Pile foundations are a common choice for construction projects that involve heavy loads and weak soil layers. These foundations are classified as deep foundations since they transfer the weight of the structure to a more solid layer of soil through the weaker strata. Piles can either be constructed on-site or pre-casted and then transported to the construction location.
Cast-in-situ pile construction involves drilling a borehole, installing a reinforcement cage, and filling it with concrete. Various pile construction techniques are available, including rotary drilling with a kelly, twin rotary drilling, and auger cast-in-situ. These methods differ in terms of the drilling process, the way the excavation is supported, and the concrete placement technique.
When selecting a piling technique, a range of factors must be considered, including the soil conditions, the availability of necessary equipment and skilled workers, the technical requirements of the site, and the overall cost. Ultimately, the chosen method should be suitable for the specific project and its requirements.
Construction Techniques for Cast-in Situ Reinforced Concrete Pile
Rotary Drilling with Kelly, Borehole Supported by Casing
A method for excavating a borehole involves the use of rotary drilling and a supporting casing. The casing is a steel tube that is slightly larger in diameter than the pile being excavated. Before the drilling begins, the casing is driven into the ground using a rotary drive. For larger piles and deeper excavations, a casing oscillator may be utilized. This technique is suitable for all soil types and can be used in constrained construction sites. One of its main advantages is that it doesn’t produce any vibrations during the drilling process. The diameter of the excavated pile can range from 60 cm to 180 cm, with depths reaching up to 40 m and even greater depths being possible.
Rotary Drilling with Kelly, Borehole Supported by Hydrostatic Pressure
The technique used in this construction method involves using rotary drilling to excavate a borehole. However, to support the wall of the borehole, either bentonite solution or polymer slurry is utilized. At the start of the excavation process, a short-length casing is used to guide the excavator, and a bucket is employed to extract the excavation spoil. This construction method is suitable for all types of soils, large pile diameters and depths, and vertical piles. Additionally, the drilling operation is free from vibrations, allowing for a deeper pile depth of around 40 meters or more, with a diameter ranging from 40 centimeters to 240 centimeters. Figure-2 provides further details of the construction procedure.
Rotary Drilling Using Twin Rotary Head
Contractors can use counter-rotating twin drives to simultaneously carry out continuous flight auger drilling and casing installation. This technique is suitable for all types of soil and can be utilized in constricted sites, even those with existing walls. The construction machinery used in this technique generates no vibration, which is particularly advantageous in urban areas. The pile’s depth can reach approximately 15 meters, and its diameter falls between 63 cm and 88 cm.
During the concrete pouring process in this technique, the auger is lifted. Consequently, the reinforcement cage is placed after the concrete placement process is complete. However, this means that a vibration or driven force is necessary to place the reinforcement cage.
Auger Cast-In-Situ Pile
The continuous auger technique is utilized to drill boreholes, lift the excavated soil, and pour concrete while moving the auger upwards. This process is known as auger cast-in-situ pile, and it eliminates the need for casing or hydrostatic pressure to prevent damage to the borehole walls. This method is well-suited for all types of soil and construction sites with limited space, as it is vibration-free and can achieve depths of up to 18 meters. The diameter of the pile created by this technique can range from 40 cm to 100 cm. Overall, auger cast-in-situ pile is an efficient and effective method for creating piles in various construction projects.
Displacement Pile Construction
Displacement pile is a pile construction technique that offers several advantages. It is particularly useful in cases where the soil is contaminated, as it prevents the need to dispose of drill spoil. It is also a suitable option for construction sites that have limited space or soft soil.
One of the primary benefits of displacement piles is their high bearing capacity, which is achieved by compacting the skin soil. This technique helps to avoid drilling spoil, resulting in a cleaner and more efficient construction process. Displacement piles also offer high performance and can be constructed with reduced concrete usage, making them a cost-effective solution.
The drilling diameter for displacement piles can vary between 42 cm, 51 cm, and 61 cm, depending on the specific requirements of the project. The depth of the piles typically ranges around 34 meters, making them suitable for a range of different applications. Overall, displacement piles offer a unique and effective approach to pile construction, which can help to optimize the construction process and reduce costs.
Grab Construction, Borehole Supported by Casing
Grab pile construction involves the use of a crawler crane and casing oscillator. This technique is typically used for soils that are well-suited to it, such as sand and gravel, and it requires casing technology. It is also appropriate for locations where the use of a chisel to break bedrock and boulders is economically feasible.
One thing to keep in mind when using grab pile construction is that a minimum distance from existing structures is necessary, as this technique does not significantly affect the surrounding area. The diameter of the pile can vary from 62 cm to 200 cm, and it can be as deep as 50 m.
FAQs
What is a cast-in-situ pile?
