Pile foundation- Definition and Types

Before we go into the detailed explanation about types of pile foundations we have to know some research behind pile foundations.

Pile foundations have been used since prehistoric times and In order to develop a reasonable design method. Analytical and experimental studies are performed over the past four to five decades, these studies led to the development of many design methods based on various soil types, loading criteria, and environmental conditions.

While developing these design methods, the piles are classified into various types. So, here we only focus on different types of pile foundations based on material types, Construction methods, Installation procedures, and load transfers.

Introduction

The load which is coming from the superstructure is transferred to the ground by means of a foundation.

Foundation is the connection between structure and the ground and helps in the transfer of loads to the soil. Foundations are of two types; shallow and deep.

Then, What is a pile foundation?

It is a type of deep foundation, They are formed by long, slender, columnar elements which are made from either timber, steel, concrete, or composite materials and which are used to transfer the load and to support the structure.

These pile foundations are used when the soil at shallow depth cannot be able to bear or resist the structural load coming from the superstructure.

When to use a pile foundation

• When the groundwater table is high.
• When the other type of foundation is costlier or not feasible for the conditions at the site.
• If the soil at the shallow depth is compressible and not suitable for resisting heavy loads.
• If there is a water body near the construction site and due to that water percolation is a big problem.
• When the structural loads are too high and uneven, etc…

Types of pile foundations (or) Classification of pile foundation

1) Types of pile foundations based on material

• Timber piles
• Concrete piles
• Steel piles
• Composite piles

2) Types of pile foundations based on function

• End bearing piles
• Friction piles
• Both End bearing and friction piles

3) Types of pile foundations based on piling methods

• Cast-in situ piling
• Precast piling

4) Types of pile foundations based on Installation

• Replacement piles
• Displacement piles

Detailed Explanation of; Types of pile foundations

The classification of piles is mainly based on material types but to understand the exact information about types of pile foundations, we need to describe the following three types of classification.

They are not exactly the types of pile foundations but helps in differentiating the properties of piles.

Function of piles

End bearing piles:

The toe of the pile rests on hard strata of rock or soil and transfers the whole load from the soft soil or top layers of ground to deep hard rock. Therefore the pile acts in a similar way as the column of the structure.

Friction piles:

The piles which transfer the load through skin friction or resistance without any end-bearing is known as friction piles. Here the load is taken by the friction that is developed between the sides of the pile and the layers of the soil.

Both end bearing and friction piles:

Here the piles which transfer the load through friction as well as end-bearing. This type of piles can resist heavy loads coming from the structure and will have a longer life.

Types of piling method or Construction method

Cast-in situ piling:

The bored cast-in-place piles are constructed by digging a hole in the ground then reinforcing it and filling the hole with concrete. So these piles are constructed at the site and no extra costs are required.

Precast piling:

These are cast and cured at the factory and transported to the site then driven into the ground by means of diesel or hydraulic hammer. These piles require a large space for curing.

Installation Type

Replacement piles:

The soil is taken out from the ground and replaced it with the concrete or with a pre-cast pile then grouted in, known as a replacement pile. In a Simple way, concrete or steel replaces the soil.

Displacement piles:

There is no removal of soil from the ground. The piles are driven by a hammer to displace the soil either vertically or radially, compacting the above layers into the surrounding soil.

Material Types

The types of pile foundations are mainly classified based on the material; Timber piles, Concrete piles, Steel piles, and Composite piles.

Timber piles:

Timber piles are the oldest type of pile foundations and are used to support the structural loads. These can be easily handled and readily cut into desired lengths on the site, Timber piles will last longer if the environmental conditions are favorable.

Timber piles are best suitable as friction piles in sands, silts, and clays. These are not suitable for dense gravel or as an end bearing piles to rock.

The length of piles ranges from 20 to 60ft (6 to 20 m) for diameters of 6 to 16 inches. (150 to 400mm), these dimensions correspond to the natural availability of tree trunks. Also, the design loads vary from 10 to 50 tons (89 to 441 kN).

