Have you ever wondered how those impossibly tall skyscrapers can stand on their own for decades? It’s because of a construction technique that uses drilled shafts. It is a foundation system constructed by filing a cylindrical excavation with reinforced concrete. The holes are then lined with steel liners and casing manufactured by structural steel and fabrication specialists.
What Are Drilled Shafts?
Drilled shafts, also referred to as drilled piers, caissons or bored piles, are deep foundation solutions used to support structures with large axial and lateral loads. This is done by excavating cylindrical shafts into the ground and filling them with concrete.
The support that drilled shafts provide works through a combination of side friction and end-bearing resistance. They range in diameter from 60cm-300cm and can be installed to depths in excess of 9000m.
What Are Casing and Liners?
Casings are tubes made of steel inserted into the drilled shafts to stabilise them. They are handy when ground conditions are so unstable that drilled holes cannot safely be stabilised using drilling slurry. Casings can be temporary or permanent steel pipes, which provides 100% stable excavation for the drilled shaft’s entire length.
Liners are light in weight and become a permanent part of the foundation. Corrugated sheet metal pipe (CMP) is the most common material used for making liners, but they can also be made using plastic or pressed fibres. While liners aren’t used much on drilled shafts compared to casings, they can still be an essential component in some situations.
Types of Casings
Casings come in two different types, namely temporary and permanent. The type of casing or liner used in the project depends on the number of factors:
As the name suggests, temporary casings are only used during the construction of drilled shafts. Once the shaft is considered stable, either during or after the placement of fluid concrete, the casing is then removed.
Contractors and structural steel and fabrication specialists ensure that the temporary casing is free of soil, lubricants, and other materials. This ensures that there is little to no shearing resistance between the casing and the column of fluid concrete placed inside it.
Permanent casings are installed similarly to temporary casings but are instead designed to become a permanent foundation. They are often used for drilled shafts that extend through very soft soils to reach a more stable underlying stratum. The casing serves as a barrier between the fluid concrete and the surrounding soft soils. This type of casing is also often used when a drilled shaft happens to be installed through a body of water. The protruding portion of the casing is then used as a form where the fluid concrete will be poured into.
Using permanent casings in building foundations can contribute to the drilled shaft’s structural capacity and bending stiffness. However, steel casing will start to corrode over time, affecting the foundation’s structural capacity.
Whether you use temporary or permanent casings on your project, one thing is certain—casings are an integral part of the construction of drilled shafts. Contractors should familiarise themselves with the pros and cons of using these casing methods as it could significantly impact the strength and stability of the shaft itself.
Phoenix Reinforcing is one of the longest operating reinforcing companies in the Sunshine Coast. If you’re looking for a structural steel specialist in Queensland, there’s no other company to go to than Phoenix Reinforcing. Contact us today to get a free estimate.