What are the types of piles and their construction methods?
by Andrew Lees, on March 11, 2021
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?
- What are pile foundations?
- Where did piling come from?
- Types of piling?
- Methods of constructing piles
- Types of pile foundation
- Installation of piles
- Piling in bridge construction
- Safe working platforms
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.
What are pile foundations?
Pile foundations are long, thin elements generally made of steel or reinforced concrete. They transfer the load through weak, compressible material onto more compact, less compressible stiffer soil or rock at greater depth. The deep insertion of these elements ensures a sturdy base on which the construction project can take place.
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
It’s important to note that the pile foundation and pile are two different things, yet they work in conjunction to ensure that piling can support strong loads. A pile foundation is typically defined as the series of columns that act as the base for structures, whereas the pile itself is the column or cylinder that aids in transferring the load to the lower subsoil.
Construction of pile foundations occurs in many different ways, with a variety of benefits and applications. However, all piles are bored or driven into the ground one way or another. 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 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, also known as replacement piles or drilled piles, are constructed when large holes are drilled in the ground and filled with concrete. Bored piles are very effective as they transfer the load above ground to the deep rock and soil layers below with minimal settlement, which is ideal for supporting structures such as bridges and flood protection.
For some bored 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.
Bored pile construction methods can vary depending on ground conditions. 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.
Rotary bored piling rig
Bored piles vs driven piles
As stated above, driven piles are a classic form of piling technique where a pile is driven into the soil, increasing its bearing capacity. Whereas a bored pile is constructed via drilling holes in the ground and filling with concrete.
The main difference between these two types of piles are that driven piles are formed off site and put in place on-site, whilst bored piles are cast in concrete on-site. Driven piles have the advantage of being rapid to build and use, however they create lots of vibrations, so aren’t suitable at compact sites. Bored piles are favoured as they don’t create this disturbance in soils, have higher bearing capacities and avoid seasonal disturbances such as frost penetration.
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, 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:
- 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.
- The aggregate is forced sideways into the surrounding soil to improve bearing capacity.
- 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.
- 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.
Pile construction and installation
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.
Piling in bridge construction
Piles are often installed alongside bridge abutments to provide the strong foundational support to withstand the weight of the structure above. For bridges constructed on land, the process is very similar to traditional forms of piling.
Constructing a bridge over a body of water can present some unique challenges. For piles to be installed, a watertight enclosure called a cofferdam is constructed. The water is then pumped out to allow for the construction of the piles.
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 the 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.
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