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.

 

[Ron]

Agree with fattdad and OG. If saturated soils were such a problem, we would not be able to build in my area of practice. We often have water tables that are at or near the surface, yet we place foundations (footings, not footers!) on these soils. Yes, there are many geotechnical considerations that should be made; however, the picture is not doom and gloom.

For off grade construction with wood joists, my concern would be wood rot. So for that reason, draining the water away from the crawl space is a good idea. Structurally...not usually a big deal.

 

[r13]

joenorm,

Your water problem is two fold - surface water and ground water. Since your house is on hillside, provides the area is predominately sandy soil, I don't think the latter is a thing to be concerned with, unless you dig a hole in the yard and see sitting/standing water in it. The water creeping through your crawl space and ponding, apparently is caused by surface water runoff. This problem will be gone, if you slope the fill, and either simply excavate a ditch, or embed a French drain, under your crawl space. Also, slop the ground, and check around the house to see whether there is any indication of movement - large cracks, stair landing is higher than your indoor floor... If nothing is find/suspicious, your foundation should be free of concern for a long time to come. Note, since you don't have basement, pump is not needed, unless you want to draw down the ground water as mentioned before.

Below is something I want to settle the misunderstanding between the geo heavy weights and the structural nuts. Please do not hesitate to correct me, as I like to advise people to keep foundation away from water, stay dry, if minimal effort and care can achieve it.

Can we put footing on saturated cohesive soil? Surely we can, Since cohesion c (Ø = 0) is the primary term in derivation of cohesive soil bearing strength, and the consideration of its use is as stated below.

The ultimate bearing capacity of saturated cohesive soils (clay and silt) with low permeability is most critical immediately after construction, before the excess porewater pressure has had time to dissipate i.e. undrained conditions. As time proceeds, consolidation occurs, the soil becomes stiffer and has more strength.

Unfortunately, for lightly build house without significant consolidation could occur, but with persistent presence of groundwater at/above the footing, will that anticipated "excess porewater pressure has had time to dissipate", and "becomes stiffer" to happen?

A study has shown the relationship between moisture content and cohesion for a cohesive soil (Kubishi clay):

Moisture Content (%) - Cohesion (kg/cm²)
0.0 - 6.5
2.6 - 4.8
5.3 - 4.3
17.5 - 3.8
19.9 - 2.6
22.3 - 1.4
25.1 - 1.1
27.4 - 0.7
30.4 - 0.6

I don't see water is very kind. It can lift a footing, also can sink.

 

[r13]

Comment to "stood 60 years without problem" - all I can say is we don't exactly know the soil below, water table, and system for drain. Also, it could be a mercy from the nature - as we all know 100 years event will occur sooner or later; earthquake destroys one house but not the next with the same built. We are engineers, not the god, practices/given pointed advices cautiously should always be in our mind.

 

[oldestguy]

I won't try to "educate" our friend. However, assuming the subject site has water, due to surface water likely is wrong. There are many reasons for soil "holding" water at certain moisture contents, among them the various internal atomic attractions can't be easily changed. Perhaps explain this one. Knowing high water content in soils of high content of certain minerals, we usually will try to dry them some so that compaction can create a good su0port situation. That weer content is considered "optimum". So a new college site of several buildings in such an area had some rather soft water laid during glacial times soil that was far above "optimum" water content. Our engineering firm sent an engineer (held a Master's in geotechnics) to test and oK soil that was then dried and compacted to optimum. The site was rather low and so soil was excavated from, what would be a future lake. It was compacted in layers at 95% of Max. to build up several building sites upon which would be footings place and then buildings. So the job was done and everything looked fine. The architect decided the roof rain water from the buildings should go to the excavation as the lake source.
Those drain lines were perforated in sand filled trenches to also collect rain water. Roof water was collected and drained to the lake in these buried pipes. Drain lines to external sanitary sewers from inside the building were buried in sand filled trenches that extended past the sand filled outside drain pipes. Low and behold that compacted under building clay in contact with water that came from the roofs decided it wanted to go back to its "natural" water content somewhat higher water content than the optimum. THEN WHAT AN UNHOLY MESS DEVELOPED. The under floor top layer of fill also had a layer of bentonite clay "for waterproofing" said the architect. HIGHLY EXPANDABLE. In addition this natural clay had high expansive mineral content. Floors were bulged up, doors would not open. Interior partitions all bent out of shape. A hill developed in each class room so the blackboards were badly out of line. Luckily footings were sufficiently deep that the "surcharge" over them minimized the swelling of soil beneath them. Fortunately a gymnasium building job was delayed until later when that compacted clay expanded to ts "natural" water content and then stayed unmoving. The common spec of 95% compaction surely did not apply to that site.
Moral: never presume anything about a site until you know ALL the details.

 

[r13]

However, assuming the subject site has water, due to surface water likely is wrong.

Never in my mind, nor attempted to. Maybe my poor wording caused some confusion? Below was one of my response before realized this house was on the hill.

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.

After more info were present, I said:

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.

And:

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.

And:

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.

If I have made any "assumption" in my responses, please point out, I shall learn better.

