What is the pile foundation?

The pile is a structural member that is made of either concrete, timber or steel. It is a small diameter column, which is driven or cast into the ground up to suitable depth. They are used to construct a foundation, which is deep and of course which cost more than shallow foundations. 

Pile foundation is used when the soil immediately below the foundation can not safely support the loads and load has to the transferred deep into the underlying soil. It is required when the soil-bearing capacity of the soil is not sufficient for the structure. The main function of the pile is to transmit loads to the lower level of the ground by the combination of friction pile and end-bearing pile at the pile point or base.

pile foundation

The Pile foundation consists of two major components: Pile cap and single pile/group of piles. Pile cap supports the single column or group of columns. Piles transfer the loads from the structures to the hard strata.

We use a pile foundation when –

  • The compressible or weak upper soil layer
  • Presence of horizontal forces
  • Presence of expansive soils
  • Subjected to uplifting forces
  • Soil erosion

A pile is a long slender foundation member, made of either timber, structural steel or concrete, which might be cast-in-situ, or driven and acts as a structural member to transfer the load of the structure to a required depth in deep foundations carrying a load which may be vertical or lateral or lateral plus vertical

Uses of Piles –

As far as deep foundations are concerned there are no. of types of deep foundations and pile foundation is among one of them; uses of Pile Foundation depends on the type of pile used, the intended function for which the pile is used, the load which is to be applied on the pile and the type of material which is used for the construction of the pile.

The uses of piles are –

  1. End Bearing or Compressive Strength

    Sometimes we use the piles to achieve the required compressive strength in the soft soil; in that case, we use the piles to transfer the load through that soft soil to a suitable bearing stratum by using the end bearing or toe bearing property of the pile

  2. Scour Depth:

    To build a structure within the water and on the water river or canal bed; we have to build the foundation through the riverbed and within the scour depth.

  3. Tension or Uplift:

    Piles are usually used to carry the compressive load through tip bearing or end bearing, but in case of tall structures or like towers there might be tension that must be resisted by piles. For example, for a tower carrying high power transmission lines the thrust of wind might produce overturning that must be resisted by the tension piles; other options include use of deep foundation or thick raft, which is sometimes uneconomical.

The pile may be classified as long or short in accordance with L/d ratio of the pile (Where, L= length, d = diameter of the pile). A short pile behaves as a rigid body and rotates as a unit under lateral loads. The load transferred to the tip of the pile bears a significant proportion of the total vertical load on the top. In the case of a long pile, the length beyond a particular depth loses its significance under lateral loads, but when subjected to vertical load, the frictional load on the sides of the pile bears a significant part to the total load.

The pile may further be classified as vertical piles or inclined piles. Vertical piles are normally used to carry mainly vertical loads and very little lateral load. When piles are inclined at an angle to the vertical, they are called batter piles or raker piles. Batter piles are quite effective for taking lateral loads, but when used in groups, they also can take vertical.

TYPES OF PILES ACCORDING TO THE METHOD OF INSTALLATION

According to the method of construction, there are three types of piles. They are

  • Driven piles,
  • Cast-in-situ piles and
  • Driven and cast-in-situ

Driven piles,

They may be driven either vertically or at an angle to the vertical. Piles are driven using a pile hammer. When a pile is driven into granular soil, the soil so displaced, equal to the volume of the driven pile, compacts the soil around the sides since the displaced soil particles enter the soil spaces of the adjacent mass, which leads to the densification of the mass. The pile that compacts the soil adjacent to it is sometimes called a compaction pile. The compaction of the soil mass around a pile increases its bearing capacity. If a pile is driven into saturated.

If a pile is driven into saturated silty or cohesive soil, the soil around the pile cannot be densified because of its poor drainage qualities. The displaced soil particles cannot enter the void space unless the water in the pores is pushed out. The stresses developed in the soil mass adjacent to the pile due to the driving of the pile have to be borne by the pore water only. This results in the development of pore water pressure and a consequent decrease in the bearing capacity of the soil. The soil adjacent to the piles is remoulded and loses to a certain extent its structural strength. The immediate effect of driving a pile in soil with poor drainage qualities is, therefore, to decrease its bearing strength. However, with the passage of time, the remoulded soil regains part of its lost strength due to the reorientation of the disturbed particles (which is termed thixotropy) and due to consolidation of the mass.

The advantages and disadvantages of driven piles are:

Advantages:-

  • Piles can be precast to the required specifications.
  • Piles of any size, length and shape can be made in advance and used at the site. As a result, the progress of the work will be rapid.
  • A pile driven into granular soil compacts the adjacent soil mass and as a result, the bearing capacity of the pile is increased.
  • The work is neat and clean. The supervision of work at the site can be reduced to a minimum. The storage space required is very much less.
  • Driven piles may conveniently be used in places where it is advisable not to drill holes for fear of meeting groundwater under pressure.
  • Driven pile are the most favoured for works over water such as piles in wharf structures or jetties.

Disadvantages:-

  • Precast or pre-stressed concrete piles must be properly reinforced to withstand handling stresses during transportation and driving.
  • Advance planning is required for handling and driving.
  • Requires heavy equipment for handling and driving.
  • Since the exact length required at the site cannot be determined in advance, the method involves cutting off extra lengths or adding more lengths. This increases the cost of the project.
  • Driven piles are not suitable in soils of poor drainage qualities. If the driving of piles is not properly phased and arranged, there is every possibility of heaving of the soil or the lifting of the driven piles during the driving of a new pile.

  • Where the foundations of adjacent structures are likely to be affected due to the vibrations generated by the driving of piles, driven piles should not be used.

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