Classification of Pile Foundations (9 Criterions) | Soil Engineering

The classification of piles based on different criteria are as follows: 1. Classification Based on Shape of Cross Section 2. Classification Based on Shape of Longitudinal Section 3. Classification Based on Material of Construction 4. Classification Based on Formation 5. Classification Based on Method of Installation 6. Classification Based on Function and a Few Others.

1. Classification Based on Shape of Cross Section:

Figure 20.2 shows different types of piles based on the shape of the cross section. Circular piles are the most common for bored cast in-situ piles. A square or rectangular section is usually convenient for precast concrete piles.

A hexagonal section may sometimes be used for precast concrete piles. Other types of piles are described as follows:

i. H Piles:

Piles with rolled steel H section are advantageous for driving due to their small cross-sectional area and consequent less resistance to driving. The use of H piles is a relatively recent development in the piling industry. The utility of H piles lies in the fact that they can withstand large impact stresses that are developed during driv­ing of the piles. They are suitable as end-bearing piles when driving up to the bed rock or great depths.

H piles are commonly used in the construction of bulk heads, trestles, retaining walls, bridges, and cofferdams. Driving of H piles does not cause high soil displacement and, hence, they can be driven close to an existing structure. Splicing of H piles is also possible, and spliced H piles have been used for depths up to 100 m. They, however, have limited uplift capacity.

ii. Pipe Piles:

Steel pipes with open or closed bottoms may be used as piles. In the case of open pipe pile, the hollow central portion may be filled with concrete, after driving the pile. Pipe piles, with closed ends, have a conical steel or cast iron shoe as the closed end. The diameter varies from 0.2 to 1.2 m, and the shell thickness, from 8 to 12 mm.

2. Classification Based on Shape of Longitudinal Section:

Figure 20.3 shows various types of piles based on the shape of the longitudinal section. The longitudinal section of the pile is usually prismatic, with uniform cross section throughout the length of the pile. If driven piles are provided with gradually increasing diameter from bottom to top, known as tapered piles, then this will provide additional load capacity by bearing resistance of the soil.

Other types of piles are described as follows:

i. Sheet Piles:

Flat timber or steel sheets, when driven adjacent to one another, form sheet piles. The leakage between the adjacent piles can be prevented by interlocking them with specially fastened sheet ends. Sheet piles are generally used to create a temporary construction area in deep soft clay deposits, to construct bridges across rivers, etc.

ii. Under-Reamed Piles:

An under-ream pile is a bored cast in-situ circular pile with an enlarged portion, known as bulb or under-ream, near the bottom end of the pile, as shown in Fig. 20.3(c). Under-reamed piles were orig­inally developed for expansive soils to provide anchorage to the foundation against vertical movements of the foundation due to the swelling and shrinkage of the soil. Under-reamed piles are usually 15-20 cm in diameter and of 3-4 m in length.

iii. Disc Piles:

A disc pile consists of a hollow metallic pipe with a cast iron disc at the bottom to provide an enlarged bearing area at the bottom of the pile. Disc piles can be used only in sandy or soft soils to permit sinking of the pile by a wet jet. Disc piles are used mostly in marine installations, where the total penetration of the pile in the ground is required to be large.

3. Classification Based on Material of Construction:

The materials used for piles include timber, steel, concrete, or composite material consisting of a combination of timber with steel casing, concrete with steel reinforcement, as explained in the following list:

i. Timber Piles:

These are the oldest type of piles, formed by using the trunk of a tree, chopping off its branches, bark, bends, splits, etc. An iron shoe at the bottom and an iron cap at the top are generally used for timber piles to prevent damage of the pile during driving. It is necessary to chemically treat the timber, before using it as a pile, to prevent its deterioration by water or to safeguard it against the attack by insects or termites. Timber piles may be used to carry a load of 100-250 kN per pile.

Timber piles are the cheapest type of piles, but they have smaller load capacity compared with other piles. With the development of concrete and steel piles, timber piles have become more or less obsolete with limited use, especially in view of the adverse effect on the environment in using timber.

ii. Steel Piles:

Steel piles are used to carry heavy loads. Splicing of steel piles to increase the length can be done easily. Steel piles are used in lengths up to 40 m, and they can carry loads up to 1800 kN. Driving of steel piles requires heavy equipment. It is required to protect the steel piles against corrosion. This can be done by painting the surface of the steal pile or encasing the pile in concrete.

Advantages of Steel Piles:

(a) Installation of piles is hassle-free and offers the ease of achieving the cutoff level.

(b) High driving force can be used for installation.

(c) Load capacity is high and smaller size sections can be used due to high material strength.

