Skip to contentUnited States Department of Transportation - Federal Highway AdministrationSearch FHWAFeedback

Geotechnical Engineering

<< PreviousContentsNext >>

Geotechnical Engineering Circular (GEC) No. 8
Design And Construction Of Continuous Flight Auger Piles
April 2007

Chapter 7: Quality Control (QC) / Quality Assurance (QA) Procedures

7.1 Introduction

Continuous flight auger (CFA) piles have a history of use in the U.S. commercial market but have been used infrequently on public works transportation projects. This under-utilization of a viable technology is at least partly the result of perceived difficulties in quality control on the part of transportation agencies. In addition, the proprietary systems used for the installation of drilled displacement piles are not easily incorporated into traditional design-bid-build delivery systems for public works projects.

The guide specification included in Chapter 8 of this document is performance-based in which the contractor is responsible for the final determination of pile lengths. The approach requires that the contractor provide the quality control and performance measurement parameters necessary to ensure that the owner is provided with the pile capacity and structural integrity that is required for the job. The key for the owner is that the specifications require measurements that provide a reliable indication of performance. With reliable performance indicators, this approach can allow contractors to exercise ingenuity and seek the most cost-effective and timely solutions to achieve the project requirements.

General quality control/quality assurance (QA/QC) practices for deep foundation installation that have been used in the U.S. and abroad are reviewed in this chapter to provide the background for understanding QA/QC issues with CFA piles. Recommended "best practices" for QA/QC for CFA piles on transportation projects are included in each section. The last section summarizes the recommended practices.

7.2 The Role Of The Inspector

The inspector on a CFA pile project has a significant role in the observation and recording of the contractor's QA/QC practices. As outlined in this chapter and in the guide specification in Chapter 8, a significant amount of testing, data collection by automated equipment, and manual data will be recorded for pile installation. The inspector will be required to: (a) understand the basic fundamentals of CFA pile installation;(b) verify that good construction practices are followed; and (c) understand the data collected. The inspector may also be collecting manual data, such as is done in much of current commercial practice, to duplicate or to backup the data recorded and submitted by the contractor.

While much of the data collected during pile installation may be recorded by the contractor, the inspector needs to ensure that the data is collected. Many agencies have standard protocols for record keeping and submittals, delineating responsibilities among the contractor and inspector. Some agencies may require that the inspector duplicate some or all manual data collection. The recommended records for pile installation are:

  1. Pile location and plumbness;
  2. Ground surface elevation;
  3. Pile toe (bottom) depth/elevation;
  4. Depth/Elevation of top of grout/concrete;
  5. Pile length;
  6. Auger diameter;
  7. Details of the reinforcing steel (number, size, and grade of longitudinal bars, size and spacing of transverse steel; outside diameter and length of cage);
  8. Flow cone efflux time and volume of grout placed, or slump and volume of concrete placed;
  9. Theoretical volume of excavation (theoretical diameter = diameter of auger);
  10. Depth/Elevation to which reinforcing steel was placed;
  11. Date/Time of beginning of drilling;
  12. Date/Time of completion of drilling;
  13. Date/Time grout or concrete was mixed;
  14. Date/Time ready-mix grout or concrete truck arrived at project site, and copies of all grout or concrete batch tickets used for the pile construction;
  15. Date/Time of beginning of grout or concrete pumping;
  16. Date/Time of completion of grout or concrete pumping;
  17. Date/Time of placement of reinforcing steel;
  18. Weather conditions, including air temperature, at time of grout or concrete placement;
  19. Identification of all grout or concrete samples taken from the pile;
  20. All other pertinent data relative to the pile installation; and
  21. All readings made by the automated measuring and recording equipment to include as a minimum:
    1. auger rotation vs. depth for every 0.6-m (2-ft) increment, or less, of pile advancement during the drilling process, and during placement of grout or concrete (if auger is rotated during this placement); and
    2. volume of grout or concrete placed versus depth of outlet orifice for every 0.6-m (2-ft) increment, or less, of pile placed;
    3. Average maximum and minimum pump stroke pressures at ground level for every 0.6-m (2-ft) increment, or less, of pile placed;
    4. Average maximum and minimum pump stroke pressure at or near the auger head for every 0.6-m (2-ft) increment, or less, of pile placed, if directed by the engineer; and
    5. Additionally, the engineer may also specify that the torque and crowd force (downward thrust on auger) measurements be made at every 0.6-m (2-ft) increment, or less, of pile advancement during the drilling process.

