What are the types of piles and their construction methods?

by Andrew Lees, on 11-Mar-2021 04:19:00

Piling is one of the cornerstones of the construction industry today and indeed, it has been so for centuries – if not millennia. The function of piles is quite simply to transfer loads from a structure or building, into the ground.

In this guide, we’ll first explore what piling is and where it came from, then consider the two types of pile foundation, the methods of constructing piles, the installation of piles and finally the role of safe working platforms in supporting piling rigs.

Use the links below to jump to the section you’re most interested in:

What is piling in construction?

Piling is the process of driving or boring pile foundations into the ground beneath a building that is under construction. These piles transfer loads from the structure to the ground, helping to support it. Pile foundations are often used where the ground is too weak to underpin the structure.

Read on to learn more about the different types of piling and the methods of constructing piles.

Where did piling come from?

As with many modern techniques, our high-tech piling systems today grew out of a clear need, hundreds and even thousands of years ago, that still exists today. Some of the first recorded examples of piling include river dwellers in Switzerland 6000 years ago, building structures on piled foundations to defend against flooding and attack, by elevating their dwellings. The Romans – inevitably – led the way with piles for shore works in Europe, and in the UK, there is proof that they used timber piles driven into the riverbed for bridges in London and Corbridge, over 2000 years ago.

In early civilisations, cities and towns wanted to build close to rivers for communication and protection, so the swampy soil needed to be fortified and strengthened with something – which is how and why piling was born. Timber piles were driven into the ground by hand, or holes were dug and filled with sand and stones. In more modern times, the ever-increasing need for buildings and infrastructure has forced authorities and development agencies to exploit land with poor soil characteristics. Piling is – as it always was – vital in our modern-day construction industry.

What are the types of piling?

The two main types of piling are end bearing and friction piles:

  • End bearing piles transmit loads directly to a deeper strong soil or rock layer some way below the surface.
  • Friction piles transfer load by friction between the surrounding soil and the surface of the pile over its full length.

Methods of constructing piles: types of pile foundation

There are a four main types of pile foundation, each with its own method of construction:

Read on to learn about these different pile foundation types, or use the links above to navigate to a particular type.

Driven piles

Driven piles are the classic type of pile foundation. They can be constructed with timber, a technique centuries old and still used across the globe. In the UK, timber piling is used mainly for coastal works, sea defence and jetties. Another option is precast concrete piles, reinforced to withstand driving stresses, they are usually pre-pressed with a square or octagonal section. Lastly, there are steel piles of tubular, box, or H section. Interlocking steel sheet piles are also widely used, predominantly for wall construction.

With each of these materials, 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. For this increase in soil strength to take place, the pore water pressures must dissipate by rapid drainage. This type of piling is not suitable for saturated or silty soils as they drain slowly and can not be compacted in the same way.

Bored piles

Bored piles are constructed when large holes are drilled in the ground and filled with concrete. For some piles, the bottom of the hole is widened or under-reamed, creating a bulb at the end. A steel reinforcement cage is lowered into the hole before the concrete is placed or dropped in after the concrete has been poured.

Within the category of bored pile foundations, the two commonly used subtypes are rotary bored piling and Continuous Flight Auger (CFA) piling:

  • Rotary bored piling is 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’ (ground and rock removed via boring process) are removed from inside the casing, then a steel reinforcement cage is installed. At this point, the hole is filled with concrete and the casing is removed.
  • Continuous Flight Auger (CFA) piling does not require the use of temporary casing – it is the most versatile, effective and commonly used type of bored pile foundation in the UK. Once the hole has been bored, concrete is pumped in, then a steel reinforcement cage is inserted.

piling-mat (1)Rotary bored piling rig

Driven and cast in-situ piles

Driven and cast in-situ piles combine the advantages of both piling construction methods.

One option is a permanent casing type, where a tubular casing (or shell) made from reinforced, corrugated thin steel is driven into the ground using a mandrel inserted into the casing. The mandrel is then withdrawn, leaving the casing in place. Finally, concrete is poured into the casing, forming a steel/concrete composite pile.

Another version of this type of pile foundation uses a temporary casing. Known as the Franki type, this is where a steel reinforcement cage is lowered into the casing, which is then withdrawn as dry concrete mix is being placed. The concrete is compacted, and some is forced out of the bottom of the casing, forming an enlarged bulb which increases the pile bearing capacity.

Aggregate piles

Aggregate piles, or stone columns, mean that compacted aggregate is used to form the pile, rather than concrete. This type of pile foundations works using the following process:

  1. A casing is inserted as the hole is bored, and aggregate is dropped into it in layers which are then vibrated or compacted as the casing is withdrawn.
  2. The aggregate is forced sideways into the surrounding soil to improve bearing capacity.
  3. Vibro-type aggregate piles create densely compacted columns made from gravel – or a similar material – using a vibrating casing. The displacement process densifies the surrounding granular soils.
  4. With Geopier-type aggregate piles, aggregate is rammed into a casing, forcing it out the bottom to create a dense bulb. This is repeated in stages as the casing is withdrawn, densifying, and improving the strength of the surrounding soil.

Installation of piles

Most piling rigs are track mounted and are specialist plant items, built to install one type of pile. Driven piles are hammered, jacked or vibrated into the ground using a percussion hammer, hydraulic driver or rams (typically used for sheet piles), or diesel-powered vibratory drivers used to reduce surrounding ground resistance and allow the pile to slide into the ground.

Bored piles use a continuous auger, very similar to a very large drill bit, between 300mm and 900mm diameter. For large diameter piles, a circular bit up to 1200mm diameter is used. Rigs are large with weights ranging up to 180 tonnes.

Safe working platforms

Piling rigs are produced in a range of sizes, depending on the type and depth of pile required. Rigs are required to move about and position accurately for each pile location. The area on which the rigs operate is known as the Piling platform. Platforms are usually formed by compacting a layer of coarse, granular fill, to a specified depth. The appropriate fill might be brought on to site or crushed, demolition rubble from site can be used for brownfield developments.

The piling platform is required to distribute the load from the tracks and avoid bearing failure, minimising settlement to ensure safe operation of the rig. Working platforms must be properly designed by a qualified engineer, using soils data and piling rig loading for the specific site. Platforms need to be properly constructed and regularly inspected and maintained by the contractor.

The Federation of Piling Specialists (FPS) state that one third of all Dangerous Occurrences reported by their members are related to piling platforms. They warn "every Dangerous Occurrence and every 'near miss', involving the platform is a potential fatality". The Principal contractor for the site is responsible for the design and construction of the piling platform and they will often contract design to a professional engineer.

Crane toppled over demonstrating the importance of safety
Photo credit: www.heavyliftnews.com

The FPS operates a certification system for working platforms and the Principal Contractor is required to sign the certificate confirming the platform has been properly designed and correctly constructed in accordance with the design, and that it will be regularly inspected and maintained. A guide to good practice, BRE Report BR470 was published in 2004 and subsequently reviewed to consider designs incorporating geosynthetics.

Tensar has developed its T-Value Method for the design of working platforms that delivers safe economic designs. Tensar International also offers design software incorporating T-Value method for use by engineers designing piling platforms, as well as offering a full piling platform design service to contractors.

To find out more you can download our Tensar Academy resource on the T-Value Method.

Got a burning question about geotechnical engineering?

Why not drop us a line and the answer to your question may feature in a future episode of our Ground Coffee blog. Email academy@tensar.co.uk