Types of Pile Foundation.

 

What is Pile?

An constructional element placed vertically or nearly so in the ground to increase the bearing capacity of soil or to resist the lateral load is known as pile.

A pile foundation is usually provided where the soil material under the base of a structure has deficient bearing power to take the load of the structure, and the soil near the ground surface is also incapable of supporting a raft foundation .Piles are used for erecting foundations of structures in order to transmit load to the underlying soil and to improve the load bearing capacity of low bearing capacity soils.

Piles are classified into two categories

1) Piles As per functionality

2) Piles As per material of construction

 1) Piles as per function  

Piles are classified as per their function as discussed below.

a)Bearing Piles.

The piles which rest on hard strata and act as columns to bear the load of the structure are known as Bearing piles. These piles are used to bear vertical loads .They take and transfer the load to the hard stratum lying underneath.

These piles are driven through soft strata and go deep to rest on hard surface and support the load by the resistance developed at their points by end bearing, and act as  long columns. In these piles the cross section should be comparatively greater to resist the buckling effect.

 

 b) Friction Piles or Floating Piles.

The piles which do not rest on hard strata and bear the loads on account of frictional resistance between their outer surface and the soil in contact are called Friction piles.

These piles are used when the soil is soft and there is no hard strata available up to a considerable depth. These are generally long in length. Frictional resistance can be increased by making the surface of these piles rough or by increasing their surface area likely to come in contact with soil.

c ) Friction Cum -bearing Piles.

The piles which rest on a hard strata and resist the structural load partly by bearing and partly by their skin friction are known as friction-cum bearing piles. These piles are used when the bearing capacity of soil strata lying under them is not sufficient to resist load of structure.

These piles are driven into hard strata and their bearing capacity is provided chiefly by friction of their side surface against the soil. This friction is called skin friction.

Here are some significant frictional resistance offered by various soil materials to the pile surface.

 

- Sand and Gravel          (4.9 to 8.8 ) Tonnes/Sq.m

-Stiff Clay                         (3.9 to 5.9 ) Tonnes/Sq.m

-Clay and sand mixed     (1.9 to 9.9 ) Tonnes/Sq.m

-Dried and compact silt  (1.0 to 1.5 ) Tonnes/Sq.m

-Silt and soft clay             (0.25 to 0.50 ) Tonnes /Sq.m

 

d) Batter piles

The piles driven at an inclination to resist inclined loads are known as batter piles.

These piles are used generally to resist lateral forces in case of retaining walls ,abutments etc.

e) Guide Piles

These piles are mainly used in the formation of cofferdams which are temporarily constructed to provide foundations under water.

f) Sheet Piles

The piles which consist of thin steel sheets, driven in the ground to enclose an area are known as sheet piles.

These piles are used to enclose soil so as to prevent the leakage of water and to enclose soft material. They are used in the construction of cofferdams. Sheet piles are not required to carry any load but should be strong enough to take the lateral pressure of earth filling ,water etc.

2) Piles As per material of construction

a) Timber Piles

The piles made of wood are called timber or Wooden Piles.

The timber to be used for their construction should be free from detects, decay etc and it should be well seasoned. These piles are circular (20 cm to 50 cm  ) in diameter or square (15 cm to 50 cm side) in cross-section. Length of these piles is generally 20 times of their diameter of side length. Top of these piles is provided with an iron ring to prevent it from splitting under blows of the hammer. The bottom is fitted with an iron shoe to facilitate sinking of the piles. These piles are driven by blows of drop hammer of a pile driving machine.

Timber piles are broadly used as they have advantage of flexibility and weightlessness, and in many places they are cheaper than other materials. Their disadvantages is lack of durability in certain conditions.

strength depends on the type of wood ,its moisture content ,and its position. In general timber piles are durable in permanently wet or permanently dry positions, but not where they are alternatively wet and dry or where the moisture content is widely variable.

