Advice for North Texas pier depth and techniques for moist conditions

[ClemsonGrad]

We are adding a walkway to a massive building built in the 20's. It is post and beam with brick walls. It is in Collin county.
Question 1. Where piers a common construction practice back then?
Question 2. How deep are most piers for commercial buildings in this area?

(We dug piers yesterday and hit water around 12 feet. First hole was so bad there was no way to even cut a bell. Drilling rig went down to 20' and it was very wet.
Drill operator said he could not provide pier holes with bells that would be suitable for pouring.)

The soils report mistakenly said water should not be a problem. Done last summer

Question 3. What other foundation system could be used in this situation? (Concrete deck 10 feet wide with canopy. Building has crawl space of about 8 feet so deck is at first floor height with stairs to grade. Owner is excavating in crawl space to pour slab for storage and future retail space.

THanks

Larry

 

[Ron]

You need a second opinion from a LOCAL geotech.

 

[Eldorado]

Your real expert on the questions you have posed is the geotechnical engineer. It is not uncommon for the geotech investigation not to encounter water, but they typically don't stick around or come back to monitor. Not knowing loads or the geotechnical recommendations, I can't really comment much, but I can say that I don't specify piers less than 20' deep with a 15' seasonal moisture variation zone that is typical in Texas. I'm not sure what you mean by adding a walkway, but this project sounds like it probably has a small budget. If the water is perched or coming in from the side walls of the pier excavation, you may be able to case the pier and drill past (eg. down to 25') if the material below is suitable for belling. Casing is added cost and this can kill a low budget project.

Get with your geotech.

 

[JAE]

Agree with above.

Also - typically when I've specified belled piers only to find that the bells can't work due to water or more sandy conditions than what the geotech thought - we've gone to straight shafts.

These straight shafts sometimes get very large and/or deep but sometimes the geotechnical engineer can help by also adding skin friction to the capacity.

 

[ClemsonGrad]

How is skin friction achieved?

 

[JAE]

Friction between the pier and soil around it.