Drilled Pier with Frost Depth

[sach4u1]

I have a drilled pier with a 3ft frost depth. The geotech report says that this depth should be neglected. I am trying to modify the pier with a concrete cap. Do I have to put this concrete cap below 3ft? Thanks.

 

[oldestguy]

You should explain more about this. What is meant by "with a 3 ft. frost depth"? Do you mean the pier is set at a depth of 3 feet which is the local assumed depth of frost?

You might explain what they mean by "this depth should be neglected".

If you have an assumed frost action to 3 feet, why neglect it?

A few more sentences will help.

Why the cap? Do you need more bearing area (than that of the pier) for your load? If that is the case, of course your bearing area should be at 3 ft. depth.

 

[HSNIC]

Neglect 3 ft frost depth? Are you advised to be more conservative or liberal in your design?

 

[dmitriy555]

It is typical for geotech reports advising to ignore top 3' or a full pier diameter for soil skin friction capacity and such. But if you want to reinforce that drilled pier with a cap, you can place it within that 3' depth, as long as you can place it on soil with adequate bearing capacity. However, you can not rely on the soil skin friction in the upper 3'. It will work in a similar manner for the uplift capacity, where you can rely on the weight of concrete and any concrete-soil interaction below 3'. I have reinforce lots of drilled piers this way. Make sure you provide adequate shear reinforcement between the existing pier and the new cap.