pile foundation

Pile Foundation

Foundation is an important component of any engineering work that greatly determines the strength and stability of the entire structure. Foundation acts as a medium to transfer the load from the superstructure to the underlying ground. In general, foundations can be classified as the shallow foundation (e.g. Isolated Footing) and deep foundation( e.g. pile foundation).
It is suitable to construct a shallow foundation when the surface soil has acceptable or adequate bearing capacity. On the other hand, pile foundation is suitable when the surface soil is weak and thus the foundation has to be taken deep down where the soil has adequate bearing capacity. One such example of a deep foundation is pile foundation which has been briefly discussed below.

 

Introduction

Pile foundation is defined as a type of deep foundation that comprises of a long, slender column used to transfer the load coming from the superstructure to the lower-lying ground thereby providing overall support to the structure.

In general, pile foundation is a long cylinder usually made up of steel or concrete and driven deep into the ground to act as a base or support to the structure built on it. Pile foundations transfer the load through skin friction or end bearing mechanism.
Sometimes, pile foundations made up of timber is also used. Pile foundations are mostly used for large structures and when the upper soil layer is weak and does not have sufficient bearing capacity to withstand the imposed load.

 

Suitability of Pile Foundation

It is desirable to use pile foundations in the following cases:

1. The surface soil layer is weak and cannot resist the load of the structure and undergoes excessive settlement.

2. When the structure to be constructed consists of very heavy concentrated loads such as in the construction of high rise buildings, water tanks, bridge etc.

3. When the groundwater table is relatively high.

4.When the construction has to be done on the riverside, river bed or seashore where there is increased risk of scouring.

5. When other types of foundations are not feasible or expensive.

6. When a canal system or deep drainage structure exists nearby the construction site.

 

Benefits of Using Pile Foundation

Some of the benefits of using a pile foundation can be listed as follows:

1. It is advantageous when the foundation has to be drilled through hard rocks or tough surface.

2. It is suitable for deep installations and is long-lasting.

3. Pile foundation can be pre-ordered as well as customized according to the need.

4. It is desirable even for wetland constructions and for all sizes of land construction.

5. It facilitated reduced construction time.

 

Factors Affecting Selection of Pile Foundation Type

The factors that affect the choice of the type of pile foundation are listed as follows:
1. Properties of soil.

2. Availability of construction materials.

3. Types of the load imposed.

4. Length of pile needed.

5. Depth of water table.

6. Type of structures existing nearby.

7. Budget available and durability desired.

8. Availability of driving equipment.

9. The intensity of groundwater flow.

 

Types of Pile Foundation

Pile foundation may be classified based on various factors which are discussed below:

A. Based on Function or Use

Depending upon the function or purpose served, pile foundation may be classified as follows:

i. Sheet Piles

Sheet piles are the type of piles that are mostly used for providing lateral support and resist the lateral pressure generally from loose soil, the flow of water etc. Mostly, such type of piles is used for the sheeting of trench, protection of shore, cofferdams etc.

Sheet piles are commonly used for the following purposes:

1. Retaining the loose soil around the foundation trenches.

2. Constructing retaining walls.

3. Constructing erosion protection structures.

4. Confining the soil and isolating the foundation from adjacent
soil layer.

“One of the limitations of sheet pile is that it cannot be used for providing vertical
support to the structure.”

sheet pile foundation

Figure 3: Sheet Piles

 

ii. Load Bearing Piles

Unlike sheet piles, load-bearing piles are the type of pile foundations that transfer the vertical loads to the underlying soil. Based upon the load transfer mechanism, load-bearing piles can be further classified into the following:

load bearing pile foundation

Figure 4: Types of Load Bearing Pile

a. End Bearing Piles

End bearing piles are the type of piles that are based on the load transfer mechanism and load is transferred through the bottom tip of the pile itself. The end bearing piles are driven into the ground such that the bottom end tip of the pile rests at the intermediate layer between the weak soil layer and the strong soil layer. On account of this end bearing piles develop most of the bearing
capacity at the bottom tip itself. Thus, end bearing piles act as a column and transfer the load coming from the superstructure to the underlying soil. The bearing capacity of such pile can be easily determined by multiplying the area of the bottom tip of the pile and the bearing capacity at that particular depth of soil where it rests.
Then, considering a certain factor of safety, the diameter of the pile can be determined.

 

b. Friction Piles

Friction pile is the type of load-bearing pile that transfers the load to the soil by the friction mechanism between the surface of the pile and the surrounding soil layer. Friction force can be developed along the entire length of the pile or certain length of the pile depending upon the strata of the soil. Unlike, end bearing piles, in friction pile, the entire pile surface functions to transfer the loads from the superstructure to the soil. The capacity of friction piles can be determined by multiplying the surface area of the pile by the friction force developed per unit area. The certain factor of safety must be considered while designing friction piles.

