advice sought on foundation walls

Hello Permies !

We've just started building our house (footprint of c. 120 sq metres, ground floor + loft, NO basement) and we've already run into serious difficulties at the earthworks stage.

The plan was to pour the foundation walls directly into trenches - dug 120 cm deep, 50 cm wide, same as the intended foundation walls - without any formwork, or in other words, the trench itself was going to be the form into which the concrete (with rebar reinforcement) was going to be poured.

The depth and thickness of the foundation walls were based on a geologist's recommendation (the ground is clay).

In the dense clay the excavator cut really neat, regular-shaped trenches. (The spoil was trucked away, to the plot of a neighbour that had to fill a hole.) However, a few hours after they were dug, before we managed to pour the concrete, the trenches started collapsing due to a heavy overnight rain. And the mess has been compounded by the rainwater that has accumulated in the (collapsed) trenches.

Now re-digging the trenches into the same shape is impossible, partly because of the already collapsed soil, partly because the machine can no longer get near the existing trench without causing even more damage. So it seems that we will have to use formwork to pour the concrete. But we would need much wider trenches in order to put the formwork in place and be able to remove it afterwards - that means that we would have to backfill the soil around the foundation after it's been poured and the forms removed - and I cannot imagine us re-filling with soil that has the consistency of a cake of soap... Perhaps we should refill with something else, like gravel or sand?

Are there other options?

Any ideas / advice would be immensely appreciated !

L_

This is why I tell people to get soil test before they dig and some jurisdictions require it. A geotech report before digging is even better. Do you have access to a lab to get a plastic index and compression test? You'll want to bring them at least two samples at your floor 120 CM deep at opposite sides of the perimeter.

It sounds like you have a high PI and your grade is not right for proper drainage. Maybe good finding the issues now vs later.

hello,
personally,i am a big fan of first evaluations.Sounds like the moisture has made the ground un-moldable.
If time is not a concern,what if you drained excess water by digging a hole in the middle of pit so that it
could collect water,then once side walls dried out you could re-shape the footers?
just my two cents, Marc

Terry Ruth wrote:This is why I tell people to get soil test before they dig and some jurisdictions require it. A geotech report before digging is even better. Do you have access to a lab to get a plastic index and compression test? You'll want to bring them at least two samples at your floor 120 CM deep at opposite sides of the perimeter.

It sounds like you have a high PI and your grade is not right for proper drainage. Maybe good finding the issues now vs later.

Our regulations do not (yet) require a geotech report in rural areas - but I still had it done, that's where I've got the 120 depth 50 width from.

Yes, plasticity index is probably very high.

But the plot is in a village with existing buildings (some new & some old), built on the same type of soil (the geologist made a remark to that effect).

We are building on a slope, BUT we flattened an area for building the house. We already had drainage in place around the area before we started digging the trenches. But obviously that wasn't enough for the amount of rain we got in one night.

Marc Levesque wrote:hello,
personally,i am a big fan of first evaluations.Sounds like the moisture has made the ground un-moldable.
If time is not a concern,what if you drained excess water by digging a hole in the middle of pit so that it
could collect water,then once side walls dried out you could re-shape the footers?
just my two cents, Marc

If you're referring to standing water in the trenches, I can drain that with a pump.

Not to sound critical or sarcastic but it sounds like your geologist has some questionable design practices, this should not have happened. PI can vary drastically on a site that is why we obtain two samples min @ depth so that we understand the variations. Looking at the general area as a guide is not good practice. Large variations in soil properties can and has caused future settling and buckling issues from high hydrostatic pressures making a compacted footing floor sub-base necessary, such as locking aggregates and/or NHLs, Pozzolans, or OPC soil stabilizers, that take PI down at the sidewalls and improve bearing strength. 105 KG/CM2 min. compression.

My guess is your PI and moisture holding capacity is too excessive (IE: PI>30) for a good foundation design that is why under liquid load the soil collapses due to not being able to handle its own weight. So getting a good soils engineer there to properly access the situation would be a first step.

Concrete needs to dry at the correct rate and having a heavy rain come along that stagnates around it can cause a complete R&R if the required compression is not reached in time, usually 30 days. Might want to consider a continuous drain tile to daylight, they and the footing floor should have positive drainage and filled with a course wash sand or river rock. Backfill the footer with a loose slightly damp low (PI<16) soil, no large clumps. Fill it uniformly around the perimeter in small courses.

Once you redo the trenches properly, you could consider ICFs. Not sure if you can find any 50 CM which sounds excessive, you only need 1 CM curbs past walls. Durisol or Faswall makes an excellent ICF although the shipping may be cost prohibitive.

Common problem. Clean up the trench best you can, pour footer at the base of the trench, then use block or forms to make a thinner wall, then backfill. It sucks, but not the end of the world.

Terry Ruth wrote:Not to sound critical or sarcastic but it sounds like your geologist has some questionable design practices, this should not have happened. PI can vary drastically on a site that is why we obtain two samples min @ depth so that we understand the variations. Looking at the general area as a guide is not good practice. Large variations in soil properties can and has caused future settling and buckling issues from high hydrostatic pressures making a compacted footing floor sub-base necessary, such as locking aggregates and/or NHLs, Pozzolans, or OPC soil stabilizers, that take PI down at the sidewalls and improve bearing strength. 105 KG/CM2 min. compression.