7.3 Pre-Construction Planning

Effective QA/QC begins with proper planning prior to construction. Under the performance-based specification model, the contractor will be required to submit design calculations, working drawings, a detailed pile installation plan, and a conformance testing plan. The owner and its engineer(s) and inspector(s) will need to review the submittals as part of the project planning process. The owner will also have to provide some level of information for the contractor to perform the design and develop their installation plans.

Owner-controlled design specifications can vary in the amount of design performed by the owner's design engineer and the amount performed by the CFA pile specialty contractor. For the method recommended in Chapter 8, the owner provides preliminary design information. The contractor designs the individual piles and pile cap connections and selects the CFA construction process and equipment.

During the bid process, qualified CFA pile contractors prepare a preliminary CFA pile design based on the owner's preliminary plans and specifications. The submittal design will occur with the bid. Once the contract is awarded, the selected CFA pile contractor prepares detailed CFA pile design calculations and working drawings and submits them for review.

7.3.1 Owner-Supplied Information

The owner should provide preliminary design information for the contractor during the pre-bid process. The complete list of items will vary according to project and local procedures, but will generally include:

  1. plans showing the pile design loadings, minimum pile diameter, pile tip elevation, minimum reinforcement, pile to footing/cap connection design, and pile layout for each footing/cap location;
  2. design criteria and requirements, such as design loads and maximum allowable displacements, safety factor;
  3. any geotechnical reports for the project containing the results of exploratory borings, test pits, or other subsurface data collected in the vicinity of the pile locations;
  4. site information, such as rights-of-way limits, utility locations, site limitation;
  5. material requirement for grout/concrete and reinforcement and testing specifications; and
  6. requirements for submittal and review of contractor design, working drawing, and construction submittals.

The subsurface conditions expected at the site can significantly impact the contractor's choice of procedures, methods, equipment, the biding process, and contract administration. The geotechnical report should be a factual document describing the subsurface conditions revealed by the investigation and should be included in the contract special provisions. This report should alert bidders of the subsurface conditions and reduce the potential for differing site conditions construction claims and disputes. By including the report in the contract special provisions, it becomes a legal part of the contract documents.

7.3.2 CFA Contractor Experience

The quality of CFA piles is highly dependent upon the skill of the contractor and the specific crew that is assigned to the project. It is essential that the contractor demonstrate competence to perform the work by providing documentation of successful completion of prior projects of a similar nature to the project being bid. Only experienced contractors will be allowed to perform the work and all contractors will be required to construct a test pile.

For transportation projects, the recommended experience requirements for contractors and their personnel are as follows.

  1. The contractor should have completed a minimum of three projects in the two-year period preceding the bid date in which CFA piles were installed successfully under subsurface and project conditions similar to those of the current project;
  2. The designated job site supervisor (foreman or crew chief) should have a minimum of three years of experience in the supervision of the installation of CFA piles;
  3. Drill rig operators should have a minimum of three years of experience installing CFA piles; and
  4. The designated project manager should have a minimum of three years experience with CFA projects of similar size and scope.

The contractor should be required to submit a list of personnel to be used on the project and provide documentation of experience.

7.3.3 Design Submittals

The CFA piles shall be designed by a licensed Professional Engineer (Design Engineer) that is licensed in the state where the project is located. The Design Engineer should have experience in the design of at least three successfully completed CFA pile projects over the past five years with CFA piles of similar capacity to those required for the project.

