What are piled foundations?
If standard strip or trench fill foundations aren’t suitable for your site, deep piled ones may be needed instead. Tim Doherty takes a closer look at how these work
On sites where standard strip or trench fill foundations aren’t appropriate, piling may be the best solution
If you have ever wondered how tower buildings stand up, then part of the answer could be piled foundations. For instance, the world’s tallest building, the Burj Khalifa in Dubai (829m high) used 45,000 cubic metres of concrete to construct 192 piles, each buried 50m into the ground. These columns are driven into the earth to transfer the weight of the building through less stable sub soils until sufficient weight bearing strata is encountered.
Whilst self built homes in the UK are unlikely to be more than three to four storeys, the same principles can apply, meaning piled foundations are often the ideal engineered solution.
When are piled foundations needed?
You’ll need a qualified assessment or a detailed soil investigation to let you know exactly what to expect in terms of the best foundation solution for the ground on your site. Generally speaking, the type of land conditions that could give rise to piling include: Unconsolidated filled materials, artificially imported, which are very difficult to properly compress. Naturally formed and highly compressible subsoils, like peat. Low bearing capacity subsoils which could include wet clay or loose and unstable sand.
High water tables that are capable of flushing subsoils, which leads to the risk of erosion and/or sink holes. Subsoils with a propensity for volume change, such as highly shrinkable clay.
How do they work?
If you are unlucky enough to find one of the above conditions on your site, then your first port of call should be a civil or structural engineer. Your building might have been designed with ordinary strip or trench fill foundations in mind, but the line loads and building weight are easy for the engineer to calculate from your architectural plans. How these are then transferred into the ground will be based on a designed number of piles, their depths, widths (diameter), type, specific layout and how they are all ultimately connected together.
For domestic projects there are essentially two different types of piles. Those that transfer load directly to solid strata are end bearing piles. Where solid strata cannot be relied upon, there’s a type that transfers load through shear strength (the overall surface area of pile and subsoil contact underground), known as friction piles. Your engineer’s job is to deliver the most appropriate designed solution based on your soil conditions using the most practical installation technique in terms of how much it costs. Accessibility and the proximity of neighbouring buildings will also play an important part in the design.
Replacement or bored piles
Piles can be either bored or driven into the ground. Bored types physically remove the subsoil spoil, creating a cylindrical tube into which concrete can be poured, so the technique is effectively replacing subsoil in favour of concrete. It is often used on sites that are in close proximity to other buildings as the excavation process is more controlled, not so noisy and causes much less ground vibration.
Boring is frequently used for friction piles as this method suits cohesive and firm soils. However, it’s an expensive option because of the various on-site processes involved. Friction piles could need temporary casings, excavation, management of the spoil, concrete pouring, rebar cage production, insertion etc, plus all of the associated equipment required for handling each process.
In terms of excavation methods for bored replacement piling, there are further choices to make. The first option is rotary drilling with a wide bladed auger head, which can remove the spoil as the drill penetrates the ground in a twisting motion (think of a flat bladed cork screw). The second would be a percussion borer that uses a hammer action to drive a tube with a cutting blade through the ground, which then fills with spoil for intermittent extraction. The third is a flight auger, which has a continuous helical cutting blade that removes the full depth of the pile spoil in one process. All excavations may need temporary tube linings to help keep the pile excavation from inwardly collapsing (depending on subsoil type) until the concrete is poured. These linings would be removed before the concrete has cured.
Once the excavation is complete, concrete is poured into the pile and then (usually) a reinforcement cage is driven into the wet concrete before any tube linings are then withdrawn. Alternatively, the rebar cage could be inserted before the concrete is poured in, but with a suitable slump and/or vibration to ensure all potential voids are properly filled.
The flight auger technique can also include grout injection, where the continuous auger blade has a hollow tube down its core through which the concrete can be poured. Once the flight auger has reached its depth, concrete is poured as the auger (and spoil) is withdrawn all in one process, which may remove the need for any temporary pile casings.
Where pile diameters are generally quite wide, sometimes a bentonite slurry will be used (basically a mixture of clay and other semi-hardeners) in order to temporarily protect the pile shaft sides from collapse until the concrete can be poured.
Displacement or driven piles
The alternative to boring would be displacement piles, which are driven into the ground with hydraulic rams and hammers. There is no excavated material to manage and instead the pile displaces the subsoil as it travels below ground. Some pros consider this to be a cheaper option and with guaranteed results, as the piles are manufactured off-site to predetermined levels of quality. For shallow depths, piles could be timber (much of Venice is founded on timber piles) but concrete or steel are more likely, certainly for deeper depths.
Concrete piles are manufactured in specific lengths (generally up to around 10m), with the bottom one fitted with a spiked cap in concrete or steel. Sections are then jointed together with a splicing collar until the full predetermined depth is reached.
Alternatively, some depths may be determined as sufficient enough when the effort required to drive the pile becomes too hard, confirming that the bearing capacity of the subsoil had been reached.
The piles are driven by percussion action, which sets the speed and frequency of weighted blows to the pile top (this must be protected by a helmet to preserve its integrity).
Steel piles come in a variety of sizes; some are cylindrical and these will usually be filled with concrete. Others could be H section (like a universal column) or of a box shape.
Pile caps, ring beams & floors
Whatever pile type has been selected, it will need to be integrated into the building’s superstructure at ground floor level. Caps are built at the top of each pile in reinforced concrete, which are then tied together via a series of reinforced concrete ring beams. These then support either an integrated reinforced concrete slab or a suspended floor of one kind or another. On sloping ground the pile caps might end up sitting quite high out of the ground and this resultant void will then be ventilated just like any other suspended floor.
How much will it cost?
There are no standard costs for piled foundations as the price is always calculated on a case-by-case basis, and must be calculated according to specific details and requirements provided by the engineer. However, deep 3m trench fill foundations can be very expensive and piling is often considered a serious alternative at these depths for both engineering and financial reasons.