De-watering

[Vinny7]

I have to design / specify foundations for a house. The ground is soft at the surface and has stone below. The stone is in a firm clay and the depth to this layer varies across the house footprint from 1.4m to 2.1m (one corner). This stratum would be adequate for bearing but at the deeper corner the is a large inflow of water (and a stream near by - approx. 10m).

If we specify foundations taken down to this stratum and dewatering with a sump and pump should be be looking at the effect the flow of water in the trenches will have on the stratum. What precausions should we take? Would it be adequate to scrap of the top layer in the trench prior to pouring concrete?

 

[lovethecold]

When you say there is a large inflow of water in the corner, what do you mean by this? Was there a test boring that went artesian? You say there are stones in the clay, are you refering to boulders and/or cobbles, gravel? This sounds like a glacial till soil to me (in my home area). How permeable is this clay? Do you know what kind of water pressure there is?

If there is a fair amount of water pressure; not sure why this would be, but don't want to neglect the potential. Or the clay has a lot of sand and voids, and therefore relatively permeable, water flow could be a problem. Also, the amount of time the excavation will be left open prior to construction of the basement and backfilling will have some impact. The longer it is left open, the more time the water has to seep out and beggin to create stability problems.

Water should not be allowed to pond on the bottom of the excavation. It will only seep into the clay and softening it. Workers then will walk over this creating a 'disturbed' bottom where the foundation or basement floor will be placed over. In which case additional excavating would be needed to obtain an undisturbed native soil. And besides, concrete should not be poured over water anyway (unless specifically designed to do so).

Hope this helps, I am looking at this how we would for our area, here we excavate below the water table quite frequnetly. Sometimes the clays are highly impermeable and the excavation is open only a short time, so not much difficulty. Other times a relatively permeable soft sandy clay or silt that will open for a long period of time, additional measures need to be recommended.

Hopefully others will give there input.

 

[lovethecold]

Additional thoughts. Are you working with a local geotechnical engineer? Find a good geotechnical engineer to help, he should be familiar with the soils and give better insight. Soil borigs should be done so the appropriate recomendations can be given.

Also, becuase this is more of a soils question, you should ask on the 'Soil Testing' forum or the Earthwork Grading' forum.

Good luck.

 

[cvg]

I believe an option would be to construct a french drain or weeping tile along the wet corner in an attempt to divert the water away from the foundation. you may or may not need a pump to do this. You should consult with a geotech as excessive water and a soft foundation under one corner of the structure could result in differential settlement. get your contractor to dig a pit at this location and observe the amount of water that comes in and have your geotech make a recommendation.

 

[fattdad]

There is no one-size-fits-all answer to question about foundations below water. If it was completly "wrong" nobody would every build a bridge or fish ladder (for example). If you need to construct a footing below the water table, get it properly designed. If proper design is more cost-effective to include a french drain (or some other such engineering modification) then sobeit.

Hope this helps. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[ShawnR]

What about de-watering with a wick drain material. Here is a company that makes wick drains and specificaly the different styles of drain.

http://www.americanwick.com/applications/detail.cfm?app_id=15&app_cat_id=7

 

[Vinny7]

Ther is no basement, this is only for a strip footing and the water is entering the excavation from the sides. I think at the base of a peat layer. The ground below would be adequate but I was wondering about the best technique from ensuring the base of the excavation doesn't become soft. The water level is possibly related to the adjacent river. A trail pit was excavated approx. 5.0m form the proposed house and alot of water entered with the sides becoming unstable. A number of other trail pits were excavated quite close to this one with no significant seepage. It is likely the house foundation will may not have the ground water problem but i would not like to excavate to close to the proposed house until the foundations are being dug.

 

[lovethecold]

I guess I am a little confused. On the one hand it sounds like the excavation for the footing has been started, on the other hand it sounds like it hasn't but there is another area that has been excavated.

 

[Mickney]

I am confused as well. Based on my interpretation, I envision several geotechnical problems including, but probably not limited to 1) Softening of the subgrade soils due to water intrusion, foot traffic, etc. and 2) temporary shoring of the foundation excavations due to caving soils.

I read a peat layer in your thread. Also, a stream that is now a river only 10m away. Do you have subsurface boring information? Sounds like an experienced local geotechnical engineer should be consulted to confirm the depth to a suitable bearing strata, to identify the need for permanent/temporary dewatering and to identify settlement issues or problems. A future homeowner will not be too happy with a new house that has water or settlement problems.

 

[msucog]

yikes! 30' away from a stream and it sounds like at or below the stream level. in addition to all the things mentioned so far, there might be insurance issues related to this problem if building in what sounds to be a likely flood zone. i suggest you and/or the owner consult a local geotech...even if it's just having them out to look at the area to gather "off-the-hip" opinions...i personally would suggest acquiring their services for a more thorough evaluation based on the details provided above.

 

[fattdad]

Then again, there are riparian and potential wetland issues.

I do this type of work, just not in cyberland.

f-d

¡papá gordo ain’t no madre flaca!