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The installation process and methods of pile foundations are critical components of any construction project, paralleling the importance of the design process. Careful consideration and selection of installation methods and equipment are essential to prevent damages to the piles and ensure the structural integrity of the foundation. This article provides a detailed examination of various pile foundation installation techniques, encompassing both displacement and non-displacement methods.
Pile Driving Methods (Displacement Piles)
Pile driving methods, a subset of displacement pile installation techniques, encompass several approaches to achieve proper pile penetration and consolidation. These methods include dropping weight, explosion, vibration, jacking (limited to micro-piling), and jetting.
Drop Hammer Method of Pile Driving
The drop hammer method involves raising a hammer, approximately matching the weight of the pile, to a suitable height before releasing it to strike the pile head. This straightforward approach is particularly suitable for light frames and test piling scenarios where bringing heavy equipment like steam boilers or compressors on-site may not be feasible. Drop hammers come in two main types: single-acting steam or compressed-air hammers and double-acting pile hammers. Single-acting hammers consist of a massive cylinder raised by steam or compressed air and then released onto the pile head. On the other hand, double-acting hammers, which can be driven by steam or compressed air, eliminate the need for a piling frame and are mainly utilized for sheet pile driving.
Figure-1: Pile driving using hammer
Pile Driving by Vibrating
Vibratory hammers, typically powered by electricity or hydraulics, employ contra-rotating eccentric masses within a housing attached to the pile head. The vibration generated by these hammers aids in breaking down the skin friction on the sides of the pile, making them particularly effective in sandy or gravelly soil conditions.
Jetting
Jetting involves using water to assist in the penetration of piles, primarily in sand or sandy gravel. However, its efficacy is limited in firm to stiff clays or soils containing coarse gravel, cobbles, or boulders.
Boring Methods ( non-displacement piles)
Non-displacement pile installation techniques, such as boring methods, offer an alternative approach to foundation construction. Continuous Flight Auger (CFA) is a prominent method in this category.
Continuous Flight Auger (CFA)
CFA equipment consists of a mobile base carrier fitted with a hollow-stemmed flight auger that is rotated into the ground to the required depth. Concrete is then pumped through the flight auger as it is withdrawn, forming the pile. Careful coordination between auger rotation and concrete flow is crucial to prevent soil collapse and ensure proper pile formation. CFA is particularly effective in soft ground conditions and allows for the installation of piles of various diameters across different soil types.
Figure 2: Continuous Flight Auger Process
Underreaming
Underreaming, a unique feature of auger bored piles, involves enlarging the base of the pile to exploit the bearing capacity of suitable strata. This technique is typically employed in stiff to hard clays, such as the London clay, where the soil can stand unsupported. After installation, inspection of the shaft and underream of the pile is essential to ensure structural integrity.
Figure 3: a) Hydraulic rotary drilling equipment b) Continuous Flight Auger, c) underreaming tool open position
The installation of pile foundations requires meticulous planning and execution to ensure the stability and longevity of structures. By understanding and implementing appropriate installation methods, construction professionals can effectively address site-specific challenges and optimize the performance of pile foundations.