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Still trying to understand affect of high water table
2

Still trying to understand affect of high water table

Still trying to understand affect of high water table

(OP)
Hello All,

I have posted and discussed on this topic here before but would like further input, thanks in advance.

I started building a single family home in western Washington state the Spring before last. During the dig for the crawlspace we encountered clay to my surprise. I was expecting sand because that is what a lot of the neighboring lots have as a sub grade.

I also noticed the dig area was pretty moist but I attributed this to surface water and having no gutters for a time during early construction.

I have noted that during the winter(last winter was more wet than normal) the water table seems to be at the surface of the crawlspace floor. I dug a five gallon bucket sump pit and it seems to be full all winter.

It is now July and the water level has dropped in the bucket but but only by perhaps a 8-10 inches, still water in the lower half of the bucket.

My question is about the safety of building a house in such soil conditions. The clay is not "expansive" to my knowledge besides the fact that all clay is to some degree. I am pouring time and money into this building project and the soil conditions have worried me a bit from day one.

I did not get a geotechnical report before the build, it is not done around here and you simply do not hear about foundation issues locally.

In the previous thread there seemed to be debate about whether it was safe to build onto saturated soils. Some people said its done all the time and there will be no problem, others said never do it and I am bound for problems.

Why is there such debate over this?

Also I am wondering if its best to try to drain this water lower or leave it as is?

thanks

RE: Still trying to understand affect of high water table

2
First off what is the experience and qualifications of the discussors? For instance there is a recent discussor on many of these rooms who is obviously an expert in certain things but generally not so in most. Yet there are comments on many other fields by him. We all know folks who are "know it alls" . So just because you see comments here from many commenters here, think about their experience. In your case are they educated in the field? Have they had soil water problem factors in their education? On their experience have they worked on these problems, as a consultant, a part of their past jobs? Remember many also so called professors may have taught soil mechanics, but never dealt with the practical situation as you have. For instance also if one always looks at high ground water situations as "never build there", a great many developed areas in this country would be vacant pastures, etc. Unless you see obvious problems, such as ground heaving or settling and causing cracks,etc, live with it. I've done so on a few places where I have lived and do so even now. A 20 year old house.

Edit: One of my past soil mechanics professors remarked about those who write a lot as having "diarrhea of the pen". Applies to many here, I suppose including me..

RE: Still trying to understand affect of high water table

Foundations are meant to be in the dry. If you dig a hole in clay you have made a slow draining and filling pool of water. The foundation drainage system is meant to drain away any water which infiltrates around the edges and can easily drain away seepage from a clay hole, assuming the system was installed properly and wont clog up with clay.

It is not common to see geotechnical report for a single standard residential house. The foundation size requirements in building codes are typically more than enough for any load in most conditions. Assuming you were able to walk in your excavation without sinking or making significant boot marks.

RE: Still trying to understand affect of high water table

Where in the world does this sort of thing come from?
Foundations are meant to be in the dry.
I can't see how any one in the poster's situation can believe this if their specialty is geotechnical.
Sure for basements, etc, you may need to have a drainage system, such as perimeter drains, but for a house with no basement, sounds very inexperienced.
I challenge the geo env guy to come up with actual cases like this where there is no basement, etc. and point to damage or other on-going problems. Just because its a clay soil area is no reason in my view. Yes it my be difficult to build there, but this is for the on-going situation. Again, my experience with this in my residences, and having seen hundreds of other cases with no problems. Let's get some proof.
Let's first see education, job descriptions,how many years experience in this subject field., etc.
And answer the opening question of my comment here.
I'll add: We are taking about conditions well after building. In this case the poster mentions no problems while building.

RE: Still trying to understand affect of high water table

(OP)
Thanks for the replies.

GeoEnvGuy you said foundations are supposed to be dry, I am in no way arguing but I wonder if you could expand on that?

My drain was put in like every other drain around here, at the base of the footer. Unfortunately that only works to drain away water right to the soil surface in the crawl space. Note if I had poured a rat slab with a bit more elevation I would likely not know the water is there. But since I can see it pooling in the low spots of the crawl I am aware of it.

Should I be trying to install a new drain that is lower than my current drain so it "de-waters" the area?

