This document discusses pile foundations and methods of installing pile foundations. It defines pile foundations as slender columns made of materials like concrete or steel that support structures by transferring loads to deeper soil layers through end bearing or skin friction. It then describes different types of piles (e.g. sheet piles, load bearing piles, end bearing piles, friction piles) and materials used (e.g. timber, steel, precast concrete, cast-in-place concrete). The document proceeds to discuss various pile installation methods like dropping weight, vibration, jetting, and boring. It concludes by describing common pile driving equipment used such as piling rigs, winches, hammers, and protective gear placed on pile heads.
2. PILE FOUNDATION
Pile foundation, a kind of deep foundation, is actually a slender column or long cylinder made of
materials such as concrete or steel which are used to support the structure and transfer the load at
desired depth either by end bearing or skin friction.
They are formed by long, slender, columnar elements typically made from steel or reinforced concrete,
or sometimes timber. A foundation is described as 'piled' when its depth is more than three times its
breadth.
Pile foundations are usually used for large structures and in situations where the soil at shallow depth is
not suitable to resist excessive settlement, resist uplift, etc.
3. When to Use Pile Foundation
When the groundwater table is high.
Heavy and un-uniform loads from superstructure are imposed.
Other types of foundations are costlier or not feasible.
When the soil at shallow depth is compressible.
When there is the possibility of scouring, due to its location near the river bed or seashore, etc.
When there is a canal or deep drainage systems near the structure.
When soil excavation is not possible up to the desired depth due to poor soil condition.
When it becomes impossible to keep the foundation trenches dry by pumping or by any other measure
due to heavy inflow of seepage.
5. Types of Pile Foundation
Sheet Piles
Usually, they resist lateral pressure from loose soil,
the flow of water, etc. They are used for cofferdams,
trench sheeting, shore protection, etc.
They are not used for providing vertical support to
the structure. They are usually used to serve the
following purpose-
Construction of retaining walls.
Protection from river bank erosion.
Retain the loose soil around foundation trenches.
For isolation of foundation from adjacent soils.
For confinement of soil and thus increase the bearing
capacity of the soil.
Load Bearing Piles
This type of pile foundation is mainly used to
transfer the vertical loads from the structure to the
soil. These foundations transmit loads through the
soil with poor supporting property onto a layer
which is capable of bearing the load.
6. Types of Pile Foundation
End Bearing Piles
The loads pass through the lower tip of the pile. The
bottom end of the pile rests on a strong layer of soil or
rock. Usually, the pile rests at a transition layer of a
weak and strong slayer.
The total capacity of end bearing pile can be
calculated by multiplying the area of the tip of the pile
and the bearing capacity of at that particular depth of
soil at which the pile rests.
Friction Pile
Transfers the load from the structure to the soil by
the frictional force between the surface of the pile
and the soil surrounding the pile such as stiff clay,
sandy soil, etc.
Friction can be developed for the entire length of
the pile or a definite length of the pile, depending
on the strata of the soil.
The surface area of the pile multiplied by the safe
friction force developed per unit area determines
the capacity of the pile.
7. Types of Pile Foundation
Timber Piles
Timber piles are placed under the water level. They
last for approximately about 30 years. They can be
rectangular or circular in shape. Their diameter or
size can vary from 12 to 16 inches. The length of the
pile is usually 20 times of the top width.
They are usually designed for 15 to 20 tons.
Additional strength can be obtained by bolting fish
plates to the side of the piles.
Steel Piles
Steel piles may be of I-section or hollow pipe. They
are filled with concrete. The size may vary from 10
inches to 24 inches in diameter and thickness is
usually ¾ inches. Because of the small sectional
area, the piles are easy to drive. They are mostly
used as end-bearing piles.
8. Types of Pile Foundation
Pre-cast Concrete Pile
The precast concrete pile is cast in pile bed in the
horizontal form if they are rectangular in shape.
Usually, circular piles are cast in vertical forms.
Precast piles are usually reinforced with steel to
prevent breakage during its mobilization from
casting bed to the location of the foundation.
After the piles are cast, curing has to be performed
as per specification. Generally curing period for pre-
cast piles is 21 to 28 days.
