Loads for drilled pier design 1

[BAGW]

Hi,

While calculating the loads for bearing check in a drilled pier design, is self-weight of the drilled pier ignored? Are the loads that needs to be considered are just the superimposed loads only?

Thanks

 

[WARose]

I typically consider the weight of the pier, or at least the net increase from the difference of the weight of concrete vs. the weight of (removed) soil.

 

[BAGW]

OK. So net increase should suffice then? So like 30pcf.

 

[WARose]

OK. So net increase should suffice then? So like 30pcf

I would think so. (Especially if we are talking undisturbed ground.) But I would ask the geotech about this because sometimes (at least with piles) they already take into account the pile weight in the tip bearing value they give you.

But with shallow piers, they sometimes give real low tip bearing capacities. (Like it was a shallow footing or something.)

It just depends on your situation.

 

[BridgeSmith]

It depends on how the bearing capacity was calculated.

I don't think I've ever seen a bearing capacity adjusted for the full weight of the shaft, but our geology guys don't really ever know what our tip elevations are, so they couldn't give us that if they wanted to.

We get values adjusted for the weight of the soil over the bearing strata, since it's more accurate for the entire depth of the layer, so we use net shaft weight (the 30 +/- pcf).

Bottom line - if it's not given, ask.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Brad805]

I have seen many geotech reports of late suggest the mass of the pile should be included in the load. I suppose for exceedingly large piles that is appropriate, but for your common piles sizes I think it suggests a degree of accuracy in the geotech report that is not consistent with reality.

 

[STrctPono]

Around here, when we work with the geotech, the weight is always taken at the top of the drilled shaft. Their reports also reference shaft capacity in line with this practice. Not all conditions are end bearing and the reliance on skin friction is warranted in some situations. Additionally, there are other factors that can complicate the situation such as the use of a partial height permanent casing, etc. The geotech can very easily add the weight of the shaft to the values that you provide them as they figure out the depth. The important thing, however, is to communicate with the geotech and make sure you are both comparing apples to apples.

 

[hokie66]

It is insignificant when you consider the lack of precision of the geotechnical conditions.