Foundation is a critical stage of building construction.
The two major types of foundations are shallow foundations and deep foundations. Just as the models sound, they are measured by their depth in the soil and the load they can carry.
You don't have to be an engineer to read this piece. It will guide you as a property owner to determine if your engineer is well informed and fit to carry out your project.
This article is simple enough for a better understanding of this early stage of building construction.
Before commencing construction of any sort of building, it is necessary to know the type of foundation suitable for building construction. Mistakes at this level are detrimental to the durability of the building and can create a failure in the whole structure.
The table below summarizes the types of foundation in building construction and where they’re used.
Types of Foundations
Where They’re Used
Individual footings or pad foundation
Recommended when the load of the building is carried in a single column
For structures with more load-bearing and boundary wall construction
Raft or mat footing
Very suitable for weak soil
Appropriate when the column is spaced closely and the soil has a low bearing capacity
When hard soil is very deep in the earth and far from reach
Caisson or drift shaft foundations
When the depth of hard stratum is between 25 feet and 300 feet
Now, let’s take a closer look at the different types.
This type of foundation does not exceed the safe bearing capacity of the soil. It is also called open footing. When constructing a shallow foundation, you first excavate the earth, till the bottom of the footing, then build the footing.
The entire footing is visible in this type of foundation. It is mostly applicable to lightweight buildings with perfect soil conditions.
Shallow foundations can get damaged from freezing. Therefore, they must be protected during cold climates. Also, they should be constructed below the frost line protected from insulation.
There are 4 types of shallow foundations;
The footings are usually circular, square, or rectangular pad of concretes, and are recommended when the load of the building is carried in a single column.
When constructing a pad foundation, the engineer divides the total weight on the column by the safe bearing capacity of the soil. The pressure will determine the type of shape suitable for construction. They are recommended when the building carries a separate column.
The individual footing is the most accessible type of footing and does not need much-skilled labor. It is economical and time-saving compared with other types of shallow foundations. This footing is applicable when the column is not spaced closely and the building load is less.
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They act as support for the weight of a wall. This footing has a broader base, which is why it is also called spread footing. Its more-extensive base helps to provide more stability as it spreads across a full range.
Strip footings are suitable for walls and bridge piers where the layer is between 10 feet, with their size and thickness depending on the type of soil in the site. The ground must be sufficient enough to support the weight of the structure.
Strip footings are suitable for structures with more load-bearing and boundary wall construction.
You can use this type of footing when you plan to construct a basement. The entire basement floor slab is the foundation. Raft footings are very suitable for weak soil. The weight of the building is evenly spread on the earth to reduce the stress on the ground per space. Engineers determine the pressure by dividing the weight by the area.
You can consider raft footing on compressed soil like soft clay where pad, strip, or pile cannot be possible without excessive excavation.
The combined footing is also suitable for constructing buildings near a property line or sewer line, and if the dimension is restricted.
Deep foundations sink deep into the earth in such a way that the building rests directly on the soil. They target hard stratum and are often reliable in deep waters.
We’ll discuss two types of deep foundations.
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Let's start by buttressing the conditions for using a pile foundation. It is suitable when hard soil is very deep in the earth and far from reach. Also, when a raft or mat foundation is too expensive. It is also suitable when the building load is massive and concentrated.
Piles are appropriate for compressible soil and marshy places. They’re recommended when constructing bridges over ample water and canals. And if there is a possibility of having irrigation in the area.
Piles are vertically driven into the ground and can be classified into two working modes. Some piles are used for bearing while others are for frictions.
Bearing piles are often suitable for places where hard stratum is not too deep. They transfer the load straight to the hard layer. Frictional piles are befitting where the soil is too soft. The skin is left rough to increase friction, but this is controversial. At some point, the abrasions wear off and lead to total failure of the structure.
Piles are materially classified into;
Interestingly, they are made from trees like Sal, Babul, Deodar, and many others. These timber piles are circular with a sharpened bottom. The diameters vary from 20cm to 50cm and are always 20 times longer than the width.
The sharpened area can be blunt or supported with a metal point, and an iron cap is set on the top. If the ground is soft, a round bottom is suitable. Otherwise, use a metal point if the earth has boulder.
They must be dug below the water table to avoid decay. This type of pile is economical and does not need much of heavy machinery before it can be driven.
Here, a shell is driven into the ground to make a hole of a specific diameter. Then, engineers fill the gap with concrete. They are also called cast-in drilled hole piles. Engineers leave the shell in the hole and sometimes fill it up with concrete. They do this to make the shell protect the cement from getting eroded by acidic water it may encounter in the sub-stratum.
Concrete piles are suitable when engineers want erect foundations through plastic soil to hard stratum. They do not allow wastage and are ideal for underwater construction.
R.C.C piles are often circular, square, or octagonal with a steel helmet at the top. These piles are 15cm to 60cm in diameter and 3m to 30m in length. R.C.C does not contain more than 4% steel. The engineers precast and sharpen them just like wooden piles. After they have been cured and seasoned, they are driven into the ground.
Unlike timber, the execution is quicker and is applicable above the groundwater table. Reinforced cement concrete piles are difficult to transport because of their heavyweight and must not get damaged during transit. A little damage can cause difficulty in transpiration. This is one of the major reasons why they need skilled labor.
Also, there is a class of piles that’s suitable as a part of the foundation structure. It’s called sheet piles. The piles are used to enclose certain areas and confine loose soils during construction. These sheets can be wooden, concrete, or R.C.C.
Caissons are often recommended in the construction of a concrete dam, repair of ships, or bridge piers for construction in rivers. They are also called the drilled shaft. They are watertight boxes or cylindrical structures that are sunk into the water and filled with concrete to form a foundation.
After filling the drum, the soil around it is scooped out with daggers so that the drum can sink appropriately into the ground. If it's too short, another length of caisson will be attached and daggers will be used to scoop the sand to let it sink deeper.
Caissons have toe or shaft resistance to resist load from structures. And can carry columns more than pile foundations. They’re befitting when the depth of hard stratum is between 25 feet and 300 feet.
Although drilled shaft foundations may seem like the strongest for underwater construction, they’re recommended when deep deposits of soft clays and loose granular soil exist.
Soil type and load bearing are critical factors to consider when choosing a suitable type of foundation. Whether it's a big or small building, an engineer must be fully aware of the surroundings to erect a very durable structure.
If you’re an engineer, there are essential steps to follow to choose the appropriate structure.
First, you must inspect the soil you want to build on. Second, calculate the live and dead load that the foundation needs to carry. This will help you to decide which footing to use. Then, design the structural specifications like size, weight, and depth.