Choosing the right piling method is one of the first major decisions on any London project with poor ground, heavy loads or a basement. The two methods we are asked about most often are mini piling and CFA (continuous flight auger) piling. Both transfer building loads through weak or variable soils into stronger strata, but they suit very different sites. This guide compares them side by side so you can understand what your structural engineer is likely to recommend, and why.
What Is Mini Piling?
Mini piles are small-diameter piles, typically 150 mm to 300 mm in diameter, installed with compact rigs. Because the equipment is small and light, mini piling is the go-to method where access is tight, headroom is limited or the work is taking place inside or right next to an existing building.
Common mini pile types include:
- Driven steel-cased piles – steel tubes driven to a set resistance, then filled with reinforcement and concrete. No spoil is produced.
- Augered (rotary bored) mini piles – bored with a small auger, reinforced and concreted.
- Mini CFA piles – a smaller-diameter version of the CFA method for sites that need low vibration but have limited access.
Typical uses include underpinning, house extensions, garden rooms, internal works, basements under existing properties and repairs to buildings affected by subsidence. You can read more about the method in our article on mini piling in London.
What Is CFA Piling?
CFA piling uses a hollow-stemmed continuous flight auger that is screwed into the ground to the design depth. As the auger is withdrawn, concrete is pumped down through the stem, so the bore is never left open. The reinforcement cage is then pushed into the wet concrete.
CFA piles are usually 300 mm to 900 mm in diameter, with larger sizes available, and can reach depths of 25 metres or more. The rigs are substantially larger than mini piling rigs and need good access and a prepared working platform.
Typical uses include new-build houses and apartment blocks, commercial developments, contiguous and secant piled walls for basements, and heavily loaded structures such as RC frame buildings.
Mini Piling vs CFA Piling: Side-by-Side Comparison
| Factor | Mini piling | CFA piling |
|---|---|---|
| Typical diameter | 150–300 mm | 300–900 mm (larger possible) |
| Load capacity per pile | Lower – more piles often needed | High – fewer, larger piles |
| Site access | Narrow gates, gardens, inside buildings | Open sites with vehicle access |
| Headroom | Low-headroom rigs available | Needs full height clearance |
| Working platform | Light preparation usually enough | Engineered working platform required |
| Noise and vibration | Low (augered) to moderate (driven) | Low |
| Spoil | None (driven) or small volumes (augered) | Significant spoil to remove |
| Speed per pile | Slower per metre of capacity | Fast installation per pile |
| Best suited to | Domestic, underpinning, restricted sites | New builds, commercial schemes, piled walls |
When Mini Piling Is the Right Choice
- Restricted access: terraced houses, rear gardens reached through a side passage, or sites with no space for a large rig.
- Low headroom: work inside existing buildings, under floors or beneath overhead obstructions.
- Underpinning existing foundations: mini piles combined with ground beams or needles transfer load from shallow, failing foundations to deeper stable ground.
- Sensitive neighbours: augered mini piles keep vibration low next to party walls and older masonry.
- Moderate loads: extensions and residential structures where loads can be shared across a larger number of smaller piles.
When CFA Piling Is the Right Choice
- Higher loads: multi-storey buildings and RC frames where fewer, larger piles are more economical.
- Open sites: new-build plots and cleared sites where a large rig, concrete pump and lorries can operate safely.
- Basement retaining walls: contiguous or secant piled walls formed with CFA piles support excavations for new basement construction.
- Waterlogged or unstable soils: because the bore is supported by the auger throughout, CFA works well in ground where an open bore would collapse.
- Low vibration with high output: CFA combines quiet installation with fast progress on larger schemes.
Ground Conditions in London
Much of London is underlain by London Clay, often with sands, gravels and made ground above it. Both methods work well in these conditions, but a few London-specific issues affect the choice:
- Clay heave and shrinkage: near trees, or where trees have been removed, clay can swell or shrink. Piles and ground beams may need heave protection, such as compressible void formers and sleeved piles.
- Made ground and obstructions: old foundations, rubble and buried services are common. Driven mini piles can struggle with obstructions, so pre-augering or an alternative pile type may be needed.
- Groundwater: water-bearing gravels favour methods that keep the bore supported, such as CFA.
A site investigation with boreholes is the only reliable basis for pile design. Guessing the ground conditions is the fastest way to over-design, or under-design, a piling scheme.
What Drives the Cost of Each Method?
Comparing price per metre alone is misleading. The real cost of a piling package depends on:
- Number of piles: mini piles carry less load each, so more of them are needed.
- Pile depth and diameter: set by the loads and the ground investigation.
- Mobilisation: large CFA rigs cost more to bring to site, which makes them less economical on small jobs.
- Working platform: CFA rigs need an engineered platform, which adds cost and programme time.
- Spoil disposal: CFA produces large volumes of spoil, and contaminated material costs more to remove.
- Testing: integrity testing and load testing add to the package but are often required by the engineer or warranty provider.
As a rule of thumb, mini piling is usually more cost-effective on small domestic and restricted sites, while CFA becomes more economical as loads, pile numbers and site size increase. For typical London price ranges, see our piling cost guide for London.
Working Platforms and Site Safety
Tracked piling rigs are heavy and have a high centre of gravity. For CFA and other large rigs, an engineered working platform must be designed and certified before piling starts, usually following BRE guidance. Mini piling rigs are far lighter, but ground stability, buried services and the proximity of excavations must still be checked. At G8 BUILDS, platform design, temporary works and services clearance are planned as part of our groundworks package, so the piling contractor arrives at a site that is ready to work.
Testing and Quality Control
Whatever the method, piles should be installed and checked against a recognised specification, typically the ICE Specification for Piling and Embedded Retaining Walls. Common checks include:
- pile installation records (depth, concrete volumes, reinforcement),
- integrity testing to identify defects along the pile shaft,
- static or dynamic load tests to confirm capacity.
Good records matter: building control, warranty providers and future buyers may all ask for them.
How G8 BUILDS Delivers Piling in London
G8 BUILDS delivers piling services in London as part of a coordinated substructure package, including pile caps and ground beams, underpinning, basements and RC frames. Working with your structural engineer, we help select the piling method that meets the design loads at the lowest overall cost, not just the lowest rate per metre.
If your project involves a basement, also read our guides to basement waterproofing to BS 8102 and party wall agreements. Ready to discuss your site? Contact our team for a free site assessment and quotation.
Frequently Asked Questions
Is mini piling cheaper than CFA piling?
On small, restricted domestic sites, mini piling is usually cheaper because mobilisation and platform costs are lower. On larger sites with higher loads, CFA is often cheaper overall because fewer piles are needed.
Can CFA piling be used next to existing buildings?
Yes. CFA piling produces low vibration, so it is often used close to neighbouring structures, provided there is enough space and access for the rig and the working platform.
Which piling method is used for underpinning?
Underpinning in London is most commonly carried out with mini piles, combined with ground beams or needle beams, because the rigs can work in confined spaces and low headroom.
Do I need a site investigation before piling?
Yes. A ground investigation with boreholes is needed so the structural engineer can design pile type, diameter and depth. Without it, the design relies on assumptions, which increases both risk and cost.
