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.

 

[joenorm]

OK.

If it's malleable then its saturated. Got it.

But soils are tested assuming saturated conditions, correct? So if the foundation was designed with the strengths of saturated clay in mind, then theoretically things are OK, Correct?

thanks

 

[joenorm]

Oldestguy,

That is a reassuring anecdote, thank you. But wouldn't it matter what type of soil old cabin on the side of the hill is sitting on? If its sitting on rock then we know why it didn't settle or have issue from the water.

 

[r13]

No. The geotechnical guy provides something called bearing capacity (actually is to limit settlement to the allowable) to the structural nuts to design the foundation. From the words, "bearing" and "settlement", you can see if a saturated clay can provide. Although sometimes we have to put something over a weak soil medium without costly, or difficult, soil modification, that something is usually deep pile foundation, or mat foundation, which are not common for residential constructions.

Pay attention to oldestguy's comment, which was caught in between our conversations. You shall explore if you have "free water supply" beneath your house. But hack, the geo guys confirmed there is no changes after 60 years, then forget about it. I don't want to start a muddy war

 

[fattdad]

I'd want a foundation drain if I had a house with a crawl space. It's more to do with indoor air quality and comfort (i.e., not to get mold).

I'd want a foundation drain if I had a house with a basement too. No reason to design for hydrostatic forces and add all that waterproofing (all concrete cracks) if a drain will help.

I just think it's important to consider our first principals when having engineering conversations.

Otherwise, it's somebody's money and they can spend it as they wish. Just need proper advice!

f-d

ípapß gordo ainÆt no madre flaca!

 

[joenorm]

Fatdad-

The house has a foundation drain to daylight. I went to great lengths to install it properly. It's rigid PVC at the base of the footer with a separate pipe for gutters.

The problem seems to be this: The crawlspace floor was left at the same elevation as the base of the footer, which is the norm around here. When water from the exterior runs along the clay layer that the foundation sits on, it easily goes underneath this pipe and to slightly (1 inch or less)lower elevations within the floor of the crawlspace.

With a little bit of gravel on the floor and a vapor barrier, I will not know the water is present.

But the question is whether its a structural concern to the foundation?

Again, what about all the houses that run 24/7 with sumps? Isn't the ground saturated in this situation? And it is considered normal, right?

 

[MTNClimber]

We will never know if there's a structural concern with the foundation unless we had a boring log and possibly even testing.

If the clay is relatively stiff, the loads are relatively light, and the clay is unsaturated or partially saturated, it shouldn't be a problem for a residence.

Water does not mean there is a problem and it also doesn't mean its not a problem. But we (and you) don't have enough information on the subsurface conditions and structural design.

Houses that run sumps 24/7 aren't considered normal but it does happen. It's a large electric bill that I personally would never want to pay. We also construct buildings and (expensive) homes under the groundwater and seal the basements to prevent water getting inside. Since the building can become buoyant, the structural, geotechnical, and civil engineers need to come together to prevent that from happening. If the building loads are not high enough, ground anchors are installed.

Are you on the hillside? Can you install an interceptor drain at the base of the excavation and divert the water around the residence? Maybe that could give you a piece of mind.

 

[joenorm]

I've already dug the interceptor drain, it flows steadily during rains so its working.

But somehow there is still water creeping in. To be clear this is not a heavy flow into the crawl.

I am not interested in running pumps either, but I read about people having to do it. I'm on a hill so I figure I should be able to manage the water.

Thanks for the responses

 

[joenorm]

What kind of machine is needed for a boring test? Would they test from crawlspace or from outside?

 

[MTNClimber]

You would need a drill rig that can collect soil samples (and normally a geotechnical firm to send someone to oversee).

An interceptor drain would be installed upstream of the residence if you had shallow groudwater. It's similar to a footing drain except it runs around the property instead of next to the house. You would then let the drain daylight downhill. Care would have to be taken as to where you discharge the interceptor drain.

If the issue is that surface water is running towards your structure, then just create proper grading and swales to divert the water around the residence.

Edit: Also cap any granular foundation fill with onsite, fine-grained soil to keep water from getting in.

