I'm starting to build my house on clay soil and the construction foreman tells me I need foundations that are 80 cm deep, but a neighbor says 60 cm is enough. Is there really a standard measurement or does it depend on the soil type and the weight of the construction?
Don't just go by what your neighbor says, because every plot of land is different and clay is tricky. The thickness depends on several factors: how compacted the clay is, how much load the house will bear (whether it's one story or two), and how the ground behaves during rainy season. The 80 cm your builder is asking for is probably safer than the 60 cm your neighbor mentions, but the ideal thing is to do a formal geotechnical study so you don't end up halfway there. Without that analysis, you're gambling that everything will work out, and in construction that's not a good bet.
Have you already had a soil study done on your property? Because your contractor is right - there's no one-size-fits-all measurement, it really depends on the type of soil (with heavy clay like yours you do need more depth), the weight of the construction, and even how much moisture there is in the area. What your neighbor says might work on his property but not necessarily on yours, so the best thing is to trust what whoever is actually looking at the site recommends.
Don't let yourself be talked into something just because your neighbor did it and it worked out for him - clay is finicky, and what worked on his lot could be a disaster on yours. What your builder is asking for makes sense: on clay soil the bearing capacity is lower, so you need more depth and thickness to properly distribute the weight of the structure. The only way to be sure is to get a geotechnical survey that measures compaction, bearing capacity, and the behavior of your specific soil; without that you're gambling, and failed foundations cost way more than the survey.
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