(d) Steel piles will be advantageous for use as end-bearing piles in hard strata.

Disadvantages of Steel Piles:

(a) Steel piles are more expensive than other piles.

(b) The piles may bend under high driving stresses.

(c) There is problem of corrosion of steel piles unless adequate measures are taken to prevent the same.

(d) Driving of steel piles leads to noise pollution.

(e) Driving the piles requires heavy equipment.

iii. Concrete Piles:

Reinforced cement concrete (RCC) piles are the most common type of piles used all over the world. Concrete piles can be either cast in-situ piles, in which concrete is cast at the construction site, or precast piles, which are manufactured under controlled conditions in predetermined size and strength.

Advantages of Concrete Piles:

(a) Concrete piles are less expensive compared with steel piles.

(b) Monolithic construction can be achieved with superstructure.

(c) There is no problem of corrosion as in the case of steel piles.

(d) Concrete piles can withstand high driving stresses, and bored cast in-situ piles eliminate noise and vibra­tion problems of pile driving.

Disadvantages of Concrete Piles:

(a) It is difficult to transport precast concrete piles, which may get damaged, if proper care is not taken.

(b) It is difficult to achieve the cutoff level, when there is difference between the design length and the actual pile length.

(c) Great care is required during concreting to achieve the required strength and quality.

iv. Composite Piles:

Piles made of a combination of two or more materials are the composite pile. The most com­monly used composite piles are the RCC piles, which consists of cement concrete core with reinforcement bars both as main reinforcement and lateral ties or helical steel. Examples of other type of composite piles are timber in a steel casing or hollow steel pipe pile with in-fill concrete, etc., are rarely used.

4. Classification Based on Formation:

Concrete piles may be classified into precast or cast in-situ depending on whether concreting is done at off-site or on-site.

i. Precast Concrete Piles:

Precast concrete piles may be circular, square or rectangular in cross section. They are cast at a central casting yard, cured and transported to the construction site. They must be designed not only for the loads they have to carry from the superstructure but also for the handling and driving stresses to which piles are subjected during transportation and installation. The common size of the cross section of precast piles varies from 0.2 to 0.3 m.

Precast piles can carry loads from 600 up to 2000 kN, depending on the length of the pile. It is essential to estimate the soil/rock profile at the site accurately for the use of precast piles because it is neither possible to add extra length of the pile if the bearing stratum is at a deeper level than estimated, nor easy to cut the length if the hard stratum is encountered at a lesser depth than predicted.

Advantages of Precast Concrete Piles:

(a) It is possible to inspect and ensure quality before driving for installation.

(b) Construction of piles is not affected by groundwater.

(c) Precast concrete piles can withstand higher bending and tensile stresses.

(d) Precast piles can be driven in longer lengths.

Disadvantages of Precast Concrete Piles:

(a) It is difficult to change the length of piles based on design changes.

(b) Piles may be damaged or broken during driving.

(c) Precast piles are uneconomical since the design is based on driving stresses rather than on working stresses.

(d) There is noise and vibrations during driving of piles.

(e) Driving of piles would affect the safety and stability of the nearby structures and service lines.

ii. Cast In-Situ Concrete Piles:

This type of pile is formed by first making a bore hole into the ground up to the required depth and then filling the hole with concrete after placing the reinforcement cage.

Advantages of Cast In-Situ Concrete Piles:

(a) The length of cast in-situ piles can easily be adjusted as per the design requirements.

(b) It is possible to adopt the required cross section such as enlarged base, under-ream, or tapered section.

(c) These piles are more economical as the design is based on rather than driving stresses.

(d) The noise and vibrations can be eliminated.

Disadvantages of Cast In-Situ Concrete Piles:

(a) It is difficult to maintain uniform cross section; also, necking is possible where temporary casing is used.

(b) Concrete cannot be inspected for quality control either during or after construction.

(c) There may be a limit on the length of piles.

(d) Special techniques are required for concreting under water.

The length and load-carrying capacity of different piles is shown in Table 20.2.

5. Classification Based on Method of Installation:

Installation of piles in position may be done by:

i. Driving the pile under the blows of a hammer – such a pile is known as a driven pile. All types of piles, such as timber, concrete, and steel piles, can be installed by driving.

ii. First making a bore hole into the soil up to the pile length and then placing the pile in position – the pile is then known as a bored pile. Usually, concrete piles are installed by boring.

iii. Screwing the pile down into the soil if the pile has a helical screw at the bottom and at other positions on the stem – such a pile is known as a screw pile. Only steel piles are installed by this method.

iv. Applying downward force on the pile by a hydraulic jack – such a pile is known as a jacked pile.