Timber piles if used longer than 30 year or thereabouts and such are usually preferred for temporary works and also for semi permanent marine structures. Timber piles should be impregnated with solignum , creosote or treated with some such anti rot compounds to enhance their service life.

preservative treatment may not be necessary for piles which will be completely and permanently submerged in water-logged ground because in this case seasoning is not necessary and piles may be stored in water prior to use.

Suitability

These piles are generally used for buildings ,bridges and cofferdams but their use is not recommended in sea water.

b) Pre-Cast Concrete Piles

Reinforced concrete piles are widely used in construction practice with and without pre-stressed reinforcement. They are usually of square or rectangular cross-section, as they are easier to cast than a circular section. The usual size is 15 to 60 cm but piles have been made up to 90 cm size with cylindrical holes inside. Hollow piles have advantages where exceptional lengths are required because they provide stiffness and large perimeter with lesser weight than solid piles.

The piles having larger than 36 x 36 cm an octagonal section is preferable to a square section . Square piles should have chamfered corners. The maximum lengths of piles are usually 12 m for 30 cm square, 15 m for 36 cm square,18 m for 40 cm square, 21 m for 46 cm square. It is preferable to keep the length less than 4o times the side for friction piles and less than 20 times the side for bearing piles.

Where the piles are considered to act as column, stresses should be calculated as for ordinary columns. To prevent damage to the head of pile, the top edges should be chamfered liberally and additional lateral reinforcement provided and kept back from the head about 50 to 75 mm according to the diameter. Concrete piles should be cured at least a month. Lifting holes should be made at one-fourth to one-fifth the length of the pile from each lifting hole. 2.5 cm diameter gas pipe ferrules may be fixed in the holes. Concrete piles are generally designed for 70 to 80 tonnes.

c ) Cast-in-situ Piles.

These piles are made with driven hollow tubes or by using heavy steel pipe castings and then withdrawing them or by boring and filling the holes formed with concrete. The tube is placed at the top of loose cast iron point before driving into the ground and is slowly and steadily drawn out of the ground as concrete is filled in. Piles are also formed by driving in a steel shell ,leaving it permanently there and filling it with concrete. The shells should be strong enough so that they are not distorted by soil pressure or the driving of adjacent piles. These piles are built in diameter of 0.4 to 2 m and in lengths of up to 50 m. The bearing capacity may be as high as 600 Ton per pile. These piles can also be made with bulb toes giving greater bearing value.

There may be many patented processes for these piles such as Franki, Simplex, Vibro.

No driving pile should be withdrawn until all piles within 3m radius have been driven. Built in place piles are made without vibrating the soil, which is particularly important for construction work near or inside existing buildings and installations. As these piles are  not intended to carry impact loads under normal conditions no reinforcement is necessary and where required it is placed in the upper part only. Any reinforcement used should be made up into cages sufficiently well wired because the bars should be openly spaced , and the lateral ties should not be closer than 15 cm center to center.

The reinforcement should be exposed for a sufficient distance to permit it to be adequately bonded into the pile cap. In bored piles care should be taken to prevent influx of soil into the castings during boring. Before placing the concrete the holes should be inspected by lowering a light for any undesirable materials or water. Placing of concrete should not be started until all the shells in a group has been completed. Bored piles unless sunk into hard and compact ground should be test loaded. Steel shells which are to be filled with concrete should be coated externally with bituminous composition or tar ,etc, before they are driven . In other cases all surfaces should be coated if tar is employed , it should be neutralized with slaked lime.

 

Reinforcement and concrete materials for the Piles

The area of the main reinforcement of longitudinal bars may be 1.5% of gross cross sectional area of the pile if length of pile is 30 times of the least width of pile , and 2 % if the lengths are 30 to 40 times , which may be increased up to 3 % for longer lengths. The links usually are 6 mm diameter up to 12 m and 10 mm diameter above 12 m are spaced 50 to 75 mm for lengths up to 3 times the side at each end of the pile, lengthening to 15 to 20 cm at the center.

Reinforced concrete piles should be of 1:1.5:3 or richer mix,  with well graded aggregate of maximum size limited to 12 mm, and a slump of about 40 mm.