Revisions to the design due to field conditions will need to be documented through submittals of revised calculations and/or working drawings in the affected portion of the project. It is recommended that the contractor be required to submit as-built drawings upon completion of the pile installation. Design Calculations

Design calculations should include, but not be limited to, the following items:

  1. A written summary report that describes the overall CFA pile design;
  2. CFA pile structure critical design cross-section(s) including soil/rock strata, piezometric levels, and location, magnitude and direction of applied loads;
  3. Design criteria, including soil/rock shear strengths (friction angle and cohesion), unit weights, unit skin friction values, and unit end-bearing values. Any additional subsurface borings, laboratory work, or other subsurface data collected for the design beyond what was provided by the owner;
  4. Safety factors used in the design;
  5. Seismic design earthquake acceleration coefficient or other seismic design criteria applicable for the geographic area of the project;
  6. Design calculation sheets (both static and seismic) with the project number, CFA pile structure location, designation, date of preparation, initials of designer and checker, and page number at the top of each page. An index page should be provided with the design calculations;
  7. Design notes including an explanation of any symbols and computer programs used in the design; and
  8. Pile to cap/footing calculations. Working Drawings

Working drawings should include, but not be limited to, the following items unless provided in the contract plans:

  1. A plan view of the CFA pile structure(s) identifying:
    1. A reference baseline datum;
    2. The offset from the construction centerline or baseline to the face of the CFA pile structure at all changes in horizontal alignment;
    3. Beginning and end station of CFA pile structures;
    4. CFA pile locations with center-to-center pile spacing;
    5. Right-of-way and permanent or temporary construction easement limits, location of all known active and abandoned existing utilities, adjacent structures or other potential interferences;
    6. The centerline of any drainage structures or drainage pipes behind, passing through, or passing under the CFA pile structure; and
    7. Subsurface exploration locations shown on a plan view of the proposed CFA structure alignment with appropriate reference base lines to fix the locations of the explorations relative to the CFA structure.
  2. An elevation view of the CFA pile structure(s) identifying:
    1. CFA pile locations and elevations with vertical and horizontal spacing; and
    2. Existing and finish grade profiles both behind and in front of the CFA pile structure.
  3. General notes for constructing the CFA piles including construction sequencing or other special construction requirements.
  4. Horizontal and vertical curve data affecting the CFA pile structure and control points, including match lines or other details to relate CFA pile structure stationing to centerline stationing.
  5. A listing of the summary of quantities.
  6. CFA pile typical sections including spacing; diameter; reinforcing bar sizes, locations, and details; centralizers and spacers; and connection details to the substructure footing/pile cap.
  7. Typical details of verification and proof load test piles, including reaction systems.
  8. Details, dimensions, and schedules for all CFA piles and reinforcing steel.
7.3.4 Pile Installation Plan

The Pile Installation Plan is used by the contractor to demonstrate the acceptability of the equipment, techniques, and source of materials to be used on the project. This plan should include, but not be limited to, the following items:

  1. List and sizes of proposed equipment, including drilling rigs, augers and other drilling tools, pumps for grout or concrete, mixing equipment, automated monitoring equipment, and similar equipment to be used in construction, including details of procedures for calibrating equipment as required;
  2. Step-by-step description of pile installation procedures;
  3. A plan of the sequence of pile installation;
  4. Target drilling and grouting parameters (along with acceptable ranges) for pile installation, including auger rotation speed, drilling penetration rates, torque, applied crowd pressures, grout pressures, and grout volume factors;
  5. Details of methods of reinforcement placement, including support for reinforcing cages at the top of the pile and methods for centering the cages within the grout or concrete column;
  6. Mix designs for all grout or concrete to be used on the project, including slump loss vs. time curves and strength development vs. time curves for mixes with fly ash and/or slag;
  7. Equipment and procedures for monitoring and recording auger rotation speed, auger penetration rates, auger depths, and crowd pressures during installation;
  8. Equipment and procedures for monitoring and recording grout or concrete pressures and volumes placed during installation;
  9. Contingency plans for equipment failures during drilling or grouting operations (grout pump, monitoring equipment, etc.);
  10. Procedures for protecting adjacent structures, on or off the right-of-way, that may be adversely affected by foundation construction operations, including a monitoring plan as required in Section 3.1; and
  11. Other required submittals shown on the plans or requested by the engineer.

