As piling contractors, we often get asked the question:
“How deep do pilings need to be?”
In all honesty, there’s not a straightforward answer. The depth of pile foundations will depend on a variety of factors, including the pile foundations types used.
Pile foundations are an example of deep foundations and were first used for large constructions. However, since smaller piles have been developed, such as mini piles, we are now able to use this foundation for domestic buildings.
Pile foundations provide support for structures. They do this by transferring their load to the surrounding layers of soil or rock that have sufficient bearing capacity. The heavier the load above ground, the deeper the foundations need to be. There are other factors that will also affect this, they include:
- The ground or soil conditions
- Proximity to other structures
- type of piles used
Different structures require different types of piling foundations, and you can even read about the piling secrets of the UK’s most famous buildings.
How Deep Do Pile Foundations Go?
Pile foundations go deep enough to transfer the load of a structure into soil or rock with sufficient bearing capacity. This depth varies from site to site.
On some projects, piles may only need to extend a few metres. On others, especially where surface soils are weak, variable or made ground is present, piles may need to go significantly deeper to reach suitable bearing strata.
Piling depth is determined by:
- Ground conditions and soil type
- Structural load and layout
- The piling method being used
Why Piling Depth Varies
It’s common to want a clear number early on, but piling depth varies for practical reasons.
Ground conditions are the biggest factor
Different soils behave very differently under load:
- Firm clay or dense gravel may support loads at shallower depths
- Soft clay, peat or made ground often requires deeper piling
- Variable ground can result in different pile depths across the same site
The aim is always to avoid settlement by transferring loads into stable ground.
Structural load also affects depth
Depth may increase where:
- Buildings are larger or multi-storey
- Loads are concentrated in specific areas
- Extensions add significant weight to existing structures
A structural engineer calculates these loads and designs pile foundations accordingly.
How Soil Type Affects Piling Depth
Often, the type of soil will determine the type or depth of the piling needed. Some soil types you may encounter include:
Clay soils
Clay can shrink and swell with moisture changes. Where clay is soft or affected by trees, piles often extend below the zone of seasonal movement.
Sandy soils
Loose sand may require deeper piles to reach denser layers with better load-bearing capacity.
Gravel
Dense gravel can provide good support at shallower depths, though loose gravel may still require deeper piling.
Made ground and infill
Made ground is unpredictable and usually unsuitable for foundations. Piles typically bypass it entirely.
Peat or very soft ground
These soils have very low bearing capacity, so piles usually need to extend significantly deeper.
Less stable soil types, such as sand, peat or clay are likely to require deeper pilings. It can be difficult to know the soil type you’re working with simply by looking at it. Contact your local building control department for information on the types of soil often found in your area.
Pile foundations types
Driven Piles
Driven piles are often used to support buildings, towers, walls and bridges. The depth of driven piles depends on the size of the structure and material, but below are some examples of types of driven piles and their maximum lengths:
- Steel
- H Section Piles – Often used in marine structures and can reach up to 36m in length.
- Screw Piles – Less soil displacement because of the ‘screwing’ method. Can reach up to 24m in length.
- Tube Piles – Used in heavy load support.
- Concrete
- Widely used piles due to versatility and suitability to most ground types. They can reach up to 30m in length.
- Timber
- Can be installed on smaller properties with restricted access. Can reach up to 12m in length.
Bored Piles
Bored piles (also known as replacement piles) are a type of piling method where the soil is removed from the ground in order for the hole to be filled with concrete, which is poured in situ. They can reach up to around 40 meters in length and are used for high buildings with heavy vertical loads.
Mini Piles
A mini pile generally has a diameter of between 100 and 600mm but can extend to depths of over 50m. They can be used in most types of soil and are often used for underpinning or strengthening existing structures. Mini piles are generally used for areas where access is restricted or confined spaces.
Geothermal Piles
Geothermal energy is a sustainable source of natural heat energy that is stored in the earth, and geothermal piles not only provide support to a building but also act as a heat source. The depth of geothermal piles depends on the ground conditions.
What Happens If Pilings Aren’t Deep Enough or Are Too Deep?
If piles don’t reach suitable bearing strata, long-term settlement and movement can occur. This is rare where ground investigation, engineering design and monitored installation are in place.
Piles can also be over-designed. Excessive depth increases cost and programme time without improving performance. Good design balances safety and efficiency.
Engineers optimise depth using investigation data, load calculations and appropriate safety margins.
Do I Need Foundation Piling for my Extension?
Not every extension needs piling, but many do.
When piling is likely to be required
- Shallow existing foundations
- Weak or variable soil conditions
- Two-storey or heavily loaded extensions
- Proximity to boundaries, drains or neighbouring buildings
When piling may not be needed
- Good bearing soil close to the surface
- Lightweight, single-storey extensions
- Stable and consistent ground conditions
The decision is made by a structural engineer and confirmed through building control.
Often, a simple house extension won\’t need deep piling. Instead, strip foundations, raft foundations or trench-fill foundations will suffice. However, in certain situations where the soil is unstable, you may need piling foundations. A good indication of this is to look at your existing foundations. In most cases, your house will simply require an extension of the current foundations.
Conclusion
Without visiting a site, it\’s impossible to advise on the type and depth of piling needed. As specialists in piling foundations, we provide unmatched advice and expertise to projects across the UK and Ireland. To discuss your options and arrange a site visit with a qualified piling contractor, please get in touch with us. Our foundation experts will assess ground conditions and provide the most appropriate and cost-effective solutions.