Foundation excavation and groundwater/mud

jim_wilson · Mar 26, 2006

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 · Mar 26, 2006

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 · Mar 26, 2006

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 · Mar 26, 2006

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 · Mar 26, 2006

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 · Mar 26, 2006

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 · Mar 26, 2006

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 · Mar 26, 2006

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 · Mar 27, 2006

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 · Mar 27, 2006

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