Groundwater close to bottom of foundation excavation

[buildhome]

I am owner-building a 30x20 story and a half home in NE Vermont on a frost protected monolithic (thickened edge slab. The site has approx. 5-6% slope, silty soils and is at the bottom of a watershed of approx. 250 acres that goes a mile back.

Excavation was done to get the topsoil off at the lower elevation, and cut back level upgrade. The excavation is about 4 feet wide - extending two feet from the outside of the 12" wide slab footing and approx. 8" to 12" from the inside of the footing. The plan was to place 8 inches of 3/4" crushed stone, then 8" 3/8" stone, then 8" of 3/4"crusher run to come up two and a half feet, then set the forms and pour the slab which will have sides 2 feet high and the footing 12" wide. When I first excavated, the groundwater was about 5-6 inches below the bottom of the trench and unfortunately did not get to compact the bottom before 3 inches of rain fell in 24 hours last weekend. Now the groundwater has come up about 4 to 6 inches so that there is about an inch of water in one corner of the excavation. We have been dry for 3-4 days and the water is receding slowly. The soils are silty - a ribbon can not be rolled smaller than 3/8 to 1/2 inch, and a meatball shines up (dilatancy). I can push a rebar in 9-10 inches with steady pressure.

What is the procedure when groundwater is hit in an excavation? I'm sure there is 4-6 inches of disturbed soil that has not been compacted. Can rock or gravel be pushed/compacted into the wet soil to give it stability or does it need to be drained before anything can be done? I am considering curtain draining the area, something I considered in early planning. Should I excavate wider to get a wider stone footing than the 4 feet I have now? I have been in touch with a local geo-eng, and my tentative is to get him out to look, maybe after I curtain drain it. I am eager to hear any other ideas. My problem is similar to that of ttowne in thread261-56444

Thanks for any help.

Tom

 

[buildhome]

A quick update, though opinions are still welcomed:

I've decided to dig the curtain drain, trying to get it at least 1 to 1-1/2 feet below the trench bottom. Reading I have done on such drains brings up different opinions as to type of backfill, 1-1/2 inch stone wrapped in fabric vs. sand, arguments being that the sand supports the trench wall better than stone and inhibits erosion and piping and the silting of the system that results. There was a thread on this thread158-88291 Claims are that sand drains outlast stone backfilled trenches. I have also heard about filter fabric clogging. I plan on backfilling in graded layers as I plan on doing under the slab: 1 foot of 3/4 stone, ( possibly a layer of 3/8 stone), then 3/4 crusher run gravel to within a foot of grade, then filter fabric over the top of the gravel, fill to grade with native soil. I am going to put in a cleanout at the uphill end of thedrain. I'm not going to try to double the drain's use as a surface drain too by bringing the gravel all the way to the surface grade since surface runoff is not much of a probem and I will also put in a waterbar further uphill for surface drainage improvement.

The soils have dried more and I can not penetrate them more than 2 inches with the rebar in the drier locations.

Thanks.

Tom

 

[pba]

First to be brutal: If you are going to hire a geo-eng, do so and listen to their advice. Don't do what you want to do and then expect the geo to agree with you. If you are not going to hire a geo then you are using Eng-Tips for free advice when a geo-eng would be otherwise required. You may well get what you pay for...

Now - The important thing is to make sure that future water can't get into you home or undermine it. This suggests that some sensible form of land drainage is required. The presence of standing water on excavated surfaces is usually sufficient cause to remove the material back to undisturbed material. I can't assess the depth of excavation - A geo-eng could...

 

[buildhome]

Thanks for the reply, pba. The geo I have been talking to offers plenty of advice but says there is not much a site visit will do to provide more information.

Though the water level has dropped to 4-5 inches below the bottom of the trench the soil is staying wet and non-compactable. The attempt at placing a curtain drain failed because there was too much material to move off the top grade in order to reduce the depth to dig the drain. and the 4-1/2 foot trench was collapsing. A dozer will be used to lower the grade to where a more shallow and manageable drain trench can be dug.

Tom

 

[DRC1]

If you are in wet soil you definetly want to install a curtian drain. I typically use 6" abs plastic with filter fabric and stone backfill. However your geotech can give you recomendation that are best suited for your site. As far as getting a stable base your best bet for the long run is to excvate the toplayers of wet soil and replace with stone. be sure the ston comminicates with the drain. How deep you need to excavate is probably best answered by your geotech, but for a house slab, I would expect to excavate 2-3 feet if the underlying material is competent. A layer of fabric under the stone is also good.
Good Luck

 

[Focht3]

Don't put stone directly against the silt! The stone will clog, and you will end up with a non-functioning drain. Use concrete sand instead. Or a filter fabric -

Find another geotech. Pay him/her to visit the site - offer to give them a check when they arrive. That should get someone to visit - which you sorely need, in my opinion.

My experience with silts has been that the non-plastic varieties will firm up when confined; this would suggest that you put down a filter fabric and 6 inches of sand first, or a geogrid (like Tensar BX-1100) and 6 inches of crushed rock. Your situation sounds like a good candidate for the filter fabric and sand - it will provide a drainage layer beneath the foundation.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[buildhome]

Thanks for the replies. I dug my foundation drain down at least 6 inches below the level of the trench I had. So, I have a trench in a trench: a 5 foot wide trench with a 1-1/2 foot wide by 6+" deep trench around its outer perimeter. This was easier than the curtain drain and should serve to keep the native soils at the bottom of the trench a bit drier. I lined the drain trench with fabric and placed 4" perforated pipe surrounded by 3/4" crushed stone. I brought the fabric up the sides of the trench as I laid in 6" of 3/4" stone and then laid the fabric over the top of the stone. I like the idea of the sand drain and that is what I will use for my leachfield curtain drain. I was going to use sand here but decided to stay with stone under the slab. The perimeter drain has cleanouts before every 90 degree elbow. I added 6" of 3/8 stone on top of that and will add about 16 inches of 3/4 inch crusher run gravel on that giving me a base above the native soils which is 2-1/2 feet deep and 5 feet wide on which I will pour my frost protected radiant heated slab. All is being compacted in 6 to 8" lifts. As further protection, if it will perform any function at all, I will place 6 mil plastic on the uphill sides of the trenches before I backfill to curtain off any lateral water flow that may, for whatever reason, rise above the level of the drain.

The extruded polystyrene frost protection is being overdone as well. From what I have read and heard from two engineers and local contractors I am comfortable with the way I have done it and thank you for the replies. I have gained useful information from Focht3, in particular, and I thank you very much.

Tom