A cast-in-situ pile is a type of pile that is constructed entirely on the project site. The process of building this type of pile involves several steps that are carried out in sequence. Firstly, a borehole is drilled into the ground at the location where the pile is to be constructed. Following this, reinforcement cages are fabricated and placed into the borehole. Finally, concrete is poured into the hole to complete the pile construction.
To save time, certain steps of the pile construction process can be carried out simultaneously. For example, while the borehole is being drilled, the reinforcement cages can be fabricated on site. This can help to reduce the overall construction time and improve the efficiency of the pile construction process.
What are the methods used to support boreholes in pile construction?
- Casing:
Casing refers to a type of pipe that is used in oil and gas drilling. It is typically made of steel and is inserted into the wellbore to protect the drilling equipment and to provide support for the well structure. Casing is an essential component of oil and gas drilling as it helps to prevent the well from collapsing and also serves as a conduit for the extraction of oil or gas.
- Hydrostatic pressure:
Hydrostatic pressure refers to the pressure that is exerted by a fluid at rest. This type of pressure is caused by the weight of the fluid and can be calculated using the equation P = ρgh, where P is the hydrostatic pressure, ρ is the density of the fluid, g is the acceleration due to gravity, and h is the depth of the fluid. Hydrostatic pressure is an important concept in a variety of fields, including fluid mechanics, engineering, and geology. It is used to determine the stability of structures such as dams, bridges, and buildings, as well as to understand the behavior of fluids in different environments.
What are the construction techniques used for cast-in-situ piling?
- a long, hollow steel pipe known as a Kelly. The Kelly is attached to a drill bit at its lower end and is rotated by a power source, typically a diesel engine. As the Kelly turns, the drill bit cuts through the ground, and the cuttings are brought to the surface by circulating drilling fluid, which is pumped down through the Kelly and up through the annular space between the Kelly and the borehole wall. Hydrostatic pressure from the drilling fluid helps to support the borehole and prevent cave-ins.
- In twin rotary head drilling, two drill bits are used instead of one, and each is driven by its own motor. This allows for faster drilling and more efficient cuttings removal. The twin rotary head drilling method is often used in harder or more complex geological formations where traditional rotary drilling methods may be ineffective.
- Auger cast-in-situ piles are a type of deep foundation that are constructed by drilling a hole into the ground using a continuous flight auger. The auger is rotated and advanced into the ground, and a fluid grout mixture is pumped through the hollow stem of the auger as it is withdrawn from the hole. The grout fills the void left by the auger and forms a pile that is reinforced with steel reinforcement.
- Displacement pile construction involves driving a hollow steel tube or pile into the ground using a drop hammer or hydraulic press. As the pile is driven into the ground, the soil is displaced radially outwards, creating a void that is then filled with concrete or grout to form the finished pile.
- Grab construction, also known as shell and auger drilling, involves the use of a clamshell or grab bucket to excavate soil or rock from the ground. The grab bucket is lowered into the ground and closed, and then raised to the surface to remove the excavated material. Casing is often used to support the borehole and prevent cave-ins. Grab construction is typically used for smaller diameter boreholes and in soils that are not too hard or rocky.
What are the factors that influence the selection of the piling technique?
The choice of piling technique is influenced by several factors, including the soil conditions at the construction site, the availability of appropriate equipment and skilled labor, the technical specifications of the project, and the overall cost considerations. These factors play a crucial role in determining the most suitable piling method to be used.
Soil conditions are a key factor in selecting the appropriate piling technique. Different types of soils, such as clay, sand, or rock, require different methods of piling. For example, cohesive soils like clay may require bored pile techniques, while cohesionless soils like sand may be better suited for driven pile methods. The geotechnical characteristics of the soil, such as its bearing capacity, shear strength, and settlement potential, also impact the choice of piling technique.
The availability of suitable equipment and skilled labor is another important consideration in selecting a piling technique. Certain piling methods may require specialized equipment or highly trained personnel, which may not be readily available at all construction sites. Factors such as the project timeline and location may affect the availability of equipment and labor, and may influence the choice of piling technique.
Technical specifications of the site, such as the structural requirements and design considerations, also play a significant role in determining the appropriate piling technique. Factors such as the required load-bearing capacity, the depth of the foundation, and the presence of any underground utilities or structures may impact the selection of the piling method. Compliance with local building codes, regulations, and environmental requirements also need to be considered in the decision-making process.
Finally, the overall cost of the project is a crucial factor in selecting a piling technique. Different piling methods have varying costs associated with materials, equipment, labor, and construction time. The budget and financial constraints of the project may influence the choice of piling technique, as it needs to be economically feasible within the project budget.
In summary, the selection of a piling technique is a complex decision that depends on various factors, including soil conditions, availability of equipment and labor, technical specifications of the site, and the overall cost considerations. All these factors need to be carefully evaluated to determine the most appropriate and cost-effective piling method for a particular construction project.