Having been an Engineer in Nigeria for more than a decade, I would say that one of the issues I have seen is, Plumbing Engineers deciding to embed their waste pipes inside the slabs to be casted. Most times, I approach the Engineers and request why did they not consider other alternatives. Their response has varied each time. Some will tell you that running your waste pipes on the surface means that you will have a higher screed level for the floor while tiling. That this will lead to higher costs. in purchasing additional sharp sands which also leads to additional cement being required.
Sometimes, the response I get is, this is the normal practice and it is perfectly safe. Only a few Engineers act puzzled or are inquisitive to request if there was a better way to route your plumbing waste pipes. To the inquisitive ones, I go ahead to explain to them the risk involved in embedding your plumbing waste pipes inside the building slab. I explain to them how Plumbing codes forbid this act. The main reason this is forbidden is for the fact that the areas where you embed those 2in or 4in waste pipes will be weak structurally.
Most slabs used in buildings are designed to not exceed 150mm (6in). This means that when you embed that 2in pipe inside the slab, you will be reducing the volume of concrete in that portion of the slab. A 2in pipe is about 60.3mm in outer diameter. This means that the concrete in that portion of the slab will be 89.7mm. This is quite small considering that there will also be reinforcements embedded in the slab. This made even worse when you consider that a 4in waste pipe is 114mm in outer diameter, thus leaving only about 36mm concrete volume around it. If the 4in waste pipe is perfectly embedded, then you will be having 18mm concrete around it on both sides.
"Most waste pipes referred to as 2in in Nigeria are actually 1-1/2in pipes. Their outer diameters are actually 48.3mm rather than 60.3mm."
This makes these areas fragile and weak structurally. It is for this reason that I agree that Waste pipes should not be embedded in building slabs.
Another reason, I do not embed waste pipes in slabs, is that waste pipes need to be sloped at 1% or 2% in order to allow for easy movement of the waste liquid, slurry or solid. A 4in waste pipe must have a slope that does not allow the liquid waste to flow ahead of the solid waste. If you have visited a slab just about to be casted with reinforcement done, you will realize that when you embed the waste pipes in between reinforcements and position them perfectly, you will realize that it is quite difficult to keep the slopes intact. It's either the slope gets enormous or the there is no slope or the slope gets reversed later on during casting. Some people might say you can make use of binding wires to keep your slope intact. However, this is difficult to setup and it is also not fail proof.
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