The geologist did a penetration test - which, I repeat, was not even mandatory. He did not look at the general area "as a guide", he made a general remark about the high clay content in the area, with which he was familiar.

I don't know what locking aggregates are.

What is the process for creating the compacted sub-base with the materials you specified?

Terry Ruth wrote: My guess is your PI and moisture holding capacity is too excessive (IE: PI>30) for a good foundation design that is why under liquid load the soil collapses due to not being able to handle its own weight. So getting a good soils engineer there to properly access the situation would be a first step.

Your guess is very likely to be correct.

Terry Ruth wrote:Concrete needs to dry at the correct rate and having a heavy rain come along that stagnates around it can cause a complete R&R if the required compression is not reached in time, usually 30 days. Might want to consider a continuous drain tile to daylight, they and the footing floor should have positive drainage and filled with a course wash sand or river rock. Backfill the footer with a loose slightly damp low (PI<16) soil, no large clumps. Fill it uniformly around the perimeter in small courses.

Okay.

What's R&R ?

Terry Ruth wrote:Once you redo the trenches properly, you could consider ICFs. Not sure if you can find any 50 CM which sounds excessive, you only need 1 CM curbs past walls. Durisol or Faswall makes an excellent ICF although the shipping may be cost prohibitive.

I think Amvic ICFs are the most commonly used here, and I'm not sure what else is available.

I haven't seen ICFs suitable for a wall thickness that gets close to 50 cm. Okay, 50 may be excessive, you think, but where do I draw the line? What is acceptable instead of 50? If it were 40 instead, do they make ICFs with that specification? NB: 50 is the thickness of the foundation wall, which is not resting on a wider footing.

Levente, my main first point is per a soils engineers specs there you should have PI's at two minimum locations at least 60 CM deep before you dug anything. Next time you know. If you see a high PI and large variations of say 10 PI you know you have issues. Yes, it was once not required here either but now after alot of failures PI is a minimum requirement. If you have variations the floor of the entire footing will see too much uneven settling shear/buckling. At that point there are several remedies, some opt for a road base to evenly distributed the loads. Get with a local transportation dept. I don't like a bitumen or tar binder, I go NHL 5, or fly ash, or lime, with some sharp rock aggregates since they lock together well. Not really alot of work or cost and in addition it adds some insulation to the under-footing and hygroscopic mass to manage that high PI lean/clay. Hi PI down low is OK, up high not, or as infill, and the floor has to be undisturbed hard packed. Another solution here is mineral wool Drainboard or FOAMGLASS that has the high compression/deflection under cast-in-place but, talk to them about the application.

R&R: Remove and Replace

I'd haul some sand or silty soil in to take the PI down for back fill, and get the drain slopes/tile installed asap in the process with river rock or pea gravel. Work with the geologist to get the PI and soil types in the right locations. If you sequence and schedule this right you should not have a complete loss and rework, that is not common nor should be. Sounds like I need to start a design business here, but never heard of this type of failures in our heavy marine continuous rain climates, that would drive builders out of business due to cost. I'd never had that happen to me nor would I allow it to. We do repair alot of foundations and we make good $$$ at it due to not having the proper analysis and knowledge, now or soil labs will not deal with anyone but licensed engineers since they are tired of builders that did not order the proper initial test blaming them and the liability $$$. So you did good by testing, interpreting test data can be tricky, and sometimes more is needed.

I think Amvic ICFs are the most commonly used here, and I'm not sure what else is available.

Foam vapor barrier ICFs have alot of issues, the ones I mentioned are natural made from clay, magnesium phosphate or chloride, and petrified cellulose that breaths that also adapts to insulation like miner wool boards easy as outsulation with much higher compression and deflection values than foams....follow their details. If your doing a slab that natural sub-base I described or under slab insulation is good too.

I haven't seen ICFs suitable for a wall thickness that gets close to 50 cm. Okay, 50 may be excessive, you think, but where do I draw the line? What is acceptable instead of 50? If it were 40 instead, do they make ICFs with that specification? NB: 50 is the thickness of the foundation wall, which is not resting on a wider footing.

50 CM sounds excessive for light wood construction on high PI high compression soil. It needs to be about 8 CM wider than the wall. I don't know what your wall looks like, if heavy mass like earth 300 MM is min but I'd get that down maybe, a rubble trench may work. I'd have too look I think you're right about 50 CM being too high for ICF's, that may be able to be stacked I think they can. I'd be looking to take my wall thicknesses down, and I would. Just guides, an Structures Engineer can do the calculations.

This is one area of a building you want to take your time and get right, Good luck!

R Scott wrote:Common problem. Clean up the trench best you can, pour footer at the base of the trench, then use block or forms to make a thinner wall, then backfill. It sucks, but not the end of the world.

Though I'm not an architect or builder, lately I've been comparing practices in this country (Romania) and those in other (especially Anglo-Saxon) countries. For self-build residential construction, and more precisely for houses without a basement, most people here usually don't pour a footing, but lay down a sub-base of rocks on the bottom of the trench, then go on to pour a thick foundation wall (thickness of 40-50 cm or more).

Here's an illustration of the process similar to the one that I intended to use - before the trenches collapsed https://www.youtube.com/watch?v=uHLVvet1ve8

Interesting video... that is about twice the thickness of concrete actually needed to support a residential building wall, but if concrete is cheap, it may be cost-effective to eliminate the formwork construction and removal. Also, if the wall is to be thick, you need a thick foundation even if it is much stronger than necessary.