There must be countless houses where the water table is high. What foundation type/system is recommended in these palaces?

To OldestGuy: as always thanks for the level headed advice. This is a great site!



RE: Still trying to understand affect of high water table

Another post by OG. Two of your questions.
My question is about the safety of building a house in such soil conditions

Also I am wondering if its best to try to drain this water lower or leave it as is

OK, on the first question. For what you describe, in my view NO PROBLEM. Just keep in mind that water can cause problems, but be prepared when building and later, in some cases. So far it doesn't sound like any problems ongoing.

Second question. Unless you see lots of changes, such as erosion causing settlements, or water affecting other things, such as dampness and wood rotting, etc. don't try to drain. Drainage will be damn difficult due to the low permeability of the soil. Ordinary perimeter sub-drains not likely to do much good. Even putting drains in the crawl space, might require spacing as close as 5 feet to be effective. If-drains are tried, do it right with proper filtering of them.

RE: Still trying to understand affect of high water table

Hi og it appears as if I hit a nerve. To your point where does foundations are meant to be in the dry come from. Foundations under constant hydrostatic pressure are required to be waterproofed in the building code. My experience with this is sewage lift station design which are waterproofed. In residential development waterproofing is to expensive so you get damp proofing which when responding to the op an electrical engineer this is relatively dry. My experience in this was 8 years in a foundation geotechnical consulting firm.

The op also mentioned western washington state which has a frost depth requirement for foundations. Not practicing in washington state I cannot speak to the depth required but can appreciate that the crawl space (basement) mentioned by the op would need to be in the sub grade excavation similar to the neighbors mentioned by the op.

As for the challenge to identifying damage or ongoing problems with no basement. Working in the bedrock confined bogs full of varying thicknesses of very soft clay in northern ontario. An unnamed major retailer of furniture hired a southern consulting firm to design the foundation for a commercial property, no basement. It was obvious the southern firm was not familiar with soft clay or the variability of the site conditions. I was involved with months of monitoring displacements, the largest displacement measured was about 100mm between two structural post and the whole building was sinking on one side by nearly 500 mm.

Edit should you dewatering the area. No. You also do not want standing water (mold) in your crawl space you may consider somewhat filling it in or sealing it off with what you mention as a rat slab.

RE: Still trying to understand affect of high water table

(OP)
The foundation was backfilled on the wet side entirely with drain rock. I doubt there is any hydrostatic pressure on the walls of the foundation.

My question and concern is in regards to the water beneath the footers.

RE: Still trying to understand affect of high water table

OG again. This is no tank or other water containing thing. It is a chunk of concrete in the ground to carry a load. Liken this to a fence post, but wider. So what, it is in saturated ground AFTER IT IS IN USE, not while building. Good thing the poster used good common engineering sense when building. No geotech report needed either.

Edit: The poster got "shook up" by a comment from an engineer who has little or no experience in residential foundations. It is these types of commenters that serve no purpose when the subject is not something even close to their background and caused this poster unneeded concern. Confusing construction water conditions with later in use conditions is very misleading.

RE: Still trying to understand affect of high water table

Quote (joenorm)

My question and concern is in regards to the water beneath the footers

If you built a house your likely fine, you build a 2 story mansion with marble floors 50/50 noticeable problem develops, you build a high rise likely to have problem. A standard residential dwelling does not have the load to cause a problem using the minimum requirements to meet local building codes. This is why standard houses are not required to be designed by engineers to meet a very specified building code.

Quote (joenorm)


There must be countless houses where the water table is high. What foundation type/system is recommended in these palaces?

Waterfront is expensive typically set back or built up, in flood plains stilts are used.

Swamps use raft foundations or thickened edge slab on grade, very similar. Both of these are more expensive than shallow footing or strip foundations.

If all else fails pile driving a deep foundation.

RE: Still trying to understand affect of high water table

Now it should be obvious that my first comment, in anticipation that there might be misleading comments, certainly came true. My old prof's statement fit the bill here. He's long dead, but likely is giggling in his grave now.

RE: Still trying to understand affect of high water table

(OP)
I also should mention that the clay appears to be very dense. It has a mix of rocks and sand and the occasional glacially deposited boulder.

But.....It drains very very poorly.

