Uplift Resistance
Uplift Resistance
(OP)
We often recommend ignoring the adhesion from the upper 1 or 2 feet of clay when calculating uplift resistance on drilled piers or footings, out of concern for desiccation cracks or disturbance. The depth of possible tension cracks in very plastic clay is substantial, but unless the soil is put in tension by excavation, I don't think those cracks develop fully. Therefore, the depth to disregard might be better based on disturbance or a limited desiccation crack depth from seasonal moisture changes. Rules of thumb aside, does anyone know of a good basis for establishing the limit of the depth to disregard for such soils?





RE: Uplift Resistance
I would say to use a depth of 12 feet for seasonal moisture change for design purposes, but uplift forces due to the clay should be modified based on density, hardness (as a check they are usually hard with pocket pen readings of 4.5+. )and depth. Some clays are very dense and can explode with high pressures when moisture content changes even at lower depths.