Foundation excavation and groundwater/mud

[jim_wilson]

Last year I started excavation on the hole for my new garage. The site is on a slight slope so to get down to a good base soil I stepped the excavation approximately 16" so that I was down to good soil for the footer on both the higher and the lower parts of the excavation. Then last fall we got a ton of rain and the lower half of the excavation filled up with about 16" of water - which never went down until just recently. I went back out yesterday and because the soil that I had gotten down to appeared to be sandy clay mix which had turned into a muck I decided I should excavate about 6" more and then backfill with a sandy gravel mix that I had previously used to put in a new driveway - I knew this sandy gravel mix very well so I figured it would make a good base for the footer.

Well it has become apparent that what I am dealing with is groundwater because overnight I got about 4" more water into the lower part of the hole, and the sandy-gravel mix I backfilled in just turns into mud when mixed with water - which leads me to believe that it will not make a very good base for a footer. Now I am in a bit of a quandary because I don't know how proceed to get this excavation ready for a footer pour.

Is there a recommended procedure for a situation like this - a different type of fill perhaps that will perform better if it is exposed to a high water table? I have seen it recommended that large crushed rock be used in muddy conditions - would maybe using some large (3" or more) crushed rock compacted into the washed sandy-gravel base make a better bed for the footer?

The problem is that the base soil appears to have a high clay content which does not drain well. We have not gotten any significant rain in quite some time so I am almost positive the water I am getting in the hole is groundwater. I know the footer is supposed to be on solid soil - I just don't know the best method to get that solid soil.

 

[Investorguy]

You may want to check out the discussion on crawl space drainage HERE.

We've been discussing a lot of the same issue and have gotten a LOT of interesting ideas. I'm pretty much in the same boat you are, slow draining soil, wondering if its the groundwater or ???, and trying to figure out how to get rid of the water.

 

[jim_wilson]

At this point I am almost positive my problem is groundwater - I was out excavating yesterday with the tractor ( I let the hole sit all winter) and after I had excavated all the mud out I started getting water seeping back into the hole. This morning I had 4 inches in one of the lower spots and the gravel I had backfilled into my excavation had all turned to mud.

I need to find some way of firming this up so I have a good base to get the footer down. I have a compactor so compaction shouldn't be an issue - I just need to find the correct way of doing this.

I will check out that other thread - thanks.

 

[jim_wilson]

Not sure if my original post was clear - this is an excavation for a garage foundation - there will be no basement, so my main goal is to prepare a good bed for the footer and foundation so that I do not get settling and cracks later. The excavation was necessary because I needed to get rid of all the organic soil and get down to a good base soil - the foundation will be backfilled on the inside when done to support the floor.

 

[LarryRB]

if possible, determine the direction the ground water is coming from and out from the proposed garage area, dig a trench with 3/4 inch minus and a 4" corrugated pipe to redirect to another area... I am dealing with this exact same situation with a neighbors addittion....

 

[jim_wilson]

I thought about doing that but there is nowhere to direct it to - the foundation hole is stepped because my lot slopes away from the house and the back end of the foundation is on the lower part of the lot. The only way I could get rid of the water by doing this would be to dig a drywell - a very deep drywell that would hopefully go down deep enough to get under the layers that are trapping the groundwater.

 

[KentT]

Jim,

Are you sure the ground isn't frozen down deeper, and the water/melt is travelling along above this frozen layer...

Here in Central Mass west of Worcester, the ground is still frozen once you get below 12-16" or so...

 

[jim_wilson]

I hadn't thought of that - it is possible I suppose that the ground could be frozen deeper down - but I think the water problem still exists because I first got a lot of water in the hole last fall when we got all of the rain (before it froze) - so I think one way or another I still need to figure out a way to backfill this hole somehow that will be stable enough to put the footer on.

 

[rambler]

There is a reason no real below-grade construction happens in MN before May 15th. I believe waiting for the frost to go away & spring sogginess to sink away will solve your problem.

Give it a month.

--->Paul

 

[SPIKER]

I was thinking the same as above tile and dirrect it away, I have similar problems, (see other posts about multi springs on the property) when I dug my electrical/water & sewer trenchs from barn to little cabin & hook ups I went down about 4~5 feet. water filled right in as I was digging at 4' I hit a gravel sand stone shale mix hard packed but porus, water ran in right behind the back hoe pretty quickly over night after 6 hrs digging I had 12"+ of water setting in there. all durring a june/july drought... spring time i started wondering if it would float the sealed conduit? my brother fixed that problem when back filling though by snagging & breaking it /forums/images/graemlins/frown.gif (used USE underground wire which can be laid un-protected in dirrect burrial so I didn't worry too much about it.)

anyhow I set a dirty water sump pump in a deeper dug section (dug a pit for it in 3 places) & started draining it with 100+ feet of hose. took 4 or 5 days of ON-OFF pumping to remove it to point it wouldn't come back . I didn't get the stuff laid in untill end of july but the bottom was leveled & gravel wasn't needed due to that was what was there anyhow... I back filled it and laid in the return lines and misc coverings. I need to go over & re-level and till the top section compleatly this year (need to pick up a rock rake first) and level off the area 3 yrs after install it is really rough sections sunken in and all... could add dirt and just level that but I have a tiller and the yard isn't much anyhow

mark M

 

[jim_wilson]

</font><font color="blue" class="small">( There is a reason no real below-grade construction happens in MN before May 15th. I believe waiting for the frost to go away & spring sogginess to sink away will solve your problem.

