Kelly Drilling vs CFA vs FDP: Choosing the Right Foundation Method

Kelly drilling vs CFA vs FDP: choosing the right foundation method

Kelly drilling, Continuous Flight Auger drilling (CFA) and Full Displacement Piling (FDP) can all produce cast-in-place foundation piles, but they do not solve the same construction problem in the same way. The correct method depends on the ground profile, groundwater, pile geometry, reinforcement, nearby structures, spoil restrictions, concrete logistics and the quality-control system available on the project.

BAUER describes Kelly drilling as a flexible multi-pass system using a telescopic Kelly bar and interchangeable tools. CFA and FDP are single-pass methods: the working string reaches the design depth in one continuous operation before concrete is placed during extraction. That difference affects productivity, spoil generation, borehole support, reinforcement installation and the rig configuration required.

Selection principle: Do not choose a method from pile diameter or headline productivity alone. Begin with the geotechnical design and construction risks, then identify the machine and tooling that can execute the approved method.

1. Executive comparison

The figures above are BAUER method-range references, not guarantees for every rig, tool, geology or project. The selected machine, mast, rotary drive, auger, Kelly bar and concrete system set the actual limits.

2. How Kelly drilling works

The rotary drive transfers torque and crowd force through a telescopic Kelly bar to a bucket, auger, core barrel or other drilling tool. The tool cuts or collects material, is lifted from the bore and emptied, and the cycle is repeated until the design depth is reached.

Where the bore cannot remain stable on its own, temporary or permanent casing, drilling slurry or a combination may be used. Casing can be installed by the rotary drive, a casing drive system or an oscillator. After the bore is cleaned and accepted, the reinforcement cage and concrete are placed in the specified sequence.

Kelly is normally examined first when

• the soil profile changes significantly with depth;
• boulders, hard layers or rock require interchangeable tools;
• large pile diameters or exceptional depths are required;
• full or partial casing is needed;
• the design requires a long or heavy reinforcement cage;
• the project needs direct inspection and cleaning of the completed bore before concreting.

Kelly limitations to plan for

Kelly drilling normally creates more spoil and involves repeated lifting cycles. It can also require casing, slurry handling, desanding, cleaning tools and additional lifting operations. Productivity therefore depends on geology, tool choice, spoil handling and the bore-support system—not just the rig’s rotary torque.

3. How CFA works

A CFA pile is drilled with a continuous auger whose central stem is hollow. The auger remains filled with soil while it reaches the design depth, helping stabilise the excavation. Concrete is then pumped through the hollow stem as the auger is withdrawn. BAUER’s B-Tronic assistance systems can monitor the extraction and concreting process on applicable equipment.

After the auger is fully removed, the reinforcement cage is inserted into the fresh concrete. BAUER’s CFA guidance states that the cage must be sufficiently stiff for this process. FHWA guidance likewise describes reinforcement being installed before the concrete or grout initially sets.

CFA is normally examined first when

• the project contains a high number of similar piles;
• the ground can be drilled continuously without disruptive obstructions;
• the required diameter and depth fit the available single-pass setup;
• a continuous concrete supply can be maintained;
• the site benefits from avoiding a separately open bore;
• production monitoring can record depth, extraction and concrete parameters.

CFA limitations to plan for

The full auger length, mast height and rig extraction capacity must match the pile. Boulders or highly variable hard inclusions can interrupt the single-pass process. Reinforcement insertion into fresh concrete can limit cage length, stiffness and congestion. The concrete mix and delivery system must remain pumpable and continuous.

BAUER states that CFA can penetrate hard layers and can form rock sockets up to 20 MPa in suitable conditions. This should not be interpreted as universal suitability for rock. Tooling, socket length, geology and rig capacity require project-specific confirmation.

4. How FDP works

In FDP, a displacement head and drilling string are rotated and pressed into the ground. Rather than removing the full pile volume as spoil, the tool displaces material laterally. Concrete is placed during controlled extraction, creating an in-situ pile.

BAUER identifies four principal method advantages: minimised excavation material, vibrationless construction, reduced excess-concrete consumption and high load-bearing capacity. BAUER also specifies that the method requires a modern rig with high torque, high crowd force, high extraction force and a tall, torsion-resistant mast.