A clearly written pile installation plan can be very effective in reducing misunderstandings between the engineer, inspector, and the CFA pile contractor and can form the basis for solving potential problems before they occur, thus keeping the project on schedule and minimizing claims. The specific time allowance for review and approval should be clearly defined in the contract documents; 14 days is considered suitable for most routine projects. In reviewing the pile contractor's submittal, the key information regarding the equipment that should be scrutinized is:

  1. the rated capacity and boom lengths of the drill rig;
  2. the torque, rotational speed and down crowd capacity of the drilling machine;
  3. the horsepower of the hydraulic power unit used to turn the auger; and
  4. the positive displacement piston-ball valve pump, pump stroke displacements, engine horsepower and pump pressures of the grout pump to be used.

With respect to the above parameters, the installation plan should include documentation that the proposed drilling equipment has been demonstrated effective on similar size piles in similar soil conditions.

7.3.5 Testing Plan

The testing plan should be a requirement of the contract provided by the owner. The CFA pile contractor should include a plan for constructing and performing the required tests to meet the requirements of the testing plan along with the Pile Installation Plan. The testing 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.

The intent of the pre-production testing program is to install test piles to establish and/or verify installation means and methods, as well as load capacity. The results of the pre-production test program will then be used during production pile installation to ensure that the contractor is consistently installing acceptable piles (i.e., all production piles are the same as a test pile). The use of automated monitoring equipment provides a means of evaluating each pile for conformance to the installation criteria. Verification load tests and structural integrity tests during construction will be used to verify that the contractor is producing acceptable piles.

Sections 7.4 through 7.6 provide detailed discussions and recommended practices of each of the components of a testing program. The remainder of this section will outline the general requirements for the contractor's pre-construction submittal. Pre-Production Testing

Pre-production load test program will generally consist of a single or multiple static load tests and will depend on the number of piles to be installed, the range of design pile capacities, and the variation of subsurface conditions at the site. Lateral and uplift load tests may be included as well. For very large projects, pre-production testing may include a single static load test supplemented with several piles tested by the rapid load test (RLT) (usually Statnamic™) or dynamic load test (DLT) methods. Performing rapid or dynamic tests during the pre-production testing program will allow these methods to be "calibrated" against static load tests results prior to production pile installation. A discussion of various load test methods is contained in Section 7.6.

Piles installed for pre-production testing (including any reaction piles required for static load testing) should include all construction, monitoring, testing, and inspection requirements of production piles. The results of the installation and testing will be used to:

  1. establish target drilling penetration rate(s) for the various subsurface conditions on the site;
  2. establish pressure/volume relations for placement of grout/concrete. The grout factor (i.e., ratio of used volume of grout/concrete to theoretical volume for the specified pile size) ±7.5% that is calculated on the test pile(s) should be used for the installation of the production piles;
  3. establish target values for torque and downward thrust or crowd for displacement or partial displacement piles;
  4. establish mix design parameters such as grout flow, necessary admixtures, etc.; and
  5. evaluate design correlations of side and base resistance with the site specific soil parameters.

Because a major advantage of CFA piles is speed of installation, the pre-production load test program may be performed concurrent with the start of production piles to reduce additional mobilization or delay costs; however, the ability to modify the design based on the results of the load test may be limited in this case. For very large projects, the ability to modify the design based on the pre-production tests may make the separate mobilization for pre-production testing less of cost consideration. Conformance Monitoring and Verification Tests

Conformance monitoring includes the use of automated measuring and recording equipment to confirm the pile installation criteria, integrity testing, and verification tests on production piles to demonstrate that the installed production piles meet the established load-deflection criteria.

