Groundwater And Mud In Foundation Hole ????

jdwilson44

Last year I started the excavation for the foundation for my new garage - turned out just as I was ready to get the concrete poured we had a ton of rain last fall and the excavation filled with a foot and a half or so of water - that never went down.

The excavation is approx 28" x 44" and is stepped 16' because the land is slightly sloped and I needed to get down to good solid - non organic earth for the footer. It is the lower half (obviously) that accumulates the water. The ground appears to be a sandy/clay mix that does drains ok but not great.

I am 99% sure this is groundwater because 1) It never went down after those rains (the ground had previously drained) - and 2) as I was out cleaning out the mud etc recently I started getting more water (no rain in approx 2 weeks) and it accumulated about 4" more water overnight.

My original intention was to over excavate the lower part of the hole and refill and compact with some well draining washed gravel that has no clay - I did this on part of the hole but the water is high enough it just turns the gravel to mud.


So my question is this - what do I do to either
1) firm up the ground so it can handle the weight of the foundation

or

2) backfill the lower part of the excavation up to match the upper part (no step)

I have seen people use 3" crushed that is compacted directly under a footer in slightly muddy conditions but I am not sure what to do to fill in 20". I do have a compactor and understand that this needs to compacted really well before the footer is laid.

jmic

I would put a sump in a low corner out of the way and get a pump working to keep the water level under control. Then lay down some Geo Stabilization Fabric around the perimeter maybe a couple of feet on either side of the footings, mayby go with a little wider footing than you were going to use. Put 6"-8" 0f 1 1/4" washed stone and compact over fabric and pour concrete over stone. That should do the trick.

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___"Remember You Never Get A Second Chance To Make A First Impression"______________________
Joe

denick

JD,
Are you going to put in a footing drain? If you are you can dig the trench and put the pipe in now to let the water run. You can do this even if it won't get used in the future. When you trench the end near the foundation should be 6" below the bottom of your footing height and pitch out to grade at least 1/4" per foot if possible. The 6" lowers the water table by that height. Let the water run for a few days. Clay sand mixes are known to firm up after a bit of time draining. You can keep the water moving towards the trench by using a shovel and creating ditches that run to the low point you created with the trench. Without added rain water it should tighten up. The ditches can be kept open during forming the footings by putting some 4" pipe and forming over it.

If this was hard to follow the simple way to overcome water is create a low point a good distance away from where you will need to work 3' or 4'. Then start opening the area up so that the water can get to that low point. Just like connecting mud puddles. Never let the water pool or lay in a spot. Get it to run to that low point. Then get the water out of the low point with a trench or pump this is really the 1st thing to do.

Check with the building inspector to see what he will accept under footings.

This is kind of the long version of what Joe said.

Nick

jdwilson44

Part of the problem is that I really don't have anywhere to drain the water to - the lower part of the excavation that gets the water is a good 2 1/2 feet below the grade of the lower part of the lot. And there is nothing lower - in other words - nowhere I could drain the water to.

Since this foundation is for a garage - (there will be no basement) and the level of the floor will be approximately 4 feet above the highest point I have seen the water - the only reasons I am worried about the water is for long term stability of the foundation and for the initial pour (when the water will interfere with the workers).

I am thinking at this point my best option might be to just fill in the lower part of the excavation and bring it up to the same level as the upper part - which would mean getting rid of the requirement for a step in the foundation and probably bring the cost of the formwork down some. I just dont know what exactly the proper procedure is and correct material would be to do about 2 feet of backfilling. I have used a 1.5" crushed rock / rock dust mix for doing some driveway work and that compacts excellent - not sure if that would be the correct material to use under a foundation footer however.

jmic

Jd,
Refer back to my post up above, It wouldn't be a good idea to fill 2.5' and expect to pour footings over it, IMO!

__________________
___"Remember You Never Get A Second Chance To Make A First Impression"______________________
Joe

denick

JD,

If this is something that needs to be inspected you need to ask the inspector what he will accept.

If you have water and mud in the hole it isn't possible to get the compaction you need using crushed stone or fill material.

Try creating a low point 15 or more inches below the height of the footing grade 2' or 3' in diameter, line it with a piece of filter fabric and put a pump in it and fill it up with stone. Let it run for a couple of days. It should dry up the area. Then you can do what ever meets the code for your area. Joes first post is a place to start.

Nick