It is
a foundation system that transfers loads to a deeper and competent soil layer.
➤Types
of pile foundation:
Piles are structural members that are made of
steel, concrete and or timber. They are used to build pile foundations, which
are deep and which cost more than shallow foundations. Different types of piles
are used in construction work, depending on the type of load to be, the subsoil
conditions and the location of the water table .Piles can be divided into
following categories:
➤ Steel
Piles:
Steel piles generally are either pile
piles or rolled steel H-section piles. Pipe piles can be driven into the
ground with their ends open or closed. Wide-flange and I-section steel beams
can also be used as piles. However, H-section piles are usually preferred
because their wed and flange thicknesses are equal.
➤
Concrete Piles:
Concrete piles may be divided into two basic
categories: (a) precise piles and (b) case-in-situ piles. Precast
piles can be prepared by using ordinary reinforcement, and they can be
square or octagonal in cross section. Precise piles can also be pre-stressed by
the use of high-strength steel pre-stressing cables. The ultimate strength of
these steel cables is about 1800 MN/m2. During casting of the piles, the cables
are pre-tensioned to about 900−1300 MN/m2. Cast-in-situ, or
cast-in-place, piles are built by making a hole in the ground and then
filling it with concrete. These piles may be divided into two broad categories:
(a) cased and (b) uncased.
➤ Timber
Piles:
Timber piles are tree trunks that have had
their branches and bark carefully trimmed off. The maximum length of most
timber piles is 30-65 ft (10-20 m). To qualify for use as a pile, the timber
should be straight, sound, and without any defects. The American Society of
Civil Engineers’ Manual of Practice, No. 17 (1959), divided timber piles
into three classifications:
1. Class A piles carry heavy loads. The
minimum diameter of the butt should be 14 in. (356 mm).
2. Class B piles are used to carry medium
loads. The minimum butt diameter should be 12-13 in. (305-330 mm).
3. Class C piles are used in temporary
construction work. They can be used permanently for structures when the entire
pile is below the water table. The minimum butt diameter should be 12 in. (305
mm). In any case, a pile tip should not have a diameter less than 6 in. (150
mm).
Timber piles cannot withstand hard driving stress;
therefore, the pile capacity is generally limited to about 25-30 tons (220−270
kN). Steel shoes may be used to avoid damage at the pile tip (bottom). The tops
of timber piles may also be damaged during the driving operation. The crushing
of the wooden fibers caused by the impact of the hammer is referred to as
booming. To avoid damage to the pile top, a metal band or a cap may be
used.
➤
Composite Piles:
The upper and lower portions of composite piles are
made of different materials. For example, composite piles may be made of steel
and concrete or timber and concrete. Steel and concrete piles consist of a
lower portion of steel and an upper portion of cast-in-place concrete. This
type of pile is the one used when the length of the pile required for adequate
bearing exceeds the capacity of simple cast-in-place concrete piles. Timber and
concrete piles usually consist of a lower portion of timber pile below the
permanent water table and an upper portion of concrete.
0 comments: