4 ms·

Foundations do not have to go to bedrock but they do need to be poured on tightly compacted dirt or the building can eventually sink and crack as the loose soil

Foundations do not have to go to bedrock but they do need to be poured on tightly compacted dirt or the building can eventually sink and crack as the loose soil gives under the weight of the structure. If you watch a lot of home builders nowadays, they don't bother compacting, which is why people tend to have settling issues and cracking in their brand new homes after a few years. Another issue is pouring a foundation on clay because it will move whether it is compacted or not because it is around 40% water by nature.

throwaway0a5e 4y ago
They don't bother doing more than a token effort at compacting because unless you have really crappy soil the foundation is likely strong enough to spread the load to enough soil to bear the weight without cracking. If foundations didn't need to do that job they'd have a lot less if any rebar. Basically the way they do it is cheaper and good enough.
engineer_22 4y ago
It's not that simple. Even pile foundations don't always go to bedrock. In fact, most of the load bearing strength of a driven pile is in the friction along the length of the shaft, and the end-bearing capacity is a fraction of that. (It's intuitive if you recognize the surface area of the end of the pile is much smaller than the surface area of the sides of the pile) Soil calculations treat the soil almost as if it is a highly viscous fluid. In regards to settling - all structures settle. It is the structural engineer's job to design a structure that anticipates settling and mitigates strain in the rest of the structure. Clay is interesting, because it has a property called consolidation. If the clay is overconsolidated, that means the overburden pressure in the past is higher than the pressure on the soil now. If a clay is overconsolidated, it settles a lot less.

4 ms·

Foundations do not have to go to bedrock but they do need to be poured on tightly compacted dirt or the building can eventually sink and crack as the loose soil

by innocentoldguy 4y ago

throwaway0a5e 4y ago
They don't bother doing more than a token effort at compacting because unless you have really crappy soil the foundation is likely strong enough to spread the load to enough soil to bear the weight without cracking. If foundations didn't need to do that job they'd have a lot less if any rebar. Basically the way they do it is cheaper and good enough.
engineer_22 4y ago
It's not that simple. Even pile foundations don't always go to bedrock. In fact, most of the load bearing strength of a driven pile is in the friction along the length of the shaft, and the end-bearing capacity is a fraction of that. (It's intuitive if you recognize the surface area of the end of the pile is much smaller than the surface area of the sides of the pile) Soil calculations treat the soil almost as if it is a highly viscous fluid. In regards to settling - all structures settle. It is the structural engineer's job to design a structure that anticipates settling and mitigates strain in the rest of the structure. Clay is interesting, because it has a property called consolidation. If the clay is overconsolidated, that means the overburden pressure in the past is higher than the pressure on the soil now. If a clay is overconsolidated, it settles a lot less.