6. Classification Based on Function:

Piles can be classified based on the purpose for which they are used.

i. Load-Bearing Piles:

Piles, which are used to transfer the load from the superstructure to the underlying hard stratum of soil or rock, are known as load-bearing piles.

ii. Sheet Piles:

Sheet piles are used to provide temporary construction area for the construction of structures over deep, soft marine clay deposits in marine areas on the sea coast. They are also used for the same purpose for the construction of bridges across rivers or other deep water bodies. Sheet piles retain soil at different heights on the either side and are also subjected to hydrostatic pressure.

iii. Anchor Piles:

Piles used to provide anchorage against the horizontal pull from sheet piling walls or other pull­ing forces are known as anchor piles.

iv. Fender Piles:

Piles, which are used to protect concrete deck or other waterfront structures from abrasion or impact of ships or barges, are called fender piles.

v. Compaction Piles:

Short timber piles, which are driven in granular soils for increasing the bearing capacity, are known as compaction piles.

7. Classification Based on Type of Load:

Figure 20.5 shows the various types of piles based on the type of load acting on the pile.

i. Compression Piles:

Piles, which are subjected to compression due to the load from the superstructure, are known as compression piles.

ii. Tension Piles:

Piles which are provided as a foundation for underwater structures and are subjected to an uplift force are called tension piles. The skin friction developed on the surface of the pile will be in the downward direction to resist the uplift force.

iii. Laterally Loaded Piles:

Piles subjected to lateral loads or moments are known as laterally loaded piles and are used as foundations for retaining walls and wharfs.

iv. Batter Piles:

When the magnitude of lateral loads is large, laterally loaded piles are driven at some angle with the vertical to derive the benefit of passive resistance of the soil and resist large horizontal or inclined forces; such piles are called batter piles. Batter piles are also known as Raker piles.

8. Classification Based on Mechanism of Load Transfer:

Piles can be classified based on the manner in which the loads from the superstructure are transferred by the piles to the foundation soil/rock.

i. End-Bearing Pile:

Piles which transmit the load to the underlying stratum of rock through their base or tip at the bottom are called end-bearing piles. End-bearing piles are used in extremely weak or compressible soils, such as soft marine clays, where the piles are taken to the level of soft or hard rock. In a completely end-bearing pile, the frictional resistance offered by the soil surrounding the surface of the pile is either nil or negligible. Figure 20.6(a) shows a schematic representation of end-bearing resistance.

ii. Friction Pile:

Friction piles resist the load coming on them by virtue of skin friction resistance offered by the soil surrounding the pile over its length. When the soil at the base or tip of the pile offers only negligible or partial resistance to the load coming on the pile, the pile will tend to downward vertical displacement, causing the development of skin friction resistance between the soil surrounding the pile and the pile surface over its length. Figure 20.6(b) shows a schematic representation of skin friction resistance.

iii. Bearing-Cum-Friction Piles:

Piles which resist the loads due to the combined action of end-bearing resistance (f_b) at the pile bottom and skin friction resistance (f_s) over its surface along the length are known as bearing-cum- friction piles. Piles terminating in medium-stiff clays or medium-dense sands are examples of this type of piles. Figure 20.6(c) shows a schematic representation of bearing-cum-friction resistance.

9. Classification Based on Displacement of Soil during Installation:

In this classification, the different types of piles are displacement piles and non-displacement piles and these are described as follows:

i. Displacement Piles:

Driving of piles in soils causes lateral displacement of the soil and such piles are known as displacement piles. Displacement piles help to density loose cohesionless soils up to a distance of 3.5 times the pile diameter and, hence, increase the load-carrying capacity. Displacement piles are not effective in dense cohesionless soils as the angle of shearing resistance decreases due to dilatancy effect.

In clays, pile driving causes remolding of the clay up to a distance of about twice the diameter of the pile, causing a reduction in their shear strength if the clays are sensitive. Also, the pore pressure developed during driving reduces the shear strength of these soils. This process helps in driving of the piles. If sufficient time is allowed after installation before the construction of the superstructure, the pore pressure will dissipate and the shear strength is regained. Examples of displacement pile are driven piles.

ii. Small- or Non-Displacement Piles:

Piles with rolled steel sections, screw piles, and open-ended hollow section piles cause a small displacement of soil during driving and such piles are called as small-displacement piles.

In the case of bored piles, a bore hole is first made into the soil and the pile is then constructed by concreting the cased or uncased bore hole after placing the reinforcement. Such piles are called as non-displacement piles. Bored cast in-situ or bored precast piles are examples of non-displacement piles.