19/02/2025
I am inclined to pen something about civil engineering today.
It often strikes me how people enjoy engineering wonders without a care about how they came about. The sheer fact that you climb a staircase is an engineering marvel. Even that the weight of the cantilever could support you on the balcony should evoke wonder. That's not to talk of the fact that in a place which used to be swampy and waterlogged just a few months ago now has a 3-storey building standing firm.
What you observed that does not seem remarkable is the equivalence of you balancing on soft mud while carrying a heavy load. Will you be able to do it? Of course not. The ground beneath you will give way, making it impossible to stand firm.
But a 3-storey building heavier than you in multiple folds is able to do it. How did that happen?
What engineering does is the equivalence of laying a big, flat board beneath your feet, spreading your weight evenly over the soft surface. Suddenly, you’re steady and stable. Right?
That’s the idea behind a raft foundation, a clever engineering solution for building on soft, swampy, or waterlogged land.
While there are several engineering solutions for constructing foundations in swamy areas, raft foundation is quite popular for many areas I have worked - from Lagos to Akwa Ibom to Abia. The Island in Lagos would be of particular interest to those looking to see this spectacle in action.
In places like Lagos, where much of the land is low-lying and swampy, raft foundations are a lifesaver. With its booming population and ever-expanding cityscape, Lagos often has no choice but to build on challenging terrain. Whether it’s the marshlands of Lekki or the reclaimed areas of Victoria Island, raft foundations play a key role in keeping buildings upright and safe.
A raft foundation is a large, flat slab of concrete that spreads the weight of a building evenly over the ground. Instead of relying on deep columns or piles to support the structure (as in some other foundation types), the entire slab “floats” on the soft soil, much like a raft on water (hence the name). It spreads like a mat. This prevents the building from sinking or settling unevenly, which could cause cracks and structural problems.
Think of it as having a nice time on the beach. To place a cup on the sand would be difficult. The liquid would just spill away. So you find a tray, level the sand with your hand as much as you can, place the tray on the sand and then place your cup on the tray. It would work, right? In the same manner, think of a raft foundation as a wide, sturdy tray that carries the building's weight. By spreading the load over a large area, the pressure on the weak soil below is reduced, making it possible to construct even heavy buildings on less-than-ideal ground.
Areas reclaimed from lagoons and swamps don't have the types of soil with the strength to hold up buildings with concentrated loads. Without a proper foundation, structures can tilt, sink, or even collapse.
Raft foundations are particularly useful in areas with high water tables, where the ground is saturated with water just below the surface. In such conditions, traditional foundations that rely on digging deep into the earth may not be practical or cost-effective. Raft foundations solve the problem by providing a stable platform that distributes the building's weight.
Before laying a raft foundation, engineers often prepare the ground by compacting the soil or adding a layer of sand or gravel to improve stability. Then, they pour a thick slab of reinforced concrete (I have explained this in a previous civil engineering post) over the prepared area. The concrete is strengthened with steel bars (rebar) to make it more durable and resistant to cracking.
In swampy areas, engineers may also use pumps to remove excess water during construction, ensuring that the site remains dry and the foundation sets properly.
In Lagos, raft foundations have been used for various types of buildings, from residential homes to office complexes. For example, I know of a luxury apartment complex in Lekki built on swampy ground using a raft foundation. The engineers designed a 1.5-meter-thick concrete slab, reinforced with steel, to ensure the building remained stable despite the waterlogged soil. 1.5meter is 5 feet. How many feet are you sef? That's the height computer calculations told us it'd be safe for further construction.
There is also a school in Ajah which faced the challenge of building on waterlogged land. The engineers added drainage pipes beneath the raft foundation to channel water away from the site, creating a stable and dry base for the school.
Like I mentioned, there are several other foundations that could be used for swampy areas, but raft could be preferred for the following benefits:
- Stability: Raft foundations prevent buildings from sinking or settling unevenly, even on soft or swampy ground.
- Cost-Effective for Certain Projects: While they require a lot of concrete and steel, they are often cheaper than deep pile foundations (I would like to explain this in a future post) for low- to medium-rise buildings.
- Water Resistance: Properly designed raft foundations can act as a barrier against water intrusion, protecting the structure above. Imagine floods that could arise and want to seep through the foundation.
- Quick to Construct: Compared to other types of foundations, raft foundations can often be completed more quickly, which is important in fast-growing cities like Lagos.
But despite their advantages, raft foundations are not without challenges. For one, they require careful planning and precise calculations. If the slab is too thin or the steel reinforcement is inadequate, the foundation can crack or fail. That's why engineers do not joke with their calculations. Employ ethical professionals with track records, I would always say. I said 'professionals' 'ethical' and 'track record' in the same sentence. If you miss anyone of those terms, anything you see, just take it laidat.
Additionally, the construction process requires skilled labor and proper supervision to ensure quality. That's where artisanship comes in. This is not the time to ask Baba Mulika, the bricklayer, to use 40 bags of cement and he says he's been working for 30 years and so 25 bags is good enough. An experienced engineer is needed on site to enforce instructions.
There are so many points of failures in a building construction, even so in delicate environments, and getting experienced professionals is nonnegotiable for all aspects of construction including for a foundation like Raft.
Raft foundation is a work of engineering wonder.
