the fluid grout/concrete). The tubing and instrumentation make downhole tests much more costly in proportion to the total cost of the pile when compared to sonic echo testing. The speed of testing is much slower than sonic echo, which adds to the final cost.
Figure 7.13: Downhole Single-Hole Sonic Logging (SSL) Concept
Figure 7.14: Gamma-Gamma Testing Via Downhole Tube
Load testing is a very important component for the effective use of CFA piles. Load tests are performed both as pre-production tests and as verification tests as part of the QA/QC of pile production (Figure 7.15). Axial compressive load tests are by far the most common; however, uplift and lateral load test can also be performed as part of a test program when evaluation of either load condition is important. As discussed in Section 7.3.5, both pre-production and verification load tests should be an integral part of transportation projects using CFA piles. A carefully planned and executed load test program can provide the following benefits:
The axial ultimate capacity of individual CFA piles are generally not larger than a few hundred tons. There are several options available in this load range for proof testing of production piles, such as RLT methods (e.g., Statnamic or Fundex systems) or DLT (e.g., drop hammer). Proof testing of CFA piles to confirm nominal axial resistance is generally not detrimental to the structural integrity or geotechnical performance of a sound pile; hence the tested pile may be used for in-service conditions.
Details of axial load testing methods will not be repeated here, although a brief discussion of methods most appropriate for CFA piles is provided. An extensive discussion of axial, uplift, and lateral load testing methods and data interpretation is provided in the following manuals:
The objectives of performing a load test program prior to the start of production pile construction are to:(a) provide measurements of site specific values of resistance; (b) correlate these values to construction methods; and (c) verify design assumptions. This is particularly important for drilled displacement piles where the contractor may be using a proprietary system or tooling for installation. The baseline parameters for drilling rate and concrete or grout placement during the construction of successful test piles are thus established for production piles. For displacement piles, control parameters may also include specific target values of torque and downthrust forces.
In a pre-production pile test program, it is important that test pile locations are selected which are representative of the dominant conditions across the project site. Subsurface information at the specific test pile locations is essential to interpret the results of the installation monitoring and load testing in a meaningful way. In cases of uniform soil and design load conditions, a single test pile may be suitable. In other cases, several test piles may be required to be installed at various locations on the project site.
The axial compressive resistance of CFA piles is normally in a range such that conventional top down static load tests are easy to perform with reaction systems that can be assembled by most contractors. If RLT or DLT methods have been calibrated to local soil and geologic conditions, these alternative methods can offer advantages of speed and economy. On a large project with many piles to be installed, a single control static test supplemented by several RLTs or DLTs on both the control and piles can be a very effective means of achieving a maximum benefit at the least test cost. The additional control static test can provide the reliability of conventional static measurements and the RLT or DLT program can provide the coverage needed to rapidly evaluate a range of conditions that may be encountered. It is always important that a site-specific correlation between static tests and RLTs and/or DLTs be established regardless of project size.
The relatively modest axial resistance of CFA piles and the availability of rapid and dynamic load test methods make proof load testing of production piles a viable option for QC/QA. Piles may be selected at random or piles of questionable quality can be chosen for proof testing. RLT and DLT methods are quite economical for loads of up to around 5 MN, (450 tons) (Figure 7.16). After RLT or DLT equipment is mobilized to the site, it is usually possible to efficiently test several piles each day.
The use of proof tests on production piles can be planned to provide the increased reliability and lower design factor of safety (or higher design resistance factor) afforded by the inclusion of load testing in the project, and thus can result in significant cost savings.
Axial load tests on production piles are not detrimental to the subsequent performance of the pile so long as the structural capacity of the pile has not been exceeded. Figure 7.17 illustrates the results of two cycles on the same pile, each of which achieved a geotechnical limit on the pile. The first load achieves a geotechnical limit according to the commonly used Davisson criterion. The second load cycle produces a load vs. deflection response that is actually stiffer than the first cycle. In general, the Davisson criterion will provide conservative results. This test result is typical of multiple loadings on a single pile, and the second load cycle is representative of the load deflection response of a pile after a static load test has been conducted. Two observations are made from these data: (a) a load test on a production pile does not adversely affect the ability of the pile to support subsequent loadings; and (b) multiple load tests on a CFA pile can result in increased pile stiffness in subsequent load cycles. The second aspect can have implications when comparing RLT or DLT methods with a conventional static load test on the same pile, since the pile may provide a stiffer response compared to whichever test method is performed second.
Figure 7.15: Static Load Test Setup on CFA Piles
Figure 7.16: Proof Testing of Production Piles with Statnamic (RLT) Device
Figure 7.17: Effect of Multiple Load Cycles on a CFA Pile
This section provides a summary of recommended QC/QA procedures for use on CFA pile projects and checklists for inspection of CFA piles.
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:
Upon arrival at the jobsite, the inspector should take the time to thoroughly review the contractor's equipment for compliance with the plans and specifications and approved installation plan. This work includes:
Additional specific details for each of the items noted above are provided in the project specifications. Guide construction specifications are provided in Chapter 8, which may serve as a preliminary specification for state DOT engineers to use in developing a state-specific CFA pile specification.
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