 

[oldestguy]

Enuff of this back and forth. Owner seems pretty good at evaluating responses. If ever in doubt, my advice is see what Ron has to say. He is pretty good at resolving differences.

 

[r13]

I think your assessment is quite plausible. Good luck.

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

Terzaghi and Peck can confirm I said goodbye (good luck) quite early on.

 

[joenorm]

I am not as concerned with rot.

My plan all along is to add drain rock to the portions of the crawl that are pooling. At that point, nobody but me will know the water is even there. After that I will neatly detail a robust vapor barrier, taping where it meets the foundation.

Very little water vapor, will make it though. If it does, the crawl is vented.

My theory about the site is as mentioned. The surrounding area drains through sand and my lot happens to be where the fly comes shallow to the surface. So that would be subsurface ground runoff, I'm not sure how you'd classify it in the business.

It's not rising from below, but is not on the surface either.

Again, the discussion is much appreciated and a nice glance into the professional world. Like all subjects in building, there seems to be differing opinions.

 

[fattdad]

all the foundation drains in the world will not stop capillary rise! Never forget about soil suction!

f-d

ípapß gordo ainÆt no madre flaca!

 

[joenorm]

FattDad,

Perhaps you can expand on how soil suction and capillary rise are relevant here?

thanks

 

[fattdad]

You will lower the phreatic surface by installing drains. Let's say, you lower the phreatic surface to below the pipe drain. Capillarity will allow water to rise above the phreatic surface. Such capillary rise, may increase the water content of the soil to the point where it will be reduced to mud upon disturbance.

The height of capillary rise is determined by the pore size in the soil. I have an equation in a book somewhere. Turns out it is something to be mindful about; however. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

Quote from above, not always a true action. "Such capillary rise, may increase the water content of the soil to the point where it will be reduced to mud upon disturbancee." Depends on many factors. For instance, why does montmorilonite swell when there is no air-water interface needed for capillary rise.?

 

[fattdad]

not taking the bait and remain with my statement.

f-d

ípapß gordo ainÆt no madre flaca!

 

[joenorm]

Update On this.

It's been really wet the last week and the "intercepter" drain runs like a small steam. This drain is about four feet from the footer and 6 inches deeper than the bottom of footer. Higher flow while raining, petering off after the rain but still flowing.

It has helped with the water infiltration into crawl but not solved it. I am wondering if that water is still entering from above as my excavation contractor backfilled with gravel all around the two wettest sids of house, essentially leaving the open to the elements.

The water comes and goes with heavy rain, so I don't think it is a permanently high water table. I just happen to be in an area where drainage is working against me. But I am on a hillside so it theoretically can drain away.

One concern is that there is an asphalt driveway on the uphill side of me on the adjacent property. A lot of the drained water off of here is likely ending up in my "intercept" drain, perhaps working it harder than it needs to work.

I'm truly puzzled as to a solution here, I cannot afford too much more excavation unless I know it will solve the problem.

In the meantime most of you have giving me some confidence that this will not turn into a structural issue which is my main concern for a newly built house.

Not sure how to proceed.

 

[MTNClimber]

Make sure that gravel backfill around the house is capped with fine-grained, low permeability soil or hardscape. I typically spec a minimum of 2 feet of low perm. soil where water can be an issue. I'm sure you already did this but make sure the downspouts are discharging far enough away where they are not causing an issue with infiltration.

 

[oldestguy]

In line with Mountain Climber. On way you can "waterproof" an area with that fine grained soil is to do some hand labor or easier with a roto tiller. Go to a distributor of well drilling products, such as a heavy hardware outlet. There you can get what they term "driller's mud". It is a dried natural clay called bentonite or montmorillonite. Be sure it is the powder variety, not granulated. Well Drillers use it mixed with water for keeping a the hole open while drilling. It is VERY expandible. Use too much in a mix with soil can cause problems. So experiment before dong much with it. Mix in to native soil maybe 3 to 5 pounds per cubic foot of natural soil. This mix will take on water and be practically impermeable. Too high a percentage mixed in will have a constant soft sloppy condition. A little goes a long ways. It is cheap, but don't over do it. When I have done this we strip off the sod and then work it into a soil layer about 3 or 4 inches with a roto tiller. Thorough mixing is required. Replace sod.

Do your experimenting with it by carefully measuring and use something like a kitchen sieve into which you put the mix. Dampen it and test how well it holds water. I'd do the test on a thin layer of soil, maybe 1 to 2 inches thick and have some way to get a "Head' of water on it. A tin can with fine holes punched in bottom might do it. Be precise with measuring. Maybe do the mixing test on site but only one or two square feet.

 

[joenorm]

Have any of you guys in the business ever specified using plastic as the water barrier around the house?

One could theoretically slope heavy-duty plastic away from the house to the same affect as the clay that you suggest above?

thanks

 

[r13]

My experience with plastic sheet was it deteriorate rather fast. I put down a sheet under my bed to prevent the drift from the cold basement and through the wood floor, it worked a season or two, then wrapped, and seemed loosing its elastic property. I wouldn't suggest outdoor application as yours.

 

[joenorm]

I didn't mention that I assumed it to be buried under gravel, so not sunlight exposed

 

[r13]

There was no sunlight under my bed, I put mattress directly over it.