Installation Automated Monitoring

Automated monitoring equipment provides "real time" evaluation of each pile on a project. Section 7.4 outlines the types of equipment and their application. Automated monitoring is a contract requirement. Therefore, the contract documents should outline the data to be collected and submitted. The installation plan should include type of monitoring equipment, manufacturer, data to be collected, current calibration records, and sample data records. As a minimum, the monitoring equipment should have the capability to monitor and record the following:

  1. auger rotation;
  2. depth of the auger injection point;
  3. torque delivered to the auger; and
  4. crowd force (downward thrust on auger).

All measurements should be referenced to (or plotted against) the depth of the auger injection point. This can be accomplished with a rotational position indicator on the auger head system and an electronic position indicator on the crane line or boom holding the auger. Torque and thrust load cells should be positioned on the auger head system.

As a minimum, the following automatic measurements should be recorded during the grouting or concreting operation:

  1. volume of grout or concrete;
  2. maximum and minimum grout or concrete pressure;
  3. auger rotation (if rotated); and
  4. depth of the injection point.

All measurements should be referenced to (or plotted against) the depth of the auger injection point. This can be accomplished with electronic flowmeters and electronic pressure transducers placed in the grout or concrete pressure line, an electronic position indicator on the crane line or boom holding the auger, and a rotational position indicator on the auger system.

Calibration should be made on all monitoring equipment at the beginning of the project in accordance with the equipment manufacturer's specifications. The values indicated by the monitoring equipment should be within three percent of the manufacturer recommendations.

Integrity Testing

Post-installation integrity tests are valuable in establishing that the contractor's procedures are producing acceptable piles on any given project. The most reliable of the post-installation integrity tests for identifying anomalies within the pile are those that use down-tube instruments, such as the cross-hole sonic (CSL) test, single-hole sonic test, the backscatter gamma test, and the fiber-optic television camera test. However, these types of tests are costly and utilize intrusive tubes, and thus not generally practical for CFA piles of less than 760 mm (30 in.) in diameter. The piles that will include any access tubes should be noted in the test program. Sonic echo tests performed from the pile top are also available to check pile integrity, and may be more practical for routine use to verify the overall structural integrity of the piles in the upper 10 to 20 diameters, though the results may not be as reliable as down-tube tests under certain conditions. Descriptions of tests applicable to CFA piles and a discussion of their use are included in Section 7.5. Piles that have installation records out of specification or that otherwise appear abnormal can be selected for integrity tests or verification load tests to determine if they should be accepted or rejected. Integrity tests along with careful monitoring of installation would then be used to verify pile capacity based on comparison to the pre-production test results. Recommended frequency of integrity testing is given in Section 7.3.3.

Verification Testing

Verification tests should be performed on a minimum of two percent of production piles, or at a greater frequency if required by the engineer. For smaller projects (i.e., less than about 50 piles), a minimum of one or two verification tests should be specified; the actual minimum is dependent upon the variability of site conditions, experience in the area, and other factors as may be considered relevant by the engineer. Verification tests can also be used to determine if a questionable pile should be accepted or rejected. Verification tests can be performed using static load tests, RLTs, or DLTs. Combinations of the various test methods may also be used as appropriate for the project. Section 7.6 includes discussions of common RLTs and DLTs available.

A single pre-production test only demonstrates the performance of the test pile. Performing verification tests periodically throughout production pile installation will verify that the pile installation techniques continue to provide adequate pile capacities. The use of RLTs (e.g., Statnamic™) or DLTs (e.g., drop hammer) can often test a large number of piles more efficiently in both time and cost compared to static load test methods. Calibrating the RLT or DLT results with static load test results during the pre-production test program should be part of the testing program, unless comparative tests have been performed on previous projects in similar soil conditions. Materials Testing

