CFA Piles VS Driven Piles
Which is the best solution – CFA Piles or Driven piles? How do you decide which piling technique is going to give you the best solution? Below we look at the key differences and compare CFA Piles Vs Driven Piles to help you understand each method and its benefits.
What are CFA piles?
CFA piles are installed by drilling a continuous flight hollow stem auger into the ground to a specified depth. The hollow stem auger provides support to the excavated shaft at all times during the drilling process, thus avoiding the need for temporary casings or drilling fluids. When reaching the pile toe level, concrete is pumped through the hollow stem to fill the cavity then the auger is extracted. After the concreting, the steel reinforcement cage is installed into the fluid concrete. While initially developed to cater to sites with soft and sandy materials beneath the water table, CFA piles are a prevalent foundation type due to their ease of installation, low vibration nature, and depth/torque capability.
Benefits of CFA:
- High production rates mean that piles are commercially attractive
- A broad range of auger sizes (300mm to 1200mm diameter) means that the most economical use of construction materials is possible
- Depths of up to 25m means that CFA piling is effective for low to mid-range loading, thus suitable for most commercial and residential projects
- Low noise emissions
- Virtually vibration-free
Disadvantages of CFA:
- Longer piles required than driven (not driven to a predetermined set)
- Removal and disposal of spoil material generated from the pile
- Drilling through large obstructions/boulders cannot be undertaken
- Strict quality control and thorough supervision is required during the pile installation
CFA Piling noise & vibration levels
It is no surprise that heavy mechanised construction works, for example, excavation, piling, and vibratory compaction, cause ground-borne vibration due to the interaction of the machines with the ground. However, CFA technology offers the ideal solution for projects in urban/sensitive locations because it eliminates vibration and disturbance to adjacent structures. This piling approach is virtually vibration-free and benefits from low noise levels.
The below data shows the Sound Power Level of each source. The estimated on-time (i.e. operating time) of each source as a typical average percentage of the daily demolition and construction works:
Auger, Crane mounted = 116 SWL (Sound Power Level) Estimated on time: 75%
Compressor =112 SWL (Sound Power Level) Estimated on time: 75%
Concrete mixer = 99 SWL (Sound Power Level) Estimated on time: 75%
Generator Power = 108 SWL (Sound Power Level) Estimated on time: 50%
Lorry = 108 SWL (Sound Power Level) Estimated on time: 25%
Ground-borne vibration levels from CFA piling vary significantly depending on the type of ground. However, even on firm ground (e.g. clay), ground-borne vibration at 20m is still only around 0.3mm/s PPV (Peak Particle Velocity), which is around the onset of perceptibility in residential properties. If proposed CFA works were approx. 18m to nearby residential properties as an example, the maximum ground-borne vibration levels from the piling would be expected to be in the order of 0.1-0.3 mm/s PPV depending on ground conditions.
CFA Piling production rate
One of the main advantages of CFA piles is the high production rate. Typical production rates on private projects for piles having diameters of 300 to 450 mm (12 to 18 in.) and lengths of less than 20 m (65 ft) are about 300 to 450m (1,000 to 1,500 ft) per day. These rates are achievable on private projects, such as large buildings, where most of the piles are relatively close together, reducing the rig’s movement between piles.
Lower production rates, such as 60 to 150 m (200 to 500 ft) per day, should be expected for transportation projects where pile groups supporting bridge bents are spread across a large project area.
CFA Piling cost
The costs for CFA piles reflect the high productivity for projects where large numbers of piles are required. Prices for CFA piles are often a few pounds per foot less than driven precast concrete piles of similar size and axial capacity, assuming both pile types meet the project performance criteria.
Interested in learning more about CFA and rotary piling solutions? Speak to our Senior Estimator Toby Lee by emailing him at TobyLee@aarsleff.co.uk
What are driven piles?
Driven precast concrete piles are of square section and produced in short lengths in one-metre intervals between 3 and 14 metres. They are precast so that they can be easily connected to reach the required length. This will not decrease the design load capacity. Reinforcement is necessary within the pile to help withstand both handling and driving stresses. With steel tubular piles or H-Section piles, sections can be welded together to extend the length of the piles or be supplied in the required driven size, which can be up to 36m lengths.
Benefits of Driven:
- Piles can be pre-fabricated off-site, which allows for efficient installation once on site
- Driven piles displace and compact the soil, which increases the bearing capacity of the pile. In contrast, other deep foundations tend to require the removal of soil, which can lead to subsidence and other structural problems
- They are cost-effective as a wide variety of materials and shapes can be easily fabricated to specified dimensions, resulting in the need for fewer piles on site
- They generally have a superior structural strength to other forms of foundation. Their high lateral and bending resistance makes them ideal for challenging conditions such as wind, water, seismic loading, and so on
- Installation usually produces little spoil for removal and disposal
Disadvantages of Driven:
- Advance planning is required for handling and driving, as well as the heavy equipment on site
- Withstand handling stresses during transportation and installation, pre-cast or pre-stressed concrete piles must be adequately reinforced
- Driven piles may not be suitable where the ground has poor drainage qualities
- Driven piles may not be suitable for compact sites, where the foundations of structures nearby may be affected by the vibrations caused by the installation
Driven piling cost
The costs for driven piles are dependent on the type of material (sheet, timber, concrete), the project type, and project size. Typically this means CFA and Driven Piles can be similar in costings, depending on the many variables and scope of the project.
CFA Piles VS Driven Piles Conclusion:
There is no one size fits all answer to piling. Different jobs require different methods to extract earth and drive the piles, whether it be because of location, budget restraints, or other restraints that may affect which method can be used effectively and safely.
Discover your perfect solution today, speak to an expert 01636611140 or email firstname.lastname@example.org with your enquiry and we will be in touch!