Belled pier heave

[haynewp]

I have been given an uplift from expansive soil to apply to the shaft part of a belled drilled pier. This is stated to be for the reinforcing design in the shaft. That is fine.

I have also been given an allowable bearing on the top of the bell to resist uplift which is due to wind or seismic, basically uplift coming from the structure itself only. OK

But, I thought I should also check that the shaft itself was anchored so that it would not move during expansion of the clay above. Basically, I was going to check that the allowable bearing on the area of the bell (actually, area of bell -area of shaft) was enough to resist the uplift on the shaft perimeter from the heave that was given in the report also. But the geotech told me I did not need to do that, I only needed to check that there was enough reinforcing in the shaft to resist the uplift. He said the uplift is internal to the shaft only and that the shaft would not heave itself out of the ground.

So I got real confused. If you look at the statics of the forces on the enitire belled pier, there is the uplift on the shaft from the heave, then the forces counteracting this are the weight of the shaft + skin friction below the heave zone + the anchorage of the bell. So how is it that the bell anchorage does not need to be checked to make sure the entire pier does not lift with the expanding soil on the upper part of the shaft?

 

[AlmostPE]

I think you are right. I think he is just ignoring it because it will be small. The skin friction from the "minimum penetration" they give you will more than likey resist the uplift from swelling. You should have told him.. what if the bell shape is so big like a footing? Shall we ignore the uplift?

 

[haynewp]

The difference isn't small. The swell uplift is twice as much as the bell bearing resistance that was given for wind or seismic. They didn't give a skin resistance below the neutral zone so I don't know if that would save the whole shaft from heaving or not. (sighs)

 

[jdonville]

haynewp,

You should demand a meeting with the geotech to address the missing information.

Jeff

 

[JAE]

haynewp - I spent many years in south Texas designing belled piers for uplift in expansive clay regions. The concept of the belled pier was actually developed down there (Willard Simpson and Terzaghi)and the bell was supposed to be the means of resisting uplift on the shaft.

However, there may be some validity in what your geotech is saying in that when the clays expand, the are "grabbing" the pier and expanding more or less in all directions..with the upward direction being a bit more since you have no resistance above.

The free body diagram would show skin friction going upward on the shaft and some skin friction going downward from some theoretical point along the shaft length. Thus, you don't have ALL the bell taking ALL the upward skin friction force, but rather a combination where the bell doesn't take everything.

This is a structural engineer's take on it...maybe BigH or HgTX or someone out there can provide a little more insight.

 

[haynewp]

JAE,

I thought about this also, but I have not been able to find any reference backing it up. I talked to another geotech that was involved with the project who would not agree or disagree with what the other one had said, but felt like it was not going to be a problem anyway for this site (said very conservative uplift values were given versus the bell anchorage allowed).

I would kind of like to know at this point just out of curiosity if there is an opposite swelling effect that can be counted on.

 

[haynewp]

http://www.hnd.usace.army.mil/techinfo/ti/818-02/81802page.htm