Requirements for sampling and testing of grout and concrete materials used on the project should be included in the owner's bid package. The general requirements for materials testing in the State DOT Standard Specifications should be referenced, along with any additional materials testing the contractor is required to perform. Requirements would include the test type and frequency. The tasks for materials testing may be performed solely by the owner, by an independent testing firm working for the contractor, or a combination, and will be addressed in the project specifications. Requirements for grout or concrete testing are discussed in Chapter 4 and in Section 7.4

7.4 Performance Monitoring And Control During Construction

The construction of continuous flight auger piles is hidden from the view of the operator as well as the inspector. Past practice has emphasized the importance of a skilled operator using visual observations of the drilling and inaccurate estimates of grout/concrete pumped to construct a "good" pile. A skilled operator and experienced CFA contractors are important to achieve a good end result, but the reliance upon visual observations alone is insufficient to provide quality assurance and quality control (QA/QC) for transportation projects. Technology is available to obtain the measurements and feedback needed to: (a) provide operators with information needed to develop judgment and control; and (b) provide inspectors and owners with documentation that the pile was constructed with proper practices in accordance with specifications. Much of the equipment used for this purpose has been described in Chapter 4 of this document. This section will summarize the requirements for performance monitoring and control during construction. Post-construction techniques for integrity and load testing are described in subsequent sections of this chapter.

It is important to remember that automated monitoring should not be viewed as the sole record for CFA pile QA/QC. Critical information that supplements automated monitoring includes: visual record of the completion of the top of the pile, notes of the workmanship of clearing debris and forming the pile top, descriptions of the successes and difficulties in installing the rebar cage, and notes of difficulties encountered and the methods used to resolve any problems. Complementary manual checks of the data collected by the automated equipment should also be performed periodically by the inspector to verify that the equipment is working reliably and accurately. The manual/visual observations outlined in this chapter that are typical of commercial practice in the United States would be appropriate for this task. The level of duplicate data will vary according to the confidence level of the owner agency and the complexity of the project. In the event that the automated equipment malfunctions during production, pile installation should stop until repairs are made.

7.4.1 Monitoring and Control of the Drilling Phase

The drilling phase of construction should be controlled to ensure that excessive flighting of soil does not occur with conventional CFA piles and that the appropriate level of soil displacement occurs with drilled displacement piles. The operator and inspector should observe and record the depth of the auger, the speed of the auger, the rate at which the auger penetrates into the ground, and the torque with which the auger is rotated.

There is always some uncertainty as to the proper rate of penetration during construction. The penetration rate will be estimated by the contractor during the design phase and included in the installation plan. Table 7.1 lists some general guidelines for penetration rates that are based on experience. The actual rate to be used can be affected by other factors, such as the pitch on the auger flights. The pre-production installation and testing of piles will either confirm the estimated penetration rate or provide the necessary data to modify the penetration rate to an acceptable value for the production piles. The same construction practices used during installation of successful test piles should be used for all production piles unless there is a significant change in subsurface conditions, such as differing soil types, soils more susceptible to mining, etc.

Table 7.1: General Guidelines for Auger Penetration Rate for CFA Piles
Soil TypeRate of Penetration (Revolutions per Auger Pitch)
Clay soils2 to 3
Cohesionless soils1.5 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

Photo showing operator with cab mounted display used to control drilling.

7.4.2 Monitoring and Control of the Grouting/Concreting Phase

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 position of the auger tip is monitored visually by observing the height of the auger in the leads;
  • the lifting speed is controlled by the operator by feel and by observing the height of the auger in the leads while timing the withdrawal using a stopwatch;
  • the volume of grout is measured by estimating the volume per stroke of the pump, and by manually counting the pump strokes; and
  • the pressure with which the grout is delivered is monitored by a gauge in the line near the pump.

