Uplift Resistance 1

[GT22]

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?

 

[dmoler]

I disagree. When the cracks in a high PI clay form due to dessication, they actually swell and seal back up when rain/water/moisture hit. Then they tend to work a pier out slowly over repeated cycles or swell up the footing area. The top soils are also the most to pay attention to with footings. Also, forensic investigations for distressed structures I did in the Dallas to Austin areas (clay subgrades) indicated that the cracks run really deep in hot summer weather. I did borings right on the crack for some of them and they went 15 to 20 feet deep.

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.