Cast-in-Place Concrete Piles
This type of pile is constructed by boring of soil up
to the desired depth and then, depositing freshly
mixed concrete in that place and letting it cure
there.
9. METHODS OF INSTALLING PILE
FOUNDATIONS
Methods of pile driving can be categorized as follows:
Dropping weight
Explosion
Vibration
Jacking (restricted to micro-piling)
Jetting
10. 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
11. Drop Hammer Method of Pile Driving
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.
12. 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.
13. 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.
14. Boring Methods
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.
15. Under reaming
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.
In its closed position, the under reaming tool is fitted inside the
straight section of a pile shaft, and then expanded at the bottom
of the pile to produce the under ream shown in fig.
Normally, after installation and before concrete is casted, a man
carrying cage is lowered and the shaft and the under ream of the
pile is inspected.
18. Piling Rigs
It composed of a series of leaders, which
are consist of tabular element or hard box,
placed and fixed on a crane base as it can
be seen from figure. Not only does the
leaders support the hammer and the pile
but also guide them when the pile is
forced into the ground.
19. Piling Winches
The prime goal of piling winches is to left the hammer and piles in addition to support tools that responsible
for leader raking and rotation.
There are different piling winches with different capacity for instance winches with double or triple drums
possess satisfactory controlling and pile driving speed whereas one drum winch does not have that
advantage. So, the former type would be favored provided that handing and driving piles with great speed
is required.
20. Hanging Leader
Hanging leaders are specifically designed to be hanged from the jib of a crane. A
steel strut, which its length can be varied as per requirements of construction site,
provides a stiff connection from the leader foot to the machine bed frame.
Moreover, crane or excavator winch units are utilized to left the hammer and piles
using separate drums.
Finally, it is substantially crucial to practice utmost precaution to hanging leader
stiffness particularly in the case of long sloped pile driving since intolerable
deformation would lead to eccentric hammer blow and possibly cause pile
fracture.
21. Hammer Guides
When it is intended to remove hanging leaders or piling frames completely, rope
suspended leaders which are commonly guided by timber or steel formwork, would
be considered.
In this technique, an independent crane needed to control the pile and establishing
the guide and hammer. it is necessary to set and secure the guide properly in order to
avoid movements specifically in the during raking pile installation. This is because
serious fatigue stress would generate if the thrust is not centered properly and the
guide might deteriorate.
Finally, it is required to prevent disproportionate bending stress development in guide
and piles because it leads to undesired results. for example, when heavy hammer is
attached to the upper end of a long pile which is driven at flat angle of rake, excessive
bending stress may be generated at support point in the guide. This problem might
be tackled by providing suitable support for the pile at proper position.
22. Piling Hammer
There are several factors that greatly influence the decision to
choose suitable piling hammer. For example, pile size and
weight, the resistance of the ground which should be
overcame in order to obtain specified penetration, construction
site space availability, noise limitation that might be imposed at
certain areas, and availability of cranes.
As for input data required for drivability analysis, piling
hammer producer provided necessary data about efficiency
and energy feature of the piling hammer. It should be bore in
mind that piling hammer efficiency is not a constant and it is
affected by number of factor for example mechanical condition
of the hammer and operation temperature.
It should be known that the mechanical condition is not
influence the efficiency of piling drop hammer. That is why
dynamic pile analysis is carried out and its results would be
used to assess the influence of different factor on the piling
hammer efficiency.
23. Helmet, Driving Cap, Dolly and Packing
Helmet is a cast steel that placed over the pile to hold the dolly that
placed between the pile and the hammer to avoid pile head
deterioration that may cause by pile driving hammer.
Dolly, which is square at the bottom and round at the top, is placed in a
square recess at the top of the helmet. Their selection is dependent on
the driving force.
As far as packing is concerned, it is placed between pile top and the
helmet in order to protect the former from the hammer blow. Different
types of packing include paper sacking, thin timber sheet, coconut
mapping, and sawdust in bags.
Regarding driving cap, it is provided as a protection for steel bearing
piles. It is necessary to place the driving cap tightly otherwise the pile
cap would suffer deterioration. That is why it is fitted with a recess for
hardwood or plastic dolly and with steel wedges to fix the cap tightly
on its position.
