I find myself in the middle of nowhere in a country where a building code is as rare a gem as an engineer and the task at hand is to build a small, roughly 65 sq m (700 sq ft) wood framed house of one level. Houses here are largely built by copying what appear to work with little more reasoning than 'it didn't fall down the last time it was done that way, so lets do it that way again'. This is a true DIY project: no heavy equipment, no tampers (that I can find yet), no surveyor, no geo site survey. I'm hoping that some experienced builders here can give me some best-practice-rule-of-thumb advice in order to up the odds of getting it right.
The issue I need to solve is foundation design. With no engineers, there is no way to get proper soil tests done, so I need a 'best guess' design that is most likely to work.
The site is a backfilled piece of swamp. Its a tropical country (extremes of rain for half year, fairly dry the other half). I did some shoveling to find that the ground starts with a 1/2 meter of gravel/rock. After the gravel there is white clay (cohesive I think) that is wet (after rain) for the next 1/2 meter or so. I could kneed it like bread dough in my hand. At a total depth of 1.5 meters it is solid/dry. There is supposedly up to 4 or 5 meters of this clay backfill over what used to be swamp. The backfill is displacing/filling the swamp and adding about a meter of elevation. The ground water is probably fairly high since it is within a half km of the coast.
The back fill has been there for a few years and with tropical rain (think of a swimming pool falling on your head in half an hour) I assume it has some moderate amount of self compaction. How much self compaction, I have no idea. Are there any 'old tricks' that can be used to get a rough idea of bearing capacity? Some people park cars on the same backfill nearby and there is no noticeable settlement but I don't know if that means much since a car is not a permanent weight. The most I can say is that the cars don't leave tire tracks nor sink while sitting and there is no noticeable settlement where they park.
So my question.... what sort of 'best guess' foundation can be built in conditions like this? I know there is no definitive answer, but I assume there are some general designs are more likely to succeed than others.
I've been asking around about how the local people build. I've heard advice such as putting 1m square pads as footings or to make a 1m wide strip foundation below ground (couple meters down) with pillars to the surface. The general consensus seems to be 'something wide below ground'. How wide and how deep seems to be a a matter of opinion.
I'll tell you what I'm thinking of doing and why, so people can correct me or make suggestions as to some approach that is more likely to succeed.
My gut says to try a reinforced monolithic slab foundation with an internal/perimeter beams (say 35cm/14" beams and a 10cm/ 4" floor?) and see if it 'floats'. The idea being that it could be over-designed enough that it will settle as one solid unit and could be leveled if needed. With soft clay near the surface though is this a likely recipe for failure? Is it more likely to float or more likely to twist, break and disappear into never-never land? The house itself will be fairly light since the roof doesn't need to support more than rain and tin.
If I just put footings, like the locals, the difference I see is the slab represents a large surface on wet clay and the footing a smaller surface on drier clay. Both are still in backfill. If the footing is sitting on dry clay I imagine it would be rather resistant to settling, but is there any guarantee the clay stays dry? I read somewhere that footing at 1m are below the 'wet line of clay' and don't uplift, but is that true or bad information? Is there any advantage to footing in clay versus a slab above the clay?
Another option perhaps would be a combination of footings below ground on the dry/drier clay and use the same slab on top. But does that gain much? Its extra weight. The footing is partially below the slab so there isn't an substantial increase in overall surface area. The footings though could act as a jack point if they are disconnected from the slab. Not sure. Are there any advantages/disadvantages to such a combination?
If you had to build in conditions like this, with all the unknowns, what would you do? I need some advice and guidance.
The issue I need to solve is foundation design. With no engineers, there is no way to get proper soil tests done, so I need a 'best guess' design that is most likely to work.
The site is a backfilled piece of swamp. Its a tropical country (extremes of rain for half year, fairly dry the other half). I did some shoveling to find that the ground starts with a 1/2 meter of gravel/rock. After the gravel there is white clay (cohesive I think) that is wet (after rain) for the next 1/2 meter or so. I could kneed it like bread dough in my hand. At a total depth of 1.5 meters it is solid/dry. There is supposedly up to 4 or 5 meters of this clay backfill over what used to be swamp. The backfill is displacing/filling the swamp and adding about a meter of elevation. The ground water is probably fairly high since it is within a half km of the coast.
The back fill has been there for a few years and with tropical rain (think of a swimming pool falling on your head in half an hour) I assume it has some moderate amount of self compaction. How much self compaction, I have no idea. Are there any 'old tricks' that can be used to get a rough idea of bearing capacity? Some people park cars on the same backfill nearby and there is no noticeable settlement but I don't know if that means much since a car is not a permanent weight. The most I can say is that the cars don't leave tire tracks nor sink while sitting and there is no noticeable settlement where they park.
So my question.... what sort of 'best guess' foundation can be built in conditions like this? I know there is no definitive answer, but I assume there are some general designs are more likely to succeed than others.
I've been asking around about how the local people build. I've heard advice such as putting 1m square pads as footings or to make a 1m wide strip foundation below ground (couple meters down) with pillars to the surface. The general consensus seems to be 'something wide below ground'. How wide and how deep seems to be a a matter of opinion.
I'll tell you what I'm thinking of doing and why, so people can correct me or make suggestions as to some approach that is more likely to succeed.
My gut says to try a reinforced monolithic slab foundation with an internal/perimeter beams (say 35cm/14" beams and a 10cm/ 4" floor?) and see if it 'floats'. The idea being that it could be over-designed enough that it will settle as one solid unit and could be leveled if needed. With soft clay near the surface though is this a likely recipe for failure? Is it more likely to float or more likely to twist, break and disappear into never-never land? The house itself will be fairly light since the roof doesn't need to support more than rain and tin.
If I just put footings, like the locals, the difference I see is the slab represents a large surface on wet clay and the footing a smaller surface on drier clay. Both are still in backfill. If the footing is sitting on dry clay I imagine it would be rather resistant to settling, but is there any guarantee the clay stays dry? I read somewhere that footing at 1m are below the 'wet line of clay' and don't uplift, but is that true or bad information? Is there any advantage to footing in clay versus a slab above the clay?
Another option perhaps would be a combination of footings below ground on the dry/drier clay and use the same slab on top. But does that gain much? Its extra weight. The footing is partially below the slab so there isn't an substantial increase in overall surface area. The footings though could act as a jack point if they are disconnected from the slab. Not sure. Are there any advantages/disadvantages to such a combination?
If you had to build in conditions like this, with all the unknowns, what would you do? I need some advice and guidance.
