Foundations – Layman Builder
The foundation is an essential part of the building. Although below ground, the foundation supports everything above ground. It is very important that the foundation be properly constructed. It is also important to note that different buildings require different foundations. You cannot expect to use the same foundation for a bungalow and a 2 storey building. A lot of the collapsed buildings cases we have seen are as a result of poor foundations. It is important that a proper soil investigation is done by the appropriate expert.
Types: There are 4 main types of foundations: strip, raft, pad and pile foundations.
These are the most commonly used type of foundations used. They are the cheapest to construct. There are 2 other variations of strip foundations apart from the normal strip: deep strip and wide strip. Deep strip foundations have a deeper footing than the normal strip. They are used where nearby trees could pose a problem or the soil is too soft. Wide strip foundations have a wider footing and are used to spread the load over a wider area, where the soil is poor.
These are used where the soil is soft, clayey or waterlogged. The foundation is a reinforced concrete slab spread out like a mat, covering the entire area of the building up to the ‘German floor’. This is more expensive to construct than strip foundations.
Separate concrete columns or pillars are casted from the ground up to the slab. The columns have a base which is called the ‘pad’. Pad foundations are used where you want to create a basement such as an underground parking lot or where the building is completely raised from the ground.
This type is rarely used for domestic purposes as they are the most expensive though the strongest type. They are used for high rise buildings such as skyscrapers and bridges. They require specialist engineers because they require precise calculations. Piles are made by boring a hole deep within the earth and filling it up with reinforced concrete.
Casting Ground Beam for Raft foundation
When constructing your raft foundation, the ground beams are very important because there take away the loads of the walls and columns of the building and spread them over the foundation slab (raft) at the edges and under load bearing walls. Therefore, it is very critical because the foundation slab actually rests on them. Here are some tips to know when casting ground beams on the casting day (the day concrete is poured):
Step 1: At the position where the ground beams will be, a ridge or hole would be dug (not too deep) which would then be blinded with concrete. Then the wooden formwork is erected and the reinforcement bars placed into the formwork following the bending schedule given by the engineer. The height of the formwork would depend on the height of the ground beam to be constructed.
Step 2: There would be reinforcement bars or upbars that will protrude to about 5 feet above the formwork located at the corners or middle of the foundation. These protruding rods will be used to form the pillars or columns that will join block walls for the superstructure of the building.
Step 3: The plumber is also going to be around to make sure pipes for the wastes and drains are installed inside the formwork to avoid breaking the floor again after it has been constructed.
Step 4: The concrete to be poured in will then be mixed by mixers (people who mix the concrete). There is a given ratio for the concrete mix, which can be ratio 1:2:4; which means 1 head pan of cement, 2 head pan of sand and 4 head pan of granite will be mixed together in addition of water to form the concrete to be poured into the formwork containing the reinforcement bars.
If the concrete mixer which is to be used in mixing the concrete is bigger than usual, then the mixture would contain one bag of cement, 4 head pan of sand and 8 head pan of aggregate. The size of the granite usually used is the three quarter (¾) inch granite, but all the pieces of granite are not actually the same. At this point, the engineer/builder who is experienced in building would then decide if more granite would be used over less sand and vice-versa. When mixing the concrete, you have to realize that the type of water used is very important e.g., salty water should not be used for mixing concrete.
Step 5: After the concrete has been allowed to set, then the formwork is removed and the ground beam construction is complete.
This is the first stage in constructing a raft foundation. Join us next week for more lessons.