FDP is normally examined first when

• excavated spoil must be reduced;
• the ground can accept displacement without unacceptable effects;
• the project contains repeated piles within the system’s geometry range;
• vibration from driven piling is undesirable;
• concrete delivery and automated process monitoring can remain continuous.

FDP limitations to plan for

The surrounding ground is displaced, so the geotechnical designer must consider heave, lateral movement, installation sequence and nearby foundations or utilities. Dense layers, obstructions and some rock conditions may control penetration. The final method must be confirmed by ground data and trial installation, not by the absence of spoil alone.

5. CFA, CCFA and FDP are not interchangeable

Cased CFA (CCFA) adds an outer casing to the continuous-auger process. BAUER states that it is used primarily for secant pile walls and highlights vertical accuracy, protection from surrounding soil and groundwater, dry spoil removal and relatively low noise compared with fully cased Kelly piles.

CCFA may therefore become relevant when the project is not merely a production-pile programme but a retaining-wall or groundwater-control task. It should be treated as a separate engineered configuration, not as a minor CFA accessory.

6. Ground and groundwater decision logic

Variable soil, boulders or rock

Kelly usually provides the greatest tooling flexibility because the crew can change between augers, buckets, core barrels and casing systems. CFA may be possible in selected hard layers, but continuous penetration must be demonstrated. FDP depends on whether the ground can be displaced and whether the rig can maintain penetration and extraction control.

Loose granular ground or groundwater

Kelly may require casing or slurry. CFA keeps the auger and excavated soil in the bore until concreting begins, avoiding a separately open excavation. FDP displaces the soil rather than creating a conventional open bore. None of these features removes the need to assess groundwater pressure, concrete continuity and stability at pile level.

Cohesive ground

CFA and FDP can both be efficient in appropriate cohesive profiles. BAUER’s current CFA tooling guidance identifies cohesive, friable soils and excludes boulders for the referenced starter configuration. FDP suitability depends on displacement response and penetration resistance. Kelly remains available where variable tools, casing or large dimensions are needed.

Contaminated ground

FDP can be attractive because BAUER identifies reduced excavation material. However, the environmental plan must still evaluate displacement, treatment of any spoil that does arise, concrete returns and the possibility of moving contaminants. No method should be described as automatically suitable for contaminated soil.

7. Concrete and reinforcement can decide the method

A method that appears suitable geotechnically may fail operationally if concrete supply or reinforcement cannot be executed reliably.

Kelly

The bore can generally receive a preassembled cage before concreting, subject to the approved construction sequence. Long cages may require splicing, lifting plans and sufficient auxiliary capacity. Tremie concreting and slurry control may be required below groundwater or support fluid.

CFA and FDP

Concrete placement and tool extraction occur together. The reinforcement cage is then installed into fresh concrete. The project must confirm:

• cage stiffness and lifting points;
• practical insertion depth;
• concrete workability and setting time;
• centralisation and concrete cover;
• available rig or auxiliary-crane capacity;
• contingency for interrupted concrete supply.

A heavily congested or full-depth cage can change the method decision even when the pile diameter and depth appear suitable for CFA or FDP.

8. Noise, vibration, spoil and neighbouring structures

No bored-pile method is impact-free. The correct comparison is between the actual configurations and the site receptors.

• Kelly: rotary drilling itself avoids impact driving, but casing installation, oscillator use, spoil handling and auxiliary equipment can affect noise and vibration.
• CFA: continuous rotary drilling can reduce the number of repeated tool-handling cycles, but auger cleaning, spoil handling and concrete pumping remain.
• FDP: BAUER describes the method as vibrationless and as minimising excavation material. Ground displacement and movement near existing assets still require engineering review.
• CCFA: BAUER states lower noise compared with fully cased Kelly construction for the referenced application, but this is a comparative method statement rather than a universal site limit.

Monitoring requirements may include vibration, settlement, heave, adjacent-pile movement, concrete volume and rig production data.

