Deep Foundation Systems: Driven and Bored Piles

When surface soil conditions are inadequate for supporting structural loads, deep foundation systems become essential for transferring loads to stronger soil layers or bedrock beneath. Among the most widely used deep foundation solutions are driven piles and bored piles, each offering distinct advantages for different construction scenarios.

Understanding Deep Foundations

Deep foundations are structural elements installed into the ground to support buildings, bridges, and infrastructure when shallow foundations would be insufficient. These foundations are made from preformed material having a predetermined shape and size that can be physically inspected prior to and during installation, or they are cast-in-place concrete piles, meaning the pile is cast on the construction site.

Driven Piles: The Displacement Method

What Are Driven Piles?

Driven piles are made from preformed material having a predetermined shape and size that can be physically inspected prior to and during installation, which is installed by impact hammering, vibrating or pushing into the earth. These piles can be manufactured from timber, steel, or precast concrete.

Construction Process

The installation process for driven piles involves using specialized equipment to force the piles into the ground. The piles are driven into the soil, pushing an equal volume of soil sideways and compacting a zone around the pile, increasing its bearing capacity. This displacement action is a key characteristic of the method.

The installation system consists of repeatedly striking off the top of the pile to drive it downwards through the soil, using pile-driving machinery that delivers repeated blows from above.

Equipment Requirements

Driven pile construction equipment typically includes a crane, hammer, piling rig, and powerpack. The equipment needed is generally more compact compared to bored pile operations.

Materials Used

Timber Piles: Centuries-old technique still used for coastal works, sea defence, and jetties.

Precast Concrete Piles: Reinforced to withstand driving stresses, usually pre-stressed with square or octagonal sections.

Steel Piles: Available in tubular, box, or H-section configurations. Interlocking steel sheet piles are also widely used, predominantly for wall construction.

Advantages of Driven Piles

Driven piles offer several practical benefits for construction projects:

High Bearing Capacity: They provide excellent resistance to vertical loads and can support heavy structural requirements.

Quality Assurance: Piles can be precast to the required specifications and inspected before installation, ensuring quality control.

Rapid Installation: The work progresses quickly once piles are manufactured, as the progress of the work will be rapid.

Soil Compaction: A pile driven into granular soil compacts the adjacent soil mass and as a result the bearing capacity of the pile is increased.

Clean Work Environment: The work is neat and clean with storage space required being very much less than other methods.

Suitable for Water Applications: Driven piles are the most favoured for works over water such as piles in wharf structures or jetties.

Versatility: Piles of variable lengths can be extended through soft, compressible, or swelling soils into suitable bearing material.

Disadvantages and Limitations

Despite their benefits, driven piles have notable drawbacks:

Noise and Vibration: The impact and noise of the pile driving can cause potential damage or disturbance to nearby structures or utilities.

Site Constraints: Installation may be difficult in sites with low headroom, soft or loose soil, or high groundwater levels.

Pre-drilling Requirements: Pre-drilling or jetting may be required to overcome obstructions or reduce friction.

Limited Customization: Limited length and diameter options depending on the availability and transportability of the pile materials.

Soil Limitations: Not suitable for saturated or silty soils as they drain slowly and cannot be compacted effectively.

Bored Piles: The Replacement Method

What Are Bored Piles?

Bored piles are cast-in-place concrete piles, meaning the pile is cast on the construction site. They are created when large holes are drilled in the ground and filled with concrete, often with steel reinforcement for increased strength and durability.

Bored piles are very effective as they transfer the load above ground to the deep rock and soil layers below with minimal settlement, which is ideal for supporting structures such as bridges and flood protection.

Construction Process

The bored pile construction is non-destructive and involves drilling a hole into the ground to the required depth. The installation process involves the use of specialized drilling rigs that carefully excavate the soil or rock to the desired depth.

As the hole is created, it is supported by temporary casings or drilling fluids to prevent collapse. Once the excavation is complete, steel reinforcement is placed within the borehole, followed by the pouring of high-quality concrete.

Types of Bored Piles

Rotary Bored Piling: Used in cases where there are significant obstructions in the ground. A temporary casing is installed by the piling rig to provide support, pile arisings are removed from inside the casing, then a steel reinforcement cage is installed before concrete is poured and the casing is removed.