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:

  • the position of the auger tip [monitored automatically by a position sensor (shown in Figure 7.2)];
  • the volume of grout [measured by an in-line flowmeter (see Figure 7.3) that provides a reliable and accurate measure of the grout/concrete that is delivered in real time];
  • the pressure with which the grout/concrete is delivered [monitored using a gauge in the line near the swivel at the top of the auger, or in the auger itself near the tip (latter option is better)];
  • the rotation of the auger [monitored by a sensor];
  • the lifting speed [controlled by the operator based on real time observation of the control parameters noted above, displayed graphically in the cab of the rig, and compared to target values]; and
  • the entire operation [recorded as a part of the documentation process].

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

Close-up photo showing depth encoder mounted on crane boom.

Figure 7.3: In-line Flowmeter

Photo depicting an in-line flowmeter.

Figure 7.4: Pressure Sensors on Hydraulics to Monitor Rig Forces

Two photos showing 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

Photo showing a display panel for observation by inspector.

Figure 7.6: Example Data Sheet from Project

Illustration showing an example of data sheet from a project.

Source: Jean Lutz S.A.

7.4.3 Finishing the Pile Top and Installing Reinforcement

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

Photo showing the dipping of 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

Photo showing the cleaning of 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

Close-up photo showing placement of reinforcing cage with plastic spacers.

7.4.4 Sampling and Testing of the Grout or Concrete

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

Photo showing 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.

7.5 Post-Construction Integrity Testing

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).

7.5.1 Use of Integrity Testing

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.

7.5.2 Integrity Testing by Surface Methods

The most commonly available, economical, and easily applied type of integrity test is the sonic echo test. The advantage of the method is that a test can be performed rapidly, inexpensively, and without any internal instrumentation or tubes in the pile. In general, the sonic echo test is the recommended method for routine testing of CFA piles of 760 mm (30 in.) diameter or less.

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

Photo and illustration showing 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

Illustration showing sonic-echo testing of long piles. Figures shows different responses as a function of whether the pile is long, short, or has a defect.

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.

7.5.3 Integrity Testing using Downhole Techniques

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

Illustration and graphic showing downhole single-hole sonic logging concept.

Figure 7.14: Gamma-Gamma Testing Via Downhole Tube

Photo showing Gamma-Gamma testing via downhole tube.

7.6 Load Testing

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:

  • Site-specific load test data provide verification of design parameters and increases reliability. This reduction in uncertainty may allow the use of lower design factors of safety employed in the ASD methodology.
  • The pile load testing program serves to establish the baseline parameters for construction of production piles, particularly with respect to target drilling penetration rate and pressure/volume relations during placement of grout/concrete. Other important parameters such as grout flow and necessary admixtures can be established during the test program.
  • The test results can be used to evaluate correlations of side and base resistance with site-specific soil parameters, and to revise or improve the design.

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:

  • "Static Testing of Deep Foundations" (Kyfor et al., 1992) - FHWA-SA-91-042;
  • "Design and Construction of Driven Pile Foundations," Volumes 1 and 2 (Hannigan et al., 2006) - NHI Course FHWA-NHI-132021;
  • "Micropile Design and Construction" (Sabatini et al., 2005) - FHWA NHI-05-039 and
  • "Drilled Shafts" (O'Neill and Reese, 1999) - FHWA-IF-99-025
7.6.1 Considerations in Planning a CFA Pre-Production Test Pile Program

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.

7.6.2 Proof Tests on Production Piles

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

Photo showing static load test setup on CFA piles.

Figure 7.16: Proof Testing of Production Piles with Statnamic (RLT) Device

Photo showing proof testing of production piles using Statnamic ® RLT device.

Figure 7.17: Effect of Multiple Load Cycles on a CFA Pile

Illustration showing the effect of multiple load cycles on a CFA pile.

7.7 Summary Of Recommended QC/QA Procedures

This section provides a summary of recommended QC/QA procedures for use on CFA pile projects and checklists for inspection of CFA piles.

