A wide variety of techniques and equipment have been used to construct continuous-flight auger (CFA) piles. Some key components of good quality construction:
Drilling Rigs – The rig must have adequate torque capacity to install the pile without excessive flighting of the soil during drilling. The torque and power of the rig will directly affect the depth to which piles can be installed, and the resulting axial capacity that can be achieved.
Drilling – In order to avoid excessive flighting, and to construct piles of consistent quality and axial capacity, target penetration rates must be established and maintained during the drilling of CFA piles. It is essential that this parameter be controlled by the rig operator and monitored for verification. Automated monitoring systems must be used to provide direct feedback to the operator and verification of performance. It is essential that the installation method used for construction of production piles be consistent with that used for construction of load test (control) piles.
Cementitious Materials – Either grout or concrete may be used for construction of CFA piles. Each has relative advantages under different circumstances. In general, it is recommended that:
- the specifications for grout/concrete materials be
performance-based verified using strength tests on either cubes or cylinders;
- testing for workability and mix temperature be routinely performed on each truck as a means of monitoring consistency. Mix proportions and characteristics should be established based on test piles or control piles, and maintained at a consistent quality throughout the project. Workability of concrete is monitored using slump tests. Workability of grout is monitored using flow cone tests with a modified opening enlarged to 0.75 inches. Workability of the mix must be maintained for the entire duration of pile construction, including rebar installation into the pile. Slump or flow cone tests should be performed at times corresponding to rebar cage installation.
Completion of the Pile Top– It is essential that the contractor continue to deliver the appropriate volume of grout/concrete to the pile when the auger is close to the surface and significant positive pressure can no longer be maintained. The completion of the pile top requires manual work to remove any debris or contaminated grout/concrete near the top of the pile before reinforcement is placed into the fluid grout/concrete. The use of a small form at the pile top extending above grade is recommended to maintain a sound surface. If below-grade cutoff is required, it is necessary to complete the pile to grade and then chip or cut the top down later. It is necessary to flush the grout/concrete to the surface of the working platform to remove any questionable or contaminated material.
Reinforcement – Installation of reinforcement requires that the grout/concrete mix retain adequate workability for the time necessary to install the cage after removal of the auger and clearing the top of the pile. The mix requirements with respect to this aspect of the work can vary with differing soil conditions, particularly with respect to the tendency of dry sandy soils to rapidly dewater the pile. The mix should be developed to demonstrate that workability is maintained within the slump or flow cone guidelines for the entire duration of time required for drilling and grouting the pile and placing the rebar cage. In addition, other measures such as anti-washout admixtures may be required if soil conditions cause excessive dewatering of the mix after casting that results in rebar installation difficulties. Designers should include reinforcement cages that use fewer heavy bars instead of many smaller bars; are no longer than the minimum necessary to provide structural capacity and anchorage; and allow the cage installation to proceed with minimum difficulty. The cage should be tied to permit handling without permanent distortion.
Installation Plan –The contractor should submit an installation plan, including details of the equipment and methods proposed for the project. Many aspects of the construction work are performance-oriented with respect to the contractor’s equipment requirements and methodology. The installation details and monitoring of the installation are key components of verifying that the performance requirements are met. Contractors should be held accountable for developing an installation plan that will achieve the required objective.
Test Piles and Test Installations – The recommended means of verifying that the installation plan will achieve the project requirements is using a carefully monitored test pile program. The program should consist of pre-production static load tests, production static and/or rapid and/or dynamic load tests, and post-installation integrity tests in sufficient quantities to provide the data necessary to demonstrate that the installed piles meet the load and deflection criteria established in the project plans, with an appropriate factor of safety. It is imperative that the demonstrated installation procedure be followed for all production pile installations.
This article is provided through the courtesy of the Federal Highway Administration, and is excerpted from its geotechnical engineering circular, “Design and Construction of Continuous Flight Auger Piles.”