Installation Process
Site Preparation and Drilling
Site preparation for continuous flight auger (CFA) pile installation begins with a comprehensive site survey to establish the soil profile, typically involving borehole sampling and testing to assess subsurface conditions such as soil type, strength, and groundwater levels. This step ensures the design depth and pile spacing are appropriate, with minimum center-to-center spacing of 3 to 5 pile diameters to avoid interference.[9] Once the soil profile is confirmed, pile locations are precisely marked using stakes based on approved working drawings, with tolerances limited to 75 mm in plan view to maintain alignment and prevent structural issues. Unsuitable surface materials, including muck, organics, or soft clays within 1.5 m of the ground surface, are excavated and backfilled with compacted granular soil (plasticity index ≤20) to at least 95% maximum dry density per AASHTO T 180, providing a stable platform.[9][1]
Rig mobilization follows, involving the transport and positioning of the CFA drilling rig—typically a hydraulic or crane-mounted unit—over the marked locations on the prepared platform to ensure stability and access for support equipment. Auger assembly entails connecting segmental hollow-stem sections, each marked at 0.3 m intervals for depth tracking, with a continuous single-helix flight uniform in diameter (at least 97% of the pile design) and a plugged base to seal the hollow stem during advancement. The rig's leads are equipped with guides to maintain plumbness within 2% deviation, and all components are verified for torque capacity (minimum 40 kN-m) and crowd force prior to starting.[9][1]
Drilling commences with the rotation of the auger at 10 to 40 rpm, applying a downward crowd force of 50 to 270 kN to excavate soil continuously to the design depth, balancing penetration to avoid over-rotation that could cause soil decompression or subsidence. In cohesive soils, the advance rate is controlled at 0.4 to 1.2 m/min, with spoils brought to the surface for observation to assess excavation quality and detect anomalies such as inconsistent soil types or obstructions.[1][9] Continuous monitoring of torque and crowd pressure via onboard sensors identifies issues like voids or hard layers, where refusal is defined as less than 0.3 m/min penetration, prompting immediate evaluation. The hollow stem remains sealed during drilling to maintain borehole stability in uncased conditions, preventing inflow of soil or water and ensuring a clean path for subsequent operations.[9][1]
Concreting and Extraction
Once the auger reaches the predetermined depth, the concreting and extraction phase commences, marking the critical transition from soil displacement to pile formation in continuous flight augering (CFA). Auger withdrawal begins slowly and steadily, typically at rates of 0.5 to 1.0 meters per minute, while concrete or grout is simultaneously pumped through the hollow stem of the auger at controlled volumes to fill the excavated void and displace surrounding soil laterally. This coordinated process ensures continuous support of the borehole walls, preventing collapse or soil relaxation that could compromise pile integrity. The withdrawal speed is matched to the pumping rate to avoid defects such as necking, with the auger lifted in increments of 150 to 300 millimeters initially before proceeding continuously.[3][9]
The mechanics of this phase rely on pressurized delivery, akin to a tremie method, where concrete is introduced from the base upward to achieve bottom-fed filling without segregation or contamination by soil. Pumping pressures, typically ranging from 1.0 to 1.7 megapascals (150 to 250 pounds per square inch) at the auger top, compensate for any soil relaxation by exerting outward force on the borehole, maintaining stability in cohesionless or soft soils. This pressure exceeds the overburden stress at the tip, ensuring the concrete displaces soil effectively while some excess flows up the auger flights. In practice, grout or concrete mixes with high fluidity (e.g., slump of 200 millimeters) are used to facilitate flow under these conditions.[9][3]
Concrete volume is rigorously monitored throughout extraction, often requiring 1.2 to 1.5 times the theoretical borehole volume to account for soil compression, bulking, and minor losses, with an initial overrun of about 15 to 20 percent recommended. For the lowermost 0.9 to 1.8 meters, volumes may double the theoretical amount to ensure complete base filling. Over-pumping, indicated by ratios exceeding 1.5, signals potential issues like soil mining or necking, prompting immediate re-drilling to 1.5 meters below the anomaly. This excess at the base can form a bulbous enlargement in softer soils, enhancing end-bearing capacity by providing additional resistance against settlement.[9][3]
Reinforcement and Finishing
In continuous flight augering (CFA), reinforcement is introduced after the auger extraction and concrete placement to provide tensile strength and structural integrity to the uncased pile. The steel cage, typically comprising 6 to 20 longitudinal deformed bars (such as #6 or #10 ASTM A615 Grade 60) arranged symmetrically with transverse ties or spirals for confinement, is fabricated off-site to specified dimensions, often 10 to 20 meters in length depending on pile depth.[2][13] These cages are equipped with non-metallic (plastic or cementitious) spacers at intervals of no more than 3 meters to ensure a minimum 75 mm clear cover from the pile exterior, preventing corrosion while maintaining concentric placement within the fluid concrete column.[2][13]
Insertion of the reinforcement cage occurs immediately after auger withdrawal and concreting, while the concrete remains fluid to allow proper bonding and embedment, typically within 30 minutes to avoid initial set.[2][13] The cage is lowered by gravity or with a gentle push using a vibratory head if permitted, without driving or excessive force to prevent distortion or displacement of the wet concrete; it is then supported at the surface with ties to timber or beams until the concrete achieves sufficient strength (e.g., a few hours or until initial set).[2][13] For piles exceeding standard cage lengths, lap splices or mechanical couplers (e.g., ASTM A722 threaded bars) are employed in the lower sections to extend reinforcement depth, ensuring continuity under tensile or bending loads without reliance on permanent casing.[2] Access tubes, such as Schedule 40 steel or PVC (one per foot of pile diameter), are secured to the cage prior to insertion to facilitate subsequent integrity testing.[2][13]
Finishing begins with trimming the pile head to the designated cut-off level, removing excess concrete, spoil, or contaminated material (often the upper 1.5 meters sieved for soil clods) to ensure a clean, level surface within tolerances of 25 mm below or 75 mm above plan elevation.[13] This step prepares the pile for connection to the foundation cap while minimizing weak zones at the top. Integrity testing follows to verify the pile's homogeneity and detect defects inherent to the uncased CFA process, such as voids, necking, or soil inclusions. Common non-destructive methods include cross-hole sonic logging (ultrasonic pulses at 62 kHz through embedded tubes), which identifies anomalies via signal delay gaps indicating inclusions or voids, and fiber-optic television inspection for visual confirmation of cracks or irregularities.[13] These tests, performed 3 days after installation on set concrete, ensure structural continuity by confirming no significant defects (e.g., signal disruptions suggesting voids larger than detectable thresholds like 0.25 mm cracks from shrinkage); piles failing tests may require remediation or replacement.[2][13]