7.7.1 Prior to Construction

The CFA pile inspector must be prepared and should have experience and knowledge of CFA pile construction techniques. Prior to construction the inspector should have and review the following:

  • Project plans and specifications;
  • Geotechnical report and/or other available subsurface information (often provided on the plans);
  • Contractor's approved installation plan;
  • Details of load test program or pre-production test pile installations;
  • Details of required automated monitoring system and control parameters;
  • Details of grout or concrete mix design and sampling and testing requirements; and
  • Reinforcing details and methods for pile top finishing and cutoff levels.
7.7.2 On-Site Review of Contractor's Equipment

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:

  • Check-in with contractor to understand day's planned work and confirm that party responsible for grout/concrete sampling is at site.
  • Document auger diameter and length, auger pitch, grout/concrete (each truck immediately when delivered), reinforcing type, and configuration, and centralizers.
  • Check calibration of grout/concrete volume monitoring equipment, and other measurement equipment for which calibration is required. Confirm that monitoring equipment is set to record at proper depth intervals as required.
  • Check equipment condition and tolerances.
7.7.3 During Drilling
  • Confirm that pile location is within horizontal tolerances and pile plumbness is within the range specified.
  • Confirm plug placed at auger bit tip.
  • Confirm the location of the pile to be constructed, so that relevant subsurface conditions, pile construction requirements (i.e., pile number, embedment depth, cut-off, and batter, if required) can be quickly and accurately referenced.
  • Document auger verticality or specified batter, as applicable. Document cuttings and auger advance rate. Confirm for consistency with conditions disclosed in geotechnical report, pile test report, and load test report.
  • Confirm removal of excessive cuttings/spoil build-up around auger.
  • Monitor behavior of the pile and surrounding ground during construction of test pile foundations. Check for indications of ground subsidence or loss of fluid grout/concrete.
7.7.4 During Grout/Concrete Placement
  • Document grout/concrete properties (batch time, temperature, additives, and flow) and those samples that have been obtained for strength testing at the intervals, per project specifications.
  • Document the pile grouting/concreting operation, including immediate start of placement and pumping of initial grout/concrete volume and pressure head, and auger withdrawal rates, per approved installation plan. Confirm (from automated monitoring data and observations) that target minimum grout/concrete factor is attained along the length of the pile. Confirm for consistency with reported conditions disclosed in geotechnical report, test pile report, and load test report.
  • Document depth at which grout/concrete return is first observed. Confirm consistency with conditions disclosed in geotechnical report, test pile report, and load test report.
  • Should a discontinuity in grout/concrete return or other questionable conditions be observed, inform the contractor of the condition and note the conditions observed on pile logs or data sheets. In some cases, the contractor may re-drill the pile to the full length. Re-drilling the pile to less than the full length to restart the grouting /concreting operation is generally considered unacceptable.
  • Confirm continuous and steady auger pull, with slow positive auger rotation if approved in the installation plan, and grout/concrete pumping until auger tip comes out of the ground. Document total volume of grout/concrete pumped, and overall grout/concrete factor. Obtain pile records from automated monitoring equipment.
  • Confirm that the pile top is cleared of any debris or contaminated grout or concrete.
7.7.5 During Reinforcement Placement and Pile Top Finishing
  • Document reinforcement installed in pile that is in accordance with specified design.
  • Document any installation difficulties, especially since they may be indicative of potential obstructions or undesired inclusions in the pile. In general, the most cost effective and time saving remedial measure is to re-drill and re-grout the pile if suspect conditions are observed.
  • Confirm that reinforcing is free of auger spoils or rust prior to insertion.
  • Confirm that reinforcing has specified extension above proposed cut-off elevation.
  • Confirm that the pile top finishing is performed in a manner consistent with the project requirements, including any forms above grade and tie-off details of the reinforcement.
7.7.6 Post-Installation
  • Check for grout subsidence.
  • Inspect pile cut-off.
  • Ensure that any post-construction integrity testing or proof load testing is scheduled and performed in a timely manner and that any questionable or rejected piles are noted and the appropriate notification is provided to the owner agency and their engineer.

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.

<< PreviousContentsNext >>


Updated: 04/07/2011

United States Department of Transportation - Federal Highway Administration