Continuous Flight Auger (CFA) Piling: The most versatile, effective and commonly used type of bored pile foundation. A hollow-stem continuous flight auger is drilled into the ground to the desired depth using a rotary drilling rig. Concrete is pumped through the hollow stem, maintaining static head pressure, to fill the cylindrical cavity created as the auger is slowly removed. The reinforcement cage is placed through the freshly placed concrete.

Equipment Requirements

Bored pile construction typically utilizes a drill rig, helper crane, spoil handling equipment, casing, slurry handling, cage erection template, concrete truck and concrete pump truck.

Advantages of Bored Piles

Minimal Noise and Vibration: The installation process generates minimal noise and vibration, making them suitable for urban and congested sites.

High Load Capacity: Bored piles have high load-carrying capacity and can support heavy structural loads effectively.

Customization: They can be customized to site conditions and design requirements in terms of length, diameter, and shape.

Deep Installation: Bored piles can be constructed to significant depths, allowing for deeper foundation support in challenging soil conditions.

No Ground Disturbance: Bored piles can avoid damage or disturbance to nearby structures or utilities due to the absence of impact during installation.

Versatile Soil Conditions: Can be installed in sites with low headroom, soft or loose soil, or high groundwater levels, and in a variety of soil conditions.

Higher Bearing Capacities: For many design situations, bored piles offer higher capacities with potentially better economics than driven piles.

Disadvantages and Limitations

Higher Cost: One key drawback is their higher cost compared to other foundation options. The specialized equipment and drilling processes involved contribute to increased expenses.

Longer Construction Time: The drilling process and the need for concrete curing can prolong project timelines.

Access Limitations: Limited access or restricted space can pose challenges for installing larger drilling rigs.

Quality Control Complexity: Proper quality control measures are crucial during installation to ensure the integrity and strength of the piles.

Soil Condition Sensitivity: May not be suitable for highly dense or hard soils, as the auger may encounter difficulties during drilling.

Continuous Flight Auger (CFA) Technology

CFA piles represent an advanced bored pile technique that has gained significant popularity. A continuous flight auger drill is used to excavate a hole and concrete is injected through a hollow shaft under pressure as the auger is extracted, creating a continuous pile without ever leaving an open hole.

CFA Construction Process

The working sequence comprises:

A hollow-stem continuous flight auger is drilled into the ground to a designed depth
Concrete or grout is pumped through the hollow stem under continuous positive pressure
The auger is slowly removed while maintaining static head pressure
Reinforcement cage is inserted through the freshly placed high slump concrete

CFA Advantages

Speed: CFA piles can be installed rapidly, which helps reduce construction time and costs.

Environmental Benefits: Less noise and vibration compared to driven piles, making them suitable for urban and sensitive environments.

Minimal Ground Displacement: Creates minimal ground displacement during installation, minimizing potential disruptions to surrounding structures.

Self-Supporting: The auger acts as a temporary casing during the drilling process, providing stability to the borehole walls.

Wide Soil Compatibility: Can be installed in cohesive soils, non-cohesive soils, and mixed ground conditions.

Comparing Driven vs Bored Piles

Key Differences

Formation Location: Driven piles are formed off site and put in place on-site, whilst bored piles are cast in concrete on-site.

Installation Method: Driven piles are installed using impact hammering, vibrating or pushing, while bored piles are created by excavating a hole and filling it with concrete.

Noise and Vibration: Driven piles create significant vibrations and noise during installation, whereas bored piles produce minimal disturbance.

Cost: Driven piles are generally cheaper than bored piles, considering that they are faster in installation and require fewer resources.

Environmental Impact: Bored piles have a lower carbon footprint in many situations and are preferred for environmentally sensitive sites.

Selection Criteria

The choice between driven piles and bored piles depends on various factors:

Site Conditions: Soil type, density, and groundwater levels significantly influence pile selection.

Design Requirements: Load capacity, settlement tolerances, and structural needs must be evaluated.

Environmental Constraints: Proximity to existing structures, noise restrictions, and vibration sensitivity are critical considerations.

Project Timeline: Construction schedule and speed requirements affect the choice.

