There are basically two types of foundations – Shallow and Deep. Pile foundation is a type of deep foundation.
In this post, I’ll discuss everything about pile foundations that you need to know as a construction professional. That includes the basics of pile foundations, its types, function, usage, and construction method.
What Is A Pile Foundation?
A pile is basically a column which is built underground.
Typically it’s a long cylinder of a strong material such as concrete or steel that is built or pushed into the ground to support structures built on top of it.
When the depth of a foundation is more than three times its breadth, we can call that a pile foundation.
A pile foundation has two parts. One is the pile itself and another is a pile cap.
A pile cap is normally built upon more than one piles. Structures are built on top of the pile cap. See the image below:
When Pile Foundations Are Used For A Structure?
Pile foundations are normally designed for a structure in the following situations:
- When there is a weak layer of soil near the ground that can’t support the load of the structure.
So we need to bypass this layer to transfer the load of the structure to a strong layer of soil or rock that is below the week layer.
- When a structure has very heavy, concentrated loads, such as highrise buildings, bridges, and urban water tanks, etc.
- If the subsoil water table is very high.
- If the structure is subjected to horizontal forces.
- Where there is a possibility to uplift forces.
- If there are deep drainage or canals near the structure.
Types Of Pile Foundations
Fundamentally, pile foundations are two types based on structural behaviors – End Bearing piles and friction piles.
End Bearing Piles: the principal of this type of pile is, the bottom end of the pile is rest on the hard layer, mainly in the intersection of the weak and hard layer.
And this pile transfers loads of the structure through it to the hard layer and thus it bypasses the weak soil layer.
So, basically it acts like a column.
Friction Piles: Friction piles are completely different than the End bearing piles.
End bearing piles normally transfer loads through its bottom end whereas fiction piles do it across its full length, by friction.
That means the entire surface of a friction pile works to transfer loads to the soil.
So, we can say, the longer a friction pile, the more amount of loads it can support.
Depending on various criteria, you can classify the pile foundation into many other types.
You need to know about those classifications as a construction professional.
Because, if someone tells a different name of a pile foundation you might be confused.
To clear this confusion, let’s see how the pile foundation can be classified based on various criteria.
Based On Materials Used To Make Piles:
- Concrete pile
- Steel pile
- Timber pile
Based On Construction Method:
- Cast-in-situ pile
- Precast driven pile
Based On The Purpose Of Use
- Sheet pile
- Load bearing pile
- Soil compaction pile
Materials Used To Make Piles
Timber, concrete, and steel are commonly used materials for constructing piles.
If you have available trees in your area that can be used to make piles, then you can make timber piles.
As two trees can’t be joined together the pile length is limited to the length of one single tree.
Concrete piles, mainly reinforced cement concrete piles, can be precast driven or cast in situ.
The shape of concrete piles can be round, rectangular, or hexagonal.
Steel also used to make piles. They are normally in H shaped. These piles can take very heavy loads. They also save construction time as it is driven into the ground directly and the casting process is eliminated.
Steel piles can be scraped away by the soil. So concrete piles should be used in areas with cohesive soil.
How Pile Foundations Are Designed And Work?
Pile foundations carry a lot of loads. So they are designed very carefully.
Engineers normally study the soil where the piles will be placed in and ensure that the soil isn’t overloaded beyond its bearing capacity.
Every pile has a zone of influence on the soil around it. That’s why loads should be distributed evenly over the soil that carries the piles, and not concentrated into a few areas.
While designing, engineers normally space the piles far enough to meet this requirement. And group a few piles together and top them with a pile cap.
This pile cap is a thick concrete footing that extends over a group of piles and works as a base of column or columns of the structure.
The main function of a pile cap is to distribute loads, imposed on it by the structure, evenly to all the piles in the group.
Construction Methods Of Piles
So far I have discussed the basics of pile foundations, its types, and how they are designed and work.
Now let’s see the construction methods of pile foundations.
There are basically two methods to construct piles.
One is Cast-In-Situ and another is Driven.
These piles are normally made of concrete.
The construction method of a cast in situ pile is simple.
You first need to bore a hole into the ground.
Then remove all soil from the borehole.
After that, lower the reinforcing cage into the borehole.
Finally, pour concrete into the hole.
I’ve talked about this before in the post how to construct a bored cast-in-situ pile. Read the post to learn every step in detail.
Precast concrete piles, timber piles, and steel piles are constructed in this method.
The steps of constructing piles in this method are:
First, the piles are cast at the ground level if they are precast concrete piles. If they are timber or steel piles just make them ready at the ground level.
Then, drive a pile into the ground using a pile driver.
A pile driver is a machine that holds the pile perfectly vertical, and then hammers it to drive into the ground blow by blow.
Each blow is hammered by lifting a heavyweight and drop it on the top of the pile.
The top of the pile is temporarily covered with a steel cap to prevent it from breaking due to hammering.
A pile driver actually performs two actions – first, it acts as a crane and lifts the pile from the horizontal position on the ground and rotates it into the vertical position and holds it there. And second, it hammers the pile to drive into the ground.
Piles should be hammered until they can’t be driven into the soil any further.
There is a little known method of constructing a pile that is used in special situations.
As we know, driving piles is very noisy and creates huge vibrations through the soil. This vibration could cause structural damage to the old buildings nearby.
For this reason, it is sometimes not possible to construct piles both in the cast in situ and driven method in some sensitive places.
Even, these methods are prohibited in some residential areas of many countries.
In such situations, the special method is used to construct piles.
In this special method, there are actually two types of piles are constructed – micro piles and helical piles.
Neither of these piles rely on hammering.
For this piling, a hole is dug into the ground using a soil boring machine. The hole is normally a little larger than the pile diameter and the full length of the pile.
A precast concrete pile is lowered or pushed into the ground and then a concrete grout is poured into the gap around the pile.
Helical piles are steel tubes that have spiral blades attached and look like giant drill bits.
The pile is driven into the ground by rotating and pushing it from above, much like we drill a screw into wood.
Once the pile is completely driven into the ground, a pile cap is constructed on the top of it.
After reading this post, you now have a basic knowledge of the pile foundation. That includes – what the pile foundations are, their types and construction methods.
With this knowledge, you can now go for constructing pile foundations.
I’ve just described the basics of constructing pile foundations. But every pile construction method needs elaborate discussion and you need to gain practical knowledge.
Practical knowledge is very essential in construction. For this visit construction sites where piling work is going on.
If this article about pile foundations helped you somehow, please do share it with your friends…