9. Decision matrix

10. Practical project examples

Bridge foundations through variable alluvium into rock

Kelly is usually the first method to study where the pile must pass through mixed layers and form a rock socket. Casing or slurry may support the upper bore, while a core barrel or rock tool develops the socket. Final selection depends on the verified rock strength, socket geometry and concrete method.

Repetitive building piles in consistent cohesive soil

CFA may offer a practical single-pass solution where the required geometry fits the rig and reinforcement can be inserted into fresh concrete. A trial pile should confirm penetration, concrete parameters and cage installation.

Industrial development with strict spoil controls

FDP may be studied where the soil can be displaced and nearby assets can tolerate the installation effects. A trial programme should measure heave, movement and concrete volume.

Deep excavation retaining wall

A secant wall may use CCFA where the geometry and ground are suitable. A diaphragm wall may be a separate alternative for deeper structural or cut-off requirements. The retaining system should be selected by the temporary-works and geotechnical design, not by drilling-rig availability.

11. BAUER equipment mapping

BAUER’s BG drilling-rig range includes H-kinematics, V-kinematics and dedicated KellyLine machines. BAUER describes H-kinematics rigs as offering wide mast outreach and fast setup, while the V range is intended for large diameters and depths. The exact method configuration depends on the mast, rotary drive, crowd system, winches, tool string and concrete equipment.

RENTEK currently catalogues the BAUER BG 30 H, BG 36 H and BG 55 as representative rotary-drilling rigs. Their presence in the catalogue must not be interpreted as confirmation that every one is configured for every method or project limit. The method, emissions configuration, attachments and availability must be confirmed in the quotation.

12. Common method-selection mistakes

1. Selecting from nominal pile diameter without checking the complete soil profile.
2. Treating CFA rock-socket guidance as permission for any rock formation.
3. Selecting FDP only to reduce spoil without assessing displacement effects.
4. Ignoring reinforcement insertion limits in single-pass methods.
5. Assuming the theoretical rig depth equals the achievable project depth.
6. Underestimating concrete-pump capacity, supply continuity and contingency planning.
7. Treating casing or slurry as interchangeable without an approved bore-support plan.
8. Omitting trial piles and instrumentation where the method is sensitive to ground response.
9. Promising production rates before tooling, spoil and concrete cycles are demonstrated.
10. Choosing a machine first and attempting to adapt the foundation design afterward.

13. Information to send with an enquiry

• geotechnical investigation and groundwater levels;
• pile schedule with diameter, depth, inclination and cut-off level;
• design loads and reinforcement drawings;
• rock strength, boulders, fill and obstruction information;
• permitted settlement, heave, noise and vibration criteria;
• spoil classification and disposal restrictions;
• concrete specification, plant output and delivery distance;
• access, working-platform data and overhead restrictions;
• adjacent structures, utilities and existing foundations;
• required production period and testing plan.

RENTEK can use this information to identify the relevant BAUER method family and request a technically appropriate rig and attachment configuration. The final method must be approved by the project’s geotechnical and structural designers and verified through the applicable construction-control plan. Price on request.

FAQ

Is Kelly drilling always slower than CFA?

No. CFA can be efficient for repeated piles in suitable ground, but obstructions, concrete interruptions or reinforcement difficulties can remove that advantage. Kelly productivity also varies greatly with geology, tooling, casing and spoil handling.

Does CFA require casing?

Standard CFA relies on the soil-filled auger flights during drilling and does not normally use conventional casing. CCFA is a separate cased system used for particular applications, including secant pile walls.

Is FDP the same as a driven displacement pile?

No. BAUER FDP is a rotary, cast-in-place displacement method. BAUER describes it as vibrationless, unlike impact-driven piling.

Which method creates the least spoil?

FDP is specifically intended to minimise conventional excavation material. Kelly and CFA bring excavated material to the surface through different cycles.

Which method is best for rock?

Kelly normally provides the broadest range of rock tools. BAUER states that CFA can form sockets up to 20 MPa in suitable conditions, but this is not universal rock capability. FDP suitability in rock or obstruction-rich ground requires specific assessment.

Can RENTEK select the final pile method?

RENTEK can help identify relevant BAUER equipment and obtain configuration information. Final method selection belongs to the project’s qualified designers and construction team.

---