Give it a month.

--->Paul )</font>

I can give it some time and the ground will dry up - however if the problem I have hit is a high groundwater table that isn't going to help me down the road - the foundation footer
will be sitting in up to a foot and a half of groundwater at certain points during the year. At this point I am thinking my best course of action is to backfill the lower part of the excavation up to the level of the upper part - and then compact it really well - that way the foundation footer should always be above the level of the groundwater - I just dont know what I should be using as a proper backfill for that part of the excavation - it needs to compact really well and drain well also.

 

[Parttimer]

Jim, There are plenty of foundation footings in the water table in "our area" mass. The bearing capacity is usually reduced by half to cover that aspect of things.(to simplify the math) Therefore doubling the footing width, or using more "Bank run gravel" compacted into the hole at 4" - 6" lifts. If you needed to remove the water temporarily to facilitate the compaction, then dig a trench just enough away from the hole and a little bit deeper fill it with crused stone and stick a sump pump in it and pump it dry until the project is back filled.

Dave

 

[CurlyDave]

the foundation footer will be sitting in up to a foot and a half of groundwater at certain points during the year

Jim:

All by itself, this is not a problem.

You really need to get a soils engineer to look at it.

When I built my house, I had a soils engineer and he explained some things to me. The load bearing capacity of any soil is rated "wet", i.e. saturated with water. Drying out increases it. This makes a lot of sense to me because I know I can drive my tractor on almost any soil dry, but only certain ones wet.

The real issue is not wet vs. dry, it is expansive vs. non-expansive. Expansive soils expand when they become wet, and they will lift a standard foundation unevenly and destroy it. You can build on expansive soils, but the techniques are different. The footer should not have a flat bottom, it should have a "knife edge". What holds it in place is friction on the sides.

In less than 20 feet soils went from 8000 pounds per sq ft capacity to 2000, and then became expansive. An engineer will charge you a few hundred $ to tell you what kind of soil you have. If you have anything expansive get a foundation engineer. Otherwise just pay attention and calculate the loading yourself. Or, get some help on this forum.

I would not touch expansive soils without a foundation engineer.

 

[slowzuki]

I'll let you know what we did. We have clay till that drains poorly and we have groundwater that seeps up. So when we planned our foundation we were ready for the water that imediately started running in.

We laid out the footing trenchs so they all slope to one corner and a trench leads away. We then laid filter fabric and added a layer of compacted clean crushed rock. Then we laid 4" drain tile all connected to run out the low corner and compacted crushed rock around and above this. We continued adding layers of compacted crush until we had a level base for our footings.

Water runs out the pipe about 2 or 3 months a year now after the initial several months of draining. This type of foundation is a modified rubble trench type foundation. Works excellent so far, acts as a curtain drain on our hill too so when we add the septic we won't have to add a curtain drain.

 

[jim_wilson]

</font><font color="blue" class="small">( the foundation footer will be sitting in up to a foot and a half of groundwater at certain points during the year

Jim:

All by itself, this is not a problem.

You really need to get a soils engineer to look at it.

When I built my house, I had a soils engineer and he explained some things to me. The load bearing capacity of any soil is rated "wet", i.e. saturated with water. Drying out increases it. This makes a lot of sense to me because I know I can drive my tractor on almost any soil dry, but only certain ones wet.

The real issue is not wet vs. dry, it is expansive vs. non-expansive. Expansive soils expand when they become wet, and they will lift a standard foundation unevenly and destroy it. You can build on expansive soils, but the techniques are different. The footer should not have a flat bottom, it should have a "knife edge". What holds it in place is friction on the sides.

In less than 20 feet soils went from 8000 pounds per sq ft capacity to 2000, and then became expansive. An engineer will charge you a few hundred $ to tell you what kind of soil you have. If you have anything expansive get a foundation engineer. Otherwise just pay attention and calculate the loading yourself. Or, get some help on this forum.

I would not touch expansive soils without a foundation engineer. )</font>

I was aware of the whole expansive soils thing - that is actually the reason why the excavation is stepped - I needed to get all of the organic soil removed and get down to a good load bearing soil. I am pretty confident that the soil I have dug down to is ok to put a footer on - at least it is absent the presence of large amounts of water - because it is the same soil my house is sitting on and that has been there for 50 years without any significant cracks in the foundation.

 

[rambler]

</font><font color="blue" class="small">( - however if the problem I have hit is a high groundwater table that isn't going to help me down the road - the foundation footer
will be sitting in up to a foot and a half of groundwater at certain points during the year. )</font>

Guess that is just normal around here in our yellow clay soils, I wouldn't have thought of it as a problem. Our footers are probably just big around here to compensate. As well we don't usually box in the footers, just dig the 4 foot trench & fill it, let the ground be the form. To me there really never is a footer, just a big fat wide foundation. (This is for out-buildings, a house with basement has many different considerations of course.)

--->Paul