 

iii. Soil Compactor Piles

Sometimes, it may be necessary to compact the soil to increase the bearing capacity of the soil. Piles used for such purpose of increasing the bearing capacity of the soil are known as soil compaction piles. The use of soil compactor piles is one of the potential methods for increasing the bearing capacity of the soil and improving the overall stability of the soil.

Figure 5: Soil Compactor Pile

 

B. Based on the construction method or construction material used

Depending upon the method of construction of the pile foundation and the materials used pile foundations may be classified into the following:

i. Timber Piles

Timber piles generally refer to the type of pile foundation that comprises of timber as the material of construction. Timber piles are mostly placed under the water level and can be rectangular or circular. They have a long life span of approximately 30 years. During the design of timber piles, commonly the diameter is kept between 12 to 16 inches and the length is taken as 20 times of the top width. Such piles can withstand load as high as 15 tons to 20 tons. Usually, fish plates are attached or bolted on the side of the timber piles to gain additional strength.

Advantages and Disadvantages

Some of the advantages of timber piles can be listed as follows:

1. Timber piles are economical because of the easy availability of timber.

2. The installation process is relatively simple and easy.

3. The timber piles can be cut to the desired length even after installation and it has a lower possibility of damage.

4. In case, timber piles have to be removed, they can be easily pulled out.

Some of the disadvantages of timber piles can be listed as follows:

1. Timber piles cannot be used as end-bearing piles.

2. The driving of timber piles is extremely difficult in the hard soil layer.

3. Spicing of timber piles is difficult.

4. If proper care is not taken, defects may occur in timber. Treatment
of timber such as application of preservative must be carried out.

timber pile

Figure 6: Timber Piles

 

ii. Concrete Piles

Concrete piles are the type of pile foundation that essentially consists of concrete as the chief element. Concrete piles may be further classified into Pre-cast concrete piles and cast-in-place concrete piles.

They are briefly described as follows:

a.  Pre-Cast Concrete Piles

Pre-cast concrete piles are one of the most commonly used pile foundations. Such piles are directly cast in the pile bed and transferred to the location of foundation. Usually, rectangular piles are cast in horizontal form whereas the circular piles are cast in vertical form. Pre-cast concrete piles may also be reinforced. Once, the piles are cast they have to be cured for at least 28 days.

Advantages and Disadvantages

Some of the advantages of pre-cast concrete piles can be listed as follows:

1. Pre-cast piles have high strength and stability.

2. They can withstand the load of high intensity.

3. Pre-cast piles have high resistance to corrosion and biological defects and remain unaffected by groundwater.

4. Such piles can be used in almost all types of construction.

5. Pre-cast concrete piles have high durability and are cost-effective.

Some of the disadvantages of pre-cast concrete piles can be listed as follows:

1. The change of length of such piles and mobilization is difficult.

2. Driving of the piles requires heavy and advanced equipment.

3. There is a high risk of breakage of piles during the handling or driving operation.

precast pile

Figure 7: Pre-cast Piles

b. Cast-in-Place Concrete Piles

Cast-in-place concrete piles are the type of concrete piles that are constructed by boring the soil up to certain desired depth, filling with freshly mixed concrete and allowing it to cure on the site itself. Generally, an outer metallic shell is first driven into the ground and filled with a fresh mix of concrete. It is then allowed to cure. Finally, the outer metallic shell is removed or pulled out.

Advantage and Disadvantage

Some of the advantages of cast-in-situ concrete piles can be listed as follows:

1. They are more flexible in terms of length variation.

2. Handling of such piles is easier.

3. Additional piles may be provided easily.

4. The installation process is simple with no possibility of breakage.

Some of the disadvantages of cast-in-situ concrete piles can be listed as follows:

5. Since the pile is cast in-situ, proper storage place must be arranged for storing the construction materials safely.

6. Quality control must be ensured during installation.

7. The difficulty may arise in casting the concrete where the underground flow of water is relatively heavy.

cast in situ pile foundation

Figure 8: Cast-in-Place Piles

 

iii. Steel Piles

Steel piles are the type of pile foundation that is made up of steel sections. Commonly used steel piles include I-section piles and hollow steel piles. This type of pile is mostly used as end-bearing piles. They are usually designed in smaller sections with a diameter ranging between 10 inches to 24 inches.

Advantage and Disadvantage

Some of the advantages of steel piles are listed as follows:

1. Due to the smaller sectional area, such piles are easy to handle.

2. Steel piles can be driven easily through hard strata of soil and can be taken up to deeper depth.

3. Steel piles can withstand heavy loads.

Some of the disadvantages of steel piles can be listed as follows:

1. Steel piles are comparatively expensive.

2. Such piles are prone to corrosion.

 

iv. Composite Piles

Composite piles generally consist of two or more different materials usually one mounted over the other such that they function as a single unit. For example, composite piles may be constructed of steel and timber or steel and concrete etc.
Such piles have very high capacity.

 

 

 

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