Basic questions : Groundwater / foundation interaction

[Engineer108]

The bottom of my foundation trenches (0.700m deep) are filled with a little groundwater (+- 5mm). 1.500m deep test holes dug at the highest point of the site are 90% filled with water and 60% filled at the lowest point of the site. The site slopes at a rate of 1.500m over 32.750m. We will be putting in steel reinforced concrete strip foundations (0.300m deep x 0.700m wide).

a) Is it normal to have more water at the top of the site than at the bottom ?

b) Is this water a problem ?

c) What will be the effect of the groundwater on the foundations / house ?

d) What will be the effect on the foundations / house if the water were to be gradually removed by a drain ("french drain" ?) ?

e) What would happen during cold seasons ?

 

[jgailla]

What type of soils are you bearing on? What is the typical frost depth or building code requirement for embedment of foundations, or failing that, where do you live? Your groundwater appears shallow. If you are getting groundwater in your footing excavations compaction could be a problem and result in excessive settlement. Groundwater naturally follows the contour of the land, so "typically", groundwater depths would be about the same at high and low elevations on the same site. Groundwater level is significantly affected by the permeability of the soils it flow through. You could have a clay seam between the top of the hill and the bottom. There is really no "normal" when it comes to soil without any parameters.

 

[DRC1]

Usually the water will collect in the low pionts. I suspect your foundations have a high amount of silt or clay. this could interupt the subsuface drainage flow. to answer your questions, I would say a little water in the trench is ok. Over excvate the trench a little and fill to the top of water or bottom of ftng (which ever is higher)with 3/4 inch crushed stone. This should solve your pour problem. If the footing is below the frost line, frost should not be a problem. High water tables will lower bearing capacity. In general, if the house is properly designed a water table near the bottom of the footing can be accomidated, however groundwater is inherently evil and should be eliminated. A good drainage system proerly designed and installed will eliminate groundwater and the best time to do it is durring construction.

 

[Engineer108]

Thanks for the info 'jgailla' and 'DRC1'.

To answer your queries :
The soil, according to the engineer is 'decomposed sandstone'.
The footings will be below the frost line.

Any ideas for a 'good' drain design - one that will last a long time without blocking up ?