Rising Groundwater and Foundation Settlement

[nbryan]

Could rising groundwater with occasional inches of ponded water contribute to settlement of a column footing? I have encountered a footing where groundwater rises to within several feet and settlement has occured. The footing bears on stiff to very stiff clay with sand. A few inches of ponded water is also present sometimes near the footing.
I understand that the soil stresses will decrease with rising groundwater, but would this cause settlement?

 

[JAE]

I do believe that saturation of soils affects the bearing capacity of soils and also the stiffness.

 

[KAM_imported]

In theory, the ultimate bearing capacity of the footing will be reduced by a rise in the water table. It would be logical to assume that this could mean some settlement could occur (due to a rise in the water table) since the ratio of applied pressure to ultimate resistance has been increased (factor of safety reduced). I have not seen this occur in Florida; however, most of the time our water tables are located near the ground surface anyway. I suggest you monitor settlement of this footing for a period of time to see if the settlement is load-related or due to groundwater issues.

 

[dik]

First I would check the load on the column and determine the bearing value. Just to see that the problem isn't due to overloading.

When clays become saturated, the shear strength of the soil is diminished, and hence the bearing capacity.

Often, most clay soils are saturated all the time (even in very dry years), except for the top few inches that dry out. This is because the fine grain size promotes capillary rise and it wicks moisture from other places. In non-fissured clay, this rise can be several feet. Also because of the fineness, the rate that water flows through clay (percolation rate) can be very low so little is lost to evapo-transpiration.