Moist/wet soils under foundation 2

[joenorm]

I am wondering if soils under foundations are typically assumed to be wet? They are in the earth, after all.

We make foundation drains, but is the point of these to keep the the supporting soils dry? Or to keep bulk water out of the foundation for other reasons?

I am still trying to understand if water migrating underneath a footer/beam foundation is a structural threat, or just inconvenient(mold, not nice to work in puddled water, etc)

Soils are sandy clay. Non-expansive as far as I know.

 

[r13]

Foundation does not like water, because it can't swim. The water washes particles beneath the footing, weaken the soil structure, and exert additional force on the foundation. The soil can be wet, but not to the extent that it is fully saturated. A big structural threatd, if not controlled.

 

[jdonville]

Wet granular soils shouldn't be a problem as long as the effective (buoyant) unit weight for that material is used to evaluate the bearing capacity. Wet cohesive soils can be a problem because excess water can soften the soil to the point where it loses strength derived from cohesion.

We often see a requirement to place a relatively thin lean concrete "mud mat" over freshly excavated cohesive foundation soils to protect the soil from ponding water.

Another detail seen is placement of geotextile on the cohesive soil (which prevents the fine-grained soil particles from migrating) with a layer of relatively free-draining granular over. The granular soil is often connected to or incorporates a drain connected to a sump pit to remove any water collecting in this layer.

 

[joenorm]

How can you tell if soil is saturated vs. wet?

What about all those millions of houses that rely on sump pumps? Aren't those soils fully saturated if they have pumps that essentially run the whole rainy season? The sump is just managing the excess water so as to not flood the house, right?

Thanks

 

[joenorm]

What would be a good solution to controlling ponding water in portions of a foundation?

these are low spots on the crawlspace floor where water migrates. If I was to fill the crawlspace with 1 inch of granular fill I would not even know the water was there.

I already have two drains in place, one at the footer that surrounds the entire building and drains to daylight.

The other I dug after the fact on the uphill side(wet)of the building. This also drains to daylight. This pipe is below the level of the footer by 6 inches. It is raining heavily now and that pipe is exiting ALOT of water, but there is still a bit creeping under the house.

The footers are on a thin layer of granular fill, but no more than an inch.

 

[r13]

The key factors to your problem are "groundwater level/range" in the area, and the frost depth. Talk to a local geotechnical engineering company, or a contractor familiar with local conditions.

For areas with high groundwater, you will see the pump runs 24/7 in certain seasons (like mine). Also, its not rare that a house is damaged after flood with pump failed - always get a spare one is wise.

 

[r13]

I think the builder of you house has done a decent job, by enclosing your foundation/house with granular back fill, and properly slopped the drain to daylight. But I don't quite get the description of "creeping under the house", since you have done correctly to fill the depression at the crawlspace - don't forget to slop the crawlspace to drain, otherwise, the puddle will return soon or later.

 

[joenorm]

I think this is an interesting case.

The water table is not necessarily "high" in the area as the soils are predominately sand. The day I dug for my foundation I was 100% expecting to be digging in a sandbox. Instead, to my surprise, it was fairly heavy clay with a mix of sand and glacial erratics.

The clay layer must rise closer to the surface just on my lot, so the sand drains free in the surrounding area and the water shows up at my place. Not ideal, but its what I got.

Everyone local builder I talked to in the area said it shouldn't be a problem, just proceed as usual. There are a lot of clay sites here. It is a rural area so Geotech's are few and far between. It is very rare to get a geotechnical involved with a residential build.

The good news is the soils "feel" solid, but that's pretty meaningless from an engineers perspective.

 

[r13]

If you are somewhat concerned, dig a hole at suspect location and observe. If necessary, you may consider construct a pumping well, intercept the sand layer, to draw down the flow.

 

[joenorm]

The crawlspace was done with an "over-dig" technique.

The entire footprint of the house is dug out and then some, to prove room to from the footer and stem wall, this also leaves room to install the drain at the base of the footer.

What I mean by "creeping under the foundation" is that the crawlspace floor is at the same level as the drain or even a little lower. So any rainwater that finds its way to the base of the footer will not be caught by the drain. Instead it will leak under the footer and into the the crawl, to the low points. Again, if I would have filled the interior of the crawl with fill like some have suggested, I would not even be seeing this water and probably not writing in this forum with concern.

 

[joenorm]

"If you are somewhat concerned, dig a hole at suspect location and observe. If necessary, you may consider construct a pumping well, intercept the sand layer, to draw down the flow."


I guess that's just what I am wondering, is if I need to be concerned or not.

 

[jdonville]

A saturated soil is a cohesive soil where all the available pore space is filled with water. Due tot he low permeability of cohesive soils, it generally takes a great deal of time for water to get in or out of these pore spaces. However, free water (ponded or within a granular soil) against the surface of cohesive soils is a problem. It may take a long time to manifest as a noticeable problem.

In the case of granular soils, they are described as wet if free water falls out of them when removed from the ground.

 

[r13]

I think your assessment is quite plausible. Good luck.

 

[fattdad]

all soil strength is determined in the laboratory (and by extension via correlation) under saturated conditions. Saturated soil strength is the design norm.

If wet soil turned to mud all embankment dams would fail.

I don't buy all the hand wringing! (Nor the internal erosion concern!)

f-d

ípapß gordo ainÆt no madre flaca!

 

[r13]

Correct, cohesive soil takes long time to dry out, meanwhile, it is highly difficult to let it reach fully saturated state, unless there is persistent high ground water present, which is not likely to happen on a hill side though. I think you should concentrate on surface water runoff at this moment. And, just to make sure there is no noticeable crack(s) around the house and on the floor.

 

[joenorm]

Thank you all for your responses.

I find it interesting that there is a split in opinions. Some people seem to say any water is bad anytime and will ultimately ruin your house.

Others point out what fattdad said, "all soil strength is determined in the laboratory (and by extension via correlation) under saturated conditions. Saturated soil strength is the design norm.

If wet soil turned to mud all embankment dams would fail."


Either way, I enjoy the discussion!

 

[r13]

The test is concerning cohesion at Θ = 0 for cohesive/clay material, if I haven't returned my learning to my professor. I still vote "no water" for/under my house, but tap water service.

 

[joenorm]

My vote is "no water" also. But dang that stuff is persistent.

Interesting to try to understand the forces at work.

thanks

 

[r13]

Hotwire,

Don't be confused with joking. You can mold a clay with water, once your kid can put finger into it, it is in a state of saturation, if not fully.

 

[oldestguy]

The "no water" folks are dreaming. Because of climate changes, one can never expect a for sure "no water' situation. I now refer to my experience as a young person when we moved to a country house where the ground was on a side hill. Out of that hill came ground water flowing and springs emerged.
The house was built over one of those springs, for "water supply" early on, before city water came. The crawl space under the house was always wet with standing eater. Made it difficult to get under there for maintenance of things. The foundations were stone with mortar extending about a foot or so under the surface because even in a cold climate the ground never froze more than an inch or so. When water freezes it gives off heat energy and the crawl space was protected from outside cold air.. No cracks or settlement noticed in the building, even over 80 years old when we moved in. Just recognize how saturation affects bearing capacity and live with it. To prove my point some 60 years after going elsewhere, I stopped in one day when going through the area. Still no change.