 

[joenorm]

see my previous post. I already have an interceptor drain on the watery side of property, its just not getting everything

 

[oldestguy]

This might best be cleared up as to what is best by using an experienced geotech engineer whom has access to or owns test boring equipment. You wouldn't have to go under, but usually one or two outside test borings outside will be all that is needed. In any case so far I do not see anyone cautioning about what can happen if the drainage system erodes soil or worse yet plugs up. The drain system should be filtered to keep from eroding fine grained soil along with flowing water.. An experienced geotech can advise on that for you for your case. Be wary of a contractor that will install the drains, yet is not an experienced engineer.

So tell us the detail of what you installed as to filter, etc.

 

[joenorm]

The drain is simply a trench dug with a slope to daylight. A rigid pipe with holes placed at the bottom, and gravel filled to the top. No filter.

This is already done so to re-do it for a filter is out of the question.

 

[oldestguy]

Just keep an eye on it. It may already have developed a filter having lost a minimum of adjacent fine material. A build up of a delta at the discharge end would tell a story.

 

[MTNClimber]

Sounds like the interceptor drain and footing drain aren’t working properly. If designed and constructed correctly, you shouldn’t see anything. Next option that I would use is a sump and sump pump.

 

[joenorm]

The intercepter drain is working very well.

It's the footing drain that is not functioning properly. But I believe it is a function of where it sits in relation to the floor of the crawlspace. How could it be expected to work if the drain sits on the same level as the floor its supposed to drain. It would have to be slightly lower. But this is not how most of the diagrams for footing drains are drawn. Some even have the drain on the top of the footer.

How in the world could a drain function to de-water the a concrete footer if its sitting on top of it?

 

[MTNClimber]

When groundwater is anticipated, it should always be below the footing elevation. I usually spec 1’ below BOF elevation.

 

[oldestguy]

One function of drains that I once check as a Master's degree project. I checked the installation of several under highways where there was a high ground water problem. I had instruments measuring water content in soil at several locations both up stream and downstream of the interceptor drains. I specifically wanted to see if a drain down gradient from a saturated zone would "draw down" the water table up there. In all cases the so called draw down of the zone up-stream of the drain was minimal. Could be due to many things,such as layering, etc. Thus, to protect any zone from high ground water the drain has to be as low as what one wants for water table down stream from the drain. For the subject house, it may be the outside "moat" of drains has to surround the house at sufficiently low elevation to accomplish that. Don't pay any attention to "standard" details that usually are not applicable to a situation such as this one. Meaning a drain located higher than the low water table you want can't expect to do any good, as you point out. Think of the "
moat" around the castle an apply that here. By the way use filtered drains especially near footings. Check out the US Corps of Engineers web sites for drainage and you will seethe gradations that serve best as drain backfill to filter the soil from eroding.

 

[oldestguy]

This usually fits the size for concrete fine aggregate and is a great filter.


Note: single size gravel is not shown here as a filter This is all sand for the most part.

 

[joenorm]

Thank you all for all the posts. Amazing what can be gleaned through the internet.

Although I must say, I'm still confused as to whether I have an actual problem to worry about. I suppose the best bet is to have a Geotech take a look.

 

[oldestguy]

While I am at it some comment on jobs done. Most of the time a perimeter drain system either inside or outside the foundation ass low as possible does a job of lowering the water table. However, in cases, maybe like this site, there is lateral seepage, maybe springs, etc. so a flow of water getting around the drain has to be collected somehow. I have seen a few cases where perimeter drains do not do much for the interior, due to seepage flow from up-hill in soil at depths below the site. The seepage gets around the up-hill drain and due to its pressure and emerges inside the foundation area. Then interior drains are needed, sometimes requiring one to cut slots in the slab for the drains. I usually would try one and see how it does. If not use more interior drains.

Edit: PEr the latest from the OP. I'd not get too excited about this, especially if you don't see any problem. Erosion under footings is darned unlikely unless the flow is open water flowing RIGHT THERE. If your UN-filtered drain is RIGHT THERE, maybe yes. Otherwise very unlikely, ,just moisture.

One more comment. Ya can't expect to always have a site where there is no ground water at shallow depths, as one commenter above would like.

I hasten to add. My preset house sits on low round not far from a swamp. It has a perimeter drain leading to a sump pump and in very wet weather that pump runs. I was not the owner when it was built 20 years ago,but assume the usual dumb construction of gravel backfill, even meeting a state spec will YA? So with water inches below my basement I do expect that maybe some day the drain will plug and a big replacement is in order. Meanwhile, no excess moisture, etc. Just normal stuff down there.