basics about pile foundation

1. Pile foundations are the most commonly used deep
foundations.
Deep foundations are employed when the soil strata
immediately beneath the structure are not capable of
supporting the load with tolerable settlement or adequate
safety against shear failure.
Pile foundations

2. CLASSIFICATION OF PILES
Piles may be classified in a number of ways based on the
different criteria indicated below:
1. Function or action
2. Material of construction and composition
3. Method of installation

3. 1. Classification Based on Function or Action
a) End-bearing piles.
b) Friction piles.
c) Combined end-bearing and friction piles.
d) Uplift piles or Tension piles.
e) Compaction piles.
f) Anchor piles.
g) Fender piles.
h) Sheet piles.
i) Batter piles.
j) Laterally-loaded piles.

4. (i) End-bearing piles.
- End bearing piles transmit the loads through
their bottom tips to a firm stratum below.
- If bed-rock is located within a reasonable
depth, piles may be extended to the rock.
- The load bearing capacity of an end-bearing
pile depends on the bearing capacity of the
rock or the hard stratum soil below.

5. (ii) Friction piles.
- Friction piles transfer the load through skin
friction between the embedded surface of the
pile and the surrounding soil.
- Friction piles are used when a hard stratum
does not exist at a reasonable depth, and hence
these piles do not rest on a hard stratum below.
- The load bearing capacity of a friction pile
depends on the skin friction along the surface
area of the pile.
- The friction piles are also known as floating
piles, as they do not rest on a hard stratum below.

6. (iii) Combined end-bearing and friction
piles.
- These piles transfer the load by a
combination of end-bearing at the bottom of
the pile and skin friction along the surface
area of the pile.
- The load bearing capacity of the pile in this
case depends on the bearing capacity of the
stratum soil below and the skin friction
along the surface area of the pile.

7. (iv) Uplift piles or Tension piles.
- These piles are used to anchor the structures subjected to uplift due to
hydrostatic pressure or to overturning moment due to horizontal forces.
- These piles are in tension and hence they are so named.

8. (v) Compaction piles.
- These piles are used to compact loose
granular soils in order to increase the
bearing capacity of such soils.
- Since these piles are not required to carry
any load, the material of construction for
these piles may not be strong.
- For example sand may be used to form piles
in this case.
- The pile tube, driven to compact the soil, is
gradually taken out and sand is filled in its
place thus forming a ‘sand pile’.

9. (vi) Anchor piles.
- Anchor piles are used to provide anchorage for sheet piles which are
subjected to earth or water pressure.
- Anchor piles provide resistance against horizontal pull from sheet
piles which are then known as anchored sheet piles.

10. (vii) Fender piles.
Fender piles are used to protect water-front structures against
impact from ships or other floating objects.

11. (viii) Sheet piles
- Sheet piles are commonly used as bulkheads, or cut-offs to reduce
seepage and uplift in hydraulic structures.
- Sheet piles are also used as fender piles.

12. (ix) Batter piles.
- Batter piles are inclined piles
which are used to support
sheet piles, especially in water
front structures.
- These piles are required to
resist horizontal and inclined
forces.

13. (x) Laterally-loaded piles.
These piles are used to support retaining walls,
bridges, dams and wharves and as fenders for
harbour construction.

14. 2. Classification Based on Material of construction and composition
(i) Timber piles.
- Timber piles are made from tree trunks after proper trimming. To
avoid damage to the top of the pile, a metal band or a cap is
provided.
- Length of timber piles - up to about 8 m;
- splicing is adopted for obtaining the piles of the required length.
Splicing of timber piles is done by using a pipe sleeve or metal straps
and bolts. The length of the pipe sleeve should be at least five times
the diameter of the pile.
- Timber piles perform well either in fully dry condition or in fully
submerged condition.
- The life of the timber piles may be increased by the use of
preservatives such as creosote oils. For timber piles the maximum
design load is about 250 kN (or 25 tonnes).

16. (ii) Steel piles.
- Steel piles are usually H-piles (rolled steel H- sections), pipe piles, or
sheet piles (rolled sections of regular shapes).
- Pipe steel piles are driven into the ground with their ends open or
closed. Steel piles are provided with a driving point or shoe at the
lower end.
- In order to reduce corrosion of the steel piles, epoxy coating are
applied in the factory during the manufacture of the piles. An
additional thickness of the steel sections used as piles is usually
recommended to take into account the corrosion.
- Sometimes concrete encasement at site is done as a protection
against corrosion. Steel piles may carry loads up to 1000 kN (or 100
tonnes) or more.

18. (iii) Concrete pile.
- Cement concrete is used for the construction of the concrete piles.
- Concrete piles may be either ‘precast’ or cast-in-situ.
- Precast piles are reinforced to withstand handling and driving
stresses. They require space for casting and storage, more time to
cure and heavy equipment for handling and driving.
- Precast piles may also be prestressed by using high strength steel
pretensioned cables. Precast piles are generally used for a maximum
design load of about 800 kN (or 80 tonnes), except for large
prestressed piles.

19. A cast-in-situ pile is constructed by making a hole in the ground
and then filling it with cement concrete. Cast-in-situ piles may be
classified as :
a) driven piles (cased and uncased)
b) bored piles (pressure piles, pedestal piles and under-reamed piles)
- A driven pile is constructed by driving a steel casing into the ground to
the desired depth and filling it with concrete.
- In the case of cased pile the casing is held embedded into the ground
and concrete is filled in it.
- In the case of uncased pile, the casing is gradually withdrawn when
concrete is filled.

20. - A bored pile is constructed by boring or drilling a hole into the ground
and filling it with concrete.
- In a pressure pile concrete is filled under pressure in the bored or
drilled hole.
- In a pedestal pile as the concrete is filled in the bored or drilled hole a
pedestal is formed at the bottom of the pile.
- Under-reamed pile is a special type of bored pile having increased
diameter or bulbs at one or more sections at intervals along its length to
anchor the foundation in expansive soil subjected to alternate expansion
and contraction.

23. (iv) Composite piles.
- Composite piles are made of two materials. These may consist of the
lower portion of timber below the permanent water table and the
upper portion of cement concrete.
- These may also consist of the lower portion of steel and the upper
portion of cement concrete.
- As it is difficult to provide a proper joint between two dissimilar
materials, composite piles are rarely used in practice.

24. Classification Based on Method of Installation
Based on the method of installation piles may be classified as follows:
(i) Driven piles. These piles are installed by driving the piles into the
ground by applying blows with a heavy hammer on their top. Timber,
steel and precast concrete piles are installed by driving, which may be
driven into position either vertically or at an inclination. If inclined
they are termed ‘batter’ or ‘raking’ piles.
(ii) Cast-in-situ piles. Only concrete piles may be cast-in-situ, in which
holes are made into the ground and these are filled with concrete.

25. USE OF PILES
The various ways in which piles are used are as follows:
(i) To carry vertical compressive loads,
(ii) To resist uplift or tensile forces, and
(iii) To resist horizontal or inclined loads.