The installation process and methods of pile foundations are equally important factors as of the design process. Pile foundation installation methods are by pile hammer and boring by mechanical auger.
To avoid damages to the piles, during design, installation Methods and installation equipment should be carefully selected.
If installation is to be carried out using pile-hammer, then the following factors should be taken in to consideration:
- Size and the weight of the pile
- Driving resistance which has to be overcome to achieve the design penetration
- Available space and headroom on the site
- Availability of cranes and
- Noise restrictions which may be in force in the locality.
Pile Driving Methods (Displacement Piles)
Methods of pile driving can be categorised as follows:
- Dropping weight
- Jacking (restricted to micro-piling)
Drop Hammer Method of Pile Driving
A hammer with approximately the weight of the pile is raised a suitable height in a guide and released to strike the pile head. This is a simple form of hammer used in conjunction with light frames and test piling, where it may be uneconomical to bring a steam boiler or compressor on to a site to drive very limited number of piles.
There are two main types of drop hammers:
- Single-acting steam or compressed-air hammers
- Double-acting pile hammers
Single-acting steam or compressed-air comprise a massive weight in the form of a cylinder. Steam or compressed air admitted to the cylinder raises it up the fixed piston rod. At the top of the stroke, or at a lesser height which can be controlled by the operator, the steam is cut off and the cylinder falls freely on the pile helmet.
Double-acting pile hammers can be driven by steam or compressed air. A piling frame is not required with this type of hammer which can be attached to the top of the pile by leg-guides, the pile being guided by a timber framework.
When used with a pile frame, back guides are bolted to the hammer to engage with leaders, and only short leg-guides are used to prevent the hammer from moving relatively to the top of the pile. Double-acting hammers are used mainly for sheet pile driving.
Figure-1: Pile driving using hammer
Pile Driving by Vibrating
Vibratory hammers are usually electrically powered or hydraulically powered and consists of contra-rotating eccentric masses within a housing attaching to the pile head.
The amplitude of the vibration is sufficient to break down the skin friction on the sides of the pile. Vibratory methods are best suited to sandy or gravelly soil.
Jetting: to aid the penetration of piles in to sand or sandy gravel, water jetting may be employed. However, the method has very limited effect in firm to stiff clays or any soil containing much coarse gravel, cobbles, or boulders.
Boring Methods ( non-displacement piles)
Continuous Flight Auger (CFA)
Equipment comprises of a mobile base carrier fitted with a hollow-stemmed flight auger which is rotated into the ground to required depth of pilling. To form the pile, concrete is placed through the flight auger as it is withdrawn from the ground.
The auger is fitted with protective cap on the outlet at the base of the central tube and is rotated into the ground by the top mounted rotary hydraulic motor which runs on a carrier attached to the mast.
On reaching the required depth, highly workable concrete is pumped through the hollow stem of the auger, and under the pressure of the concrete the protective cap is detached.
While rotating the auger in the same direction as during the boring stage, the spoil is expelled vertically as the auger is withdrawn and the pile is formed by filling with concrete.
In this process, it is important that rotation of the auger and flow of concrete is matched that collapse of sides of the hole above concrete on lower flight of auger is avoided. This may lead to voids in filled with soil in concrete.
The method is especially effective on soft ground and enables to install a variety of bored piles of various diameters that are able to penetrate a multitude of soil conditions. Still, for successful operation of rotary auger the soil must be reasonably free of tree roots, cobbles, and boulders, and it must be self-supporting.
During operation little soil is brought upwards by the auger that lateral stresses are maintained in the soil and voiding or excessive loosening of the soil minimize. However, if the rotation of the auger and the advance of the auger are not matched, resulting in removal of soil during drilling-possibly leading to collapse of the side of the hole.
Figure 2: Continuous Flight Auger Process
A special feature of auger bored piles which is sometimes used to enable to exploit the bearing capacity of suitable strata by providing an enlarged base. The soil has to be capable of standing open unsupported to employ this technique.
Stiff and to hard clays, such as the London clay, are ideal. In its closed position, the underreaming tool is fitted inside the straight section of a pile shaft, and then expanded at the bottom of the pile to produce the underream shown in fig. 3.
Normally, after installation and before concrete is casted, a man carrying cage is lowered and the shaft and the underream of the pile is inspected.
Figure 3: a) Hydraulic rotary drilling equipment b) Continuous Flight Auger, c) underreaming tool open position