(Hopefully) Simple Drilled Pier Question

[thomasbode]

I am not sure if this is the correct area or not but here goes:

I will be putting in some drilled piers (caissons, whatever you want to call them) that are about 5' to 11' in diameter and 9' to 27' in depth. The question is if they will need to be installed using slurry. The question I have personally for my own personal education is which is more important in trying to decide how well the hole will hold up, the PI or the Blow Count. Also, is there any kind of quick rule of thumb to determine the vertical stability of the hole using those vales since they are typically included in the boring logs for geotech reports.

On a side note, this project is broken up into 2 phases and thus has two geotech investigations. The problem is that the northern half of the project was completed by one company and the southern by another company. At the mid point of the project both companies had bore locations that were close to each other, relavitely speaking. The issue is that both companies came up with different results. While there is certainly the possibility of a geologic change between the north section and the south section, I find it suspicious that the change occurred right at the point of handover from one company to the next.

Anyways, any help would be appreciated.

 

[FixedEarth]

The soil stratigraphy can change at any distance, so the overlapping borings being off is usual. Couple of years ago I had two borings that were within 15 feet apart. One boring showed cohesive soils and the other revealed Sand & Gravel.

P.I. is better indicator of how well the drilled shaft will stay open. As a rule of thumb, P.I. value greater than 10 means the soils have substantial amount of clay like particles. Also, look at the boring logs and look for "qu" or unconfined compressive strength value of 1.5 TSF or greater.

 

[BigH]

thomasbode - how different were the "results" of the two companies? In what way - description of stratigraphy (which shouldn't be much unless the site truly is quite variable) or the foundation system to be used. In the later, give 5 geotechnical engineers the same data and you'll likely come up with 6 slightly to more different recommendations - as we are all prisoners of our and our mentor's experiences. I'd like to see a bit of details on the differences that you have alluded to.

As for standup time, the earlier advice is reasonable. Depends a lot on the nature of the material deposition/degradation (varved clays, for example), previous stress histories (slickensided zones). You might wish to speak with a couple of drilled shaft contractors in your area and find out how they have done this before in the same general vicinity. There experience is probably more valuable than from those of us who don't have the local knowledge.

 

[oldestguy]

A lot also depends on the ground water table location. Going into saturated earth is significantly different than unsaturated earth.

I'd suspect any move in the direction of stabilizing the hole with a fluid will be an on-site decision in marginal areas, between obviously bad conditions vs. stable materials. It sure is a lot more complicated an operation and to be avoided where possible.