Advantages of Timber piles;

• Timber piles of regular sizes are available and can be easily cut into the required sizes.
• Easily installed at the site
• These piles are economical as the material cost is very less.
• The possibility of damage is very less.
• Timber piles don’t require anti-corrosion protection.
• They are eco-friendly as being made from naturally available material.

Disadvantages of Timber piles;

• Piles of longer lengths are not available.
• These are not to be used as end-bearing piles.
• If the soil strata are very hard, timber piles cannot be driven through and there is a need to look for another material.
• Material deterioration is a big concern. Decay due to fungi, insect attacks, and mechanical wear are the primary factors to be taken care of. so these piles have to be treated with preservatives to sustain for the long run.

Concrete piles:

Concrete piles are made from concrete and based on installation methods concrete piles are two types; Precast concrete piles and cast-in-place concrete piles.

1) Precast concrete piles:

As the name suggests that, these piles are cast, cured, and stored at the factory then transported to the construction site and mostly driven into the ground.

These piles are available in various cross-sectional shapes such as circular, octagonal, or square with chamfered corners and these may have drilled core holes at the centers to save weight.

Pre-cast concrete piles are designed to withstand both service loads and driving stresses (Stresses due to hammering or driving force)

These are designed to withstand heavy loads (typically up to 300 tons, or 2670kN) and are reinforced to resist bending and uplift.

Precast concrete piles are used either as end-bearing piles or friction piles.

Advantages of Precast piles;

• High resistance towards chemical and biological attacks.
• The strength of the material is controlled properly.
• The precast piles are usually strong.
• If there is any sight of damage in the material they are easily replaced before installation.
• Precast piles can also be driven under the water.
• The whole process requires less time than cast-in-place piles.

Disadvantages of Precast piles;

• The length of the pile is fixed and cannot be increased or decreased at the site.
• These Pre-cast concrete piles require heavy and expensive machinery to drive.
• There is a possibility of breakage while driving the piles.
• These are not readily available in the market, based on site conditions the sizes have to be ordered and bought to the site.

2) Cast-in-place concrete piles:

In this method the piles are driven either by driving, boring, jetting, coring, or a combination of these and other methods then concrete is placed in the hole itself to allow it to cure for few days and the structure is constructed over the concrete piles.

These piles don’t require casting and storing space, also they are only designed for service loads because there is no need of driving and lifting.

Pile lengths and dimensions can be adjusted as per the site requirements and therefore predetermination of pile length is not critical.

Advantages of cast-in-place piles;

• The length of piles can be determined at the site and can be varied easily.
• No possibility of breaking while placing the pile.
• Additional piles can be provided if required.
• The cost of the material also is saved, as we are designing only for service loads.

Disadvantages of cast-in-place piles;

• The bottom of the pile is uneven and the concrete placed may not be symmetrical.
• It is very difficult to construct cast-in-situ concrete piles where the groundwater table is high.
• If the piles are un-reinforced then we can expect the pile may fail in tension.

Steel piles:

Steel piles are very easy to handle and can be cut into any required shape at the site itself. Also it is able to bear heavy loads which makes steel piles efficient and reliable.

Steel piles may be of hollow or I sections. These piles are filled with concrete after driving into the ground. The steel piles are mostly circular and rectangular and size may vary from 10-24 inches in diameter, and thickness is generally ¾ inches.

The cross-sectional area is small, so they are easily driven into the ground. Steel piles are used as end-bearing piles.

Advantages of steel piles;

• These can be easily handled and installed at the site.
• They are capable of reaching greater depth as compared to other piles.
• These piles can be cut into any size required at the site.
• They can carry heavy loads.

Disadvantages of steel piles;

• The pile material is prone to corrosion.
• Special treatment techniques are needed for protecting the material from deterioration.
• The steel piles may even bend when they hit hard-strata during the driving process.

Composite piles:

Composite piles are made by joining different sections of materials so that the advantages of both metals can be utilized.

We can use dissimilar materials together like concrete and wooden piles, concrete and steel sections, concrete-filled with steel piles, etc… based on the required properties.

But it is very difficult to form a good bond between dissimilar joints like concrete and wooden piles, so there needs to be proper research and conclusions while you are using composite piles.