Budget: Cost considerations including materials, equipment, installation, and overall project economics.

Access and Space: Available headroom and working space may limit certain options.

Load Transfer Mechanisms

Both driven and bored piles transfer loads through two primary mechanisms:

End Bearing: The pile delivers the majority of the pile load to the bearing stratum at its base.

Skin Friction: Friction piles depend upon the transfer of load from their surface to the surrounding soil by means of friction throughout the length of the pile.

Most piles utilize a combination of both mechanisms, with the dominant load transfer method depending on soil conditions and pile type.

Quality Assurance and Testing

For Driven Piles

Visual inspection before installation
Driving records and blow counts
Pile integrity testing
Load testing when specified

For Bored Piles

Concrete quality control (slump, strength testing)
Drilling parameter monitoring (depth, auger rotation, penetration rate)
Concreting pressure and extraction rate tracking
Integrity testing using non-destructive methods
Load-settlement performance evaluation

Modern drilling rigs are equipped with sensitive state-of-the-art instrumentation that monitors all aspects of pile construction, producing individual logs for each pile with complete traceability.

Applications and Use Cases

Driven Piles Are Ideal For:

Projects over water (wharves, jetties, marine structures)
Dense or compact soil conditions
Projects requiring rapid installation
Situations where piles can be precast and quality-verified before installation
Sites where groundwater under pressure may be encountered

Bored Piles Are Ideal For:

Urban environments with noise restrictions
Sites adjacent to existing structures
High-rise buildings requiring deep foundations
Soft or loose soil conditions
Projects with high groundwater levels
Large diameter and deep foundation requirements
Bridge construction and flood protection structures

CFA Piles Are Ideal For:

Multi-story buildings
Places where high noise of pile installation is not allowed
Projects where speed of installation is important
Unstable soil conditions with high groundwater tables
Large numbers of piles required
Low headroom conditions
Pile-supported embankments

Economic Considerations

The economic viability of each pile type varies significantly:

Driven Piles: Generally more economical for standard applications due to faster installation and lower material costs. The use of pre-manufactured components reduces on-site labor requirements.

Bored Piles: While initial costs are higher due to specialized equipment and longer installation times, they may prove more economical when considering:

Reduced noise mitigation requirements
Ability to avoid damage to nearby structures
Higher load capacities requiring fewer piles
Elimination of mobilization costs for multiple visits

Environmental Impact

Construction environmental considerations have become increasingly important:

Driven Piles:

Higher noise pollution
Significant ground vibration
Potential disruption to nearby structures
Lower concrete consumption in some applications

Bored Piles:

Minimal noise and vibration
Spoil management and disposal requirements
Higher concrete consumption
Chemical additives for drilling fluids in some applications

Future Trends and Innovations

The deep foundation industry continues to evolve with several emerging trends:

Advanced Monitoring: Real-time monitoring systems providing detailed installation data and quality assurance.

Hybrid Systems: Combining advantages of multiple pile types for optimal performance.

Sustainable Materials: Development of low-carbon concrete mixes and recycled materials for pile construction.

Automation: Increased use of automated positioning and installation systems improving accuracy and efficiency.

Load Testing Technology: Non-destructive testing methods becoming more sophisticated and reliable.

Conclusion

Both driven and bored piles serve essential roles in modern construction, each offering distinct advantages for specific applications. Driven piles excel in providing rapid, economical solutions with high bearing capacity, particularly for projects over water or in dense soils. Bored piles, including advanced CFA technology, offer versatile, low-noise alternatives ideal for urban environments and challenging soil conditions.

The selection between these deep foundation systems should be based on a comprehensive evaluation of site conditions, design requirements, environmental constraints, project timelines, and budget considerations. Consulting with qualified geotechnical engineers and foundation contractors is essential to determine the optimal solution for each unique project.

As construction technology advances, both methods continue to improve in efficiency, environmental performance, and cost-effectiveness, ensuring that engineers have robust options for addressing the diverse foundation challenges encountered in modern infrastructure development.

Deep foundation system - driven and bored piles

For expert guidance on selecting and implementing deep foundation systems for your construction project, consult with qualified geotechnical engineers who can assess your specific site conditions and requirements.