What sorts of problems can water cause? I am less and less concerned for my own site after reading the comments here, but just curious.

RE: Still trying to understand affect of high water table

OK on the site conditions. In general great stuff to build on. Not so good for a garden. This old guy, now 92, has seen a lot of residences with high groundwater. Other than this stuff heaving in the winter, not much of any problem comes to mind. Of course if at wet seasons you have a wet floor, cardboard boxes not so good there. Untreated wood sitting on that wet area decay is likely. Very poorly vented and a lot of water surface exposed can be damp and storing stuff in that area may spoil, etc. I'd want a vent, screened,to give a little fresh air, except for below zero outside when might as well shut it off since below freezing for a length of time inside may cause shallow depth freezing.. If footing is not founded at exposed surface, but a foot or more down, probably no problem with heave. Observing of course can dictate any corrections needed. There would,have to be one heck of a lot of problems before I'd consider draining the crawl space soil. That would be a major job. It has to be done right with proper filtering. Just crushed rock is not sufficient. That till soil just doesn't drain by gravity without a lot of drains. A rule of thumb any drains have to lower the measured ground water free water level at three feet or more OUTSIDE to avoid heave in northern states.

Edit: Forgot one important thing about ground freezing. Where do we find, in winter the least depth of freezing? A lake. Under same circumstances where the deepest depth of freeing? That's in clean sand with minimal water content. So in your case a wet crawl space won't have deep freezing.

RE: Still trying to understand affect of high water table

A final summary by OG. This room subject apparently before had conflicting comments by some. Again this post had a conflict comment right off, yet that commenter failed to expound there and also failed to answer a basic question. BUT, he did sneak in a differing comment at the end, in effect reversing his introduction. Since the purpose in the post was to clear up some things of the earlier post, that being differing comments about water in crawl space. It now appears that the reversed opinion by one commenter should clear up any remaining questions. So, water in the crawl space may not be the best thing, but one can live with it.

RE: Still trying to understand affect of high water table

(OP)
Follow up from the OP.

I hired an excavator to install my septic tank. He dug a 10 foot deep hole about 8-10 feet away from my foundation on the downhill side to drop the tank into.

The hole has been open for about 4 days now and NO water is present in the hole.

Meanwhile, in my crawlspace, about 15 feet away I have a small hole dug into the ground. There is water about 8 inches below the crawlspace floor surface!! How is this even possible.

The hole for the septic tank is much deeper, I totally expected to see water collect in the bottom after a little bit of time.

What is going on here?

RE: Still trying to understand affect of high water table

Yes strange. This calls for an expert on site. Might be a lot of things. Some not so good and some no problem. Now the question is where does the water you have come from?

RE: Still trying to understand affect of high water table

(OP)
I always believed the water came form the hillside above. I even had an intercepter drain dug on the "wetter" side of the building.

This drain is still just barely dripping right now(mid July).

The water in the soil is just very perplexing to me, how could it be present or not present just 15 feet apart?

thanks

RE: Still trying to understand affect of high water table

Do know the site's specific soil as a soil situation, such as residual from granite, from shale rock, glacial outwash, glacial lake area, etc. Tight clays of low permeability may be the case. What exactly was the material dug out for the septic tank? Just saying "clay" is meaningless to me. Soil grain sizes vary from boulders to residual clay, with many different "clay sizes. Put a chunk of any "clay" in a pan of water. Does it fall apart? If so how long to disintegrate?

The little checking I could do would indicate the site is a glacial moraine. In that case the bulldozer action of the glacier shoved all sorts of stuff there, including tight glacial lake clay. Thus one side of the lot may be sand and the other clay possibly tight fine grained lake clay.
Edit: While the septic tank hole is open look at a few things. Do you see beds of soil? Are they horizontal of tipped. A moraine is a bunch of soil stirred up in all directions. Beds may not even be visible due to mixing. Assuming the site is a moraine, just live with it.


RE: Still trying to understand affect of high water table

(OP)
Seems pretty mixed up. Clay mixed with rocks and likely some sand. Clay varies from light to dark grey/blue.

Without being trained, its hard to describe the exact soil structure to you. Could there be a little "spring" nearby?

thanks for all the interesting responses!

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