Foundation excavation and groundwater/mud

jim_wilson · 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 )

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 · Mar 27, 2006

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 · Mar 27, 2006

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 · Mar 27, 2006

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 · Mar 27, 2006

( 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. )

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 · Mar 27, 2006

( - 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. )

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