Deep Foundation Bearing Pressure 2

[DCBII]

We started excavating to pour a square spread footing and found some unexpected underground construction in the area that interferes with our spread footing. A 24" drilled pier would fit, but we would need a higher bearing pressure.

The site has bedrock overlain by gravely clay. Right now the geotechnical engineer has given us 4,000 psf for spread footings on bedrock. He says that if we used a deep foundation we could get 50,000 psf out of the soil. How deep is "deep"? We've excavated 2' down and I don't know if we could go much deeper than 4' more without hitting bedrock. 50,000 psf seems like a huge increase in bearing strength. How is the soil stronger than the bedrock at any depth?

 

[BAretired]

Ask your geotech how deep is "deep". His comment, taken by itself does not seem to make sense.

What is the nature of the bedrock? What do the boreholes show?

BA

 

[DCBII]

The geotechnical report bore log shows basaltic bedrock at depths ranging from 3 feet to 12 feet near the area.

 

[wiktor]

4,000 psf even for weathered bedrock seems extremely conservative. The other statement (50,000) was apparently not describing the soil capacity, but the rock bearing capacity, which you will have likely after excavating 2'-3" into rock.

 

[Qshake]

Agree with Wiktor on achieving the required higher bearing once you've socketed into rock. The nice thing is that it sounds like rock is very close to the surface. the downfall might be that drilling is a specialty your contractor may not have. Hence he'll have to get a sub on board.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[DCBII]

I just talked to the geotechnical engineer again. He said 4000 psf was assuming a fill material beneath the foundation to get down to bedrock. 50,000 psf is for a driven pile down to competent bedrock. I asked what kind of range I might expect out of a drilled pier down to bedrock and he wasn't very forthcoming with answers. I only need about 15,000 psf.

 

[msquared48]

Why can't you excavate to the bedrick and pin directly to it, using it for bearing and forgetting the soil? Doesn't sound very deep.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BigH]

Very strange that a geotechnical engineer would even have thoughts of "driving" a pile to rock if the rock was only 3 to 12 ft deep! Mmmmm You indicated that a 24 inch drilled pier would fit - could you go bigger and do a hand dug caisson (as it is very shallow)? We did a similar thing in Laos and even were able to bell the caisson a bit to get more bearing area. - and yes, you will need to support, etc. (safety).

 

[msucog]

there's a big difference in bearing on fill, residuum, top of bedrock and in to bedrock. if it's within 10' or so, i'd just extend the foundation to the top of rock with conventional equipment unless it is more cost effective to bring in someone to drill caissons. plus if you get to bedrock, your seismic site class may improve the desing parameters dramatically (assuming that it isn't too far in to or even matters for your project). either way, pay for a little extra concrete instead of mobilizing drilling equipment. or if it's a big job, look at drilling (but dont forget to account for any added geotechnical/structural design/evaluation/testing costs associated with this).

 

[BigH]

Based on the OP, I wasn't under the impression that this was a big job which is why I was suggesting manual - very costly to be mobilizing big equipment. of course, if the job is big and can handle it, many of the replies give a good idea of what you can do.