Uncased cast in situ piles are a cost-effective alternative to cased piles, particularly in areas where the soil or water may not disturb the pile hole. While not as efficient as cased piles, they do not require as much equipment or storage space, making them a practical option in many situations.
However, it is important to note that the installation of uncased cast in situ piles requires skilled workers. Additionally, once the installation is complete, no inspection can be carried out, which can be a significant disadvantage in certain contexts. Overall, while uncased cast in situ piles may not be the most efficient option available, they do offer a number of benefits that make them a viable choice in many cases.
Types of Uncased Cast-in-Situ Concrete Piles
Following are the different types of uncased cast-in-situ concrete piles and their uses:
- Simplex pile
- Frankie pile
- Vibro pile
- Pedestal pile
Simplex Pile
Simplex piles are a versatile solution for both soft and hard soils. This type of pile is created by driving a steel tube with a cast iron shoe into the ground to the desired depth. Once the tube is at the correct depth, reinforcement is added to it and then filled with concrete. After this, the tube is slowly removed, leaving the cast iron shoe at the bottom of the pile.
However, attaching a new cast iron shoe for each pile can be time-consuming and labor-intensive. To address this issue, cast iron shoes with alligator jaw points are now used. With the alligator jaw point, the shoe can be removed along with the tube, making the process of creating simplex piles more efficient.
It is essential that the soil can hold fresh concrete after the tube is removed. This is because the success of the pile depends on the integrity of the concrete and its bond with the soil. By ensuring that the soil can hold the concrete, the pile will be strong and reliable, providing a sturdy foundation for whatever structure is being built.
Frankipile
Frankie pile is a type of foundation suitable for granular soils and for soils with a bearing stratum of limited thickness located at a reasonable depth. This type of pile has a unique mushroom-shaped bottom that acts as a spread footing.
To construct a Frankie pile, the process begins by placing a pipe shell consisting of concrete or gravel at the bottom of the pile onto the ground with the help of leads. A drop hammer weighing between 20 to 30 kN is then used with diesel machines to strike the concrete bottom. The entire arrangement is then driven into the ground until it reaches the hard strata.
Once the concrete bottom reaches the hard strata, it expands and gains a mushroom-like shape due to the hammer blows. After that, a proportion of concrete is placed in the tube and ramming is done with the hammer. This process is repeated for every 300-mm layer until the top of the pile is reached.
In cases where reinforcement is needed, the hammer is inserted into the reinforcement cage. Frankie piles have the capacity to carry up to 900kN.
Vibro Pile
Vibro piles are a type of foundation system that can be used in soft soils that offer some degree of frictional resistance. There are two main types of vibro piles: standard vibro piles and expanded vibro piles.
The standard vibro pile is a type of foundation system that is used to transfer loads from a structure to the soil. It consists of a cylindrical steel casing that is inserted into the soil using a vibratory hammer. The soil around the casing is compacted as the casing is driven into the ground, which creates frictional resistance and helps to support the load.
The expanded vibro pile is similar to the standard vibro pile, but it has a wider base. This wider base is created by using a special tool that expands the base of the pile as it is inserted into the soil. This helps to increase the bearing capacity of the pile and provides additional support for heavy loads.
Standard Vibro Pile
The standard vibro pile is a type of pile that has a uniform diameter along its entire length. The installation process involves using a steel tube that is attached to a cast iron shoe and driven into the ground using a mechanical hammer weighing between 20 to 25 kN. The hammer strikes the tube from a height of 1.4 meters at a rate of 40 blows per minute.
Once the required depth is reached, the tube and hammer are connected to extracting links. Concrete is then poured into the tube from the top, and ramming is done using the hammer at a rate of 80 blows per minute. As the concrete fills the tube, the tube moves upwards. In this process, the cast iron shoe remains at the bottom of the pile.
Vibro piles are available in three different diameters – 35 cm, 45 cm, and 50 cm – and can be installed to a length of 25 meters. They are designed to withstand loads of 600 to 700 kN.
Expanded Vibro Pile
Expanded vibro piles are a popular choice when the depth of driving is limited or falls short of the required depth. The technique involves enhancing the soil’s bearing capacity by expanding the diameter of the vibro pile. A steel tube equipped with a cast iron shoe is driven into the ground. Once the tube reaches the intended depth, it is partially filled with concrete, leaving some space. The next step involves removing the steel tube entirely, and attaching a flat shoe to the tube’s bottom.
The flat shoe makes contact with the concrete and applies pressure to it, causing the concrete to compress and bulge, resulting in a bulb-like shape. As this process continues, the tube with the flat shoe penetrates the concrete, creating a deep hole. This hole is where the reinforcement cage is inserted, and the entire arrangement is filled with concrete after removing the tube. With this, the expanded vibro pile is completed, and its effectiveness in increasing soil bearing capacity can be relied upon.
Pedestal Pile
The pedestal pile is another type of foundation similar to the Frankie pile, designed for use in soils that have a limited thickness of bearing stratum at a reasonable depth. Like the Frankie pile, it can act as a spread footing. However, the pedestal pile involves the use of a steel core inside the tube for driving into the soil. Once the pile has been driven to the required depth, the core is removed, and a certain amount of concrete is poured into the space left behind. The core is then replaced on top of the concrete, and the whole tube and core are removed with a sudden burst of pressure. This allows the concrete at the bottom to expand and form a bulb-like shape, known as the pedestal. The process is repeated until the top of the pile is reached, at which point the tube and core are removed, and the pedestal pile is complete.
