Depth of Borehole 2

[ONENGINEER]

There are two silos with a distance of 10 ft. The silo diameter is B=40 ft and the foundation pressure from silo is just under 3000 psf. Two boreholes have been decided. I am advising on the depth for field investigation. It seems the site is mainly on clayey soils.

I appreciate a feedback if a depth of 40to 50 ft suffice instead of 2B = 80 ft. Thank you.

 

[LRJ]

Has a desk study been done? Do you have any idea what might be there? Preliminary capacity analyses - even if conservative - are a useful tool to justify your decision. I'm sure whatever you do the designer will not be happy though - you can never win!

 

[ONENGINEER]

Yes the site is fat clay to 6 ft and the rest is expected to be clayey soils to a considerable depth according to desk study. I want to meet the minimum standard as it is unlikely that the client questions about the depth.

 

[oldestguy]

The usual rule of thumb for this question is run the boring to the depth where added stresses are 10 percent of existing.

 

[BigH]

Silos - in Canada have had a share of problems. Transcona Grain Elevator comes to mind among others (see Tschbotarioff, 1951 for another). I do not think that 80 ft is too shallow by any means. In Sarnia, boreholes in the clays at the various refineries typically went down about 100 ft. In Toronto, I always thought that a 100 ft for a borehole was pretty deep; until I came to Asia and have seen boreholes put down to over 50 m and sometimes 100 m. Oldestguy is right in that normally you would investigate to where the vertical stress depth is 10%. For a square of round footing this would be 2B where B is the foundation width; for strip footings it would be more like 4B. You have indicated that there will be two tanks - will they be on the same or different foundations? if the same, you would be having a rectangular foundation and 2B may not be good enough; you may need to go a bit deeper.

Now that is out of the way - you should always be prepared to adjust your programme and hole depths. Obviously, if you run into bedrock at 50 ft, you would core 10 ft (to prove bedrock and not a boulder - and do at least 10 ft. A geo firm in Toronto starting with a "Gol. . ." did 5 ft one time and it was ledge rock and not bedrock - oops. Similarly, you may find a reason to go deeper. Now, having said that your programme should be flexible enough to adjust borehole depths, you should also be wanting to adjust your sampling intervals and sample retrievals. When I was in India, we had 6 m of very soft clay (Su < 20 kPa); the typical borehole would be top take a sample at 1.5 m (SPT),3 m (thin walled tube), 4.5 m (SPT) and at 6 m (thin-walled tube). I thought this was ridiculous given that we would only have 3 samples in the strata that was the most important. I adjusted the investigation to take a thin walled tube at 1 m intervals (100 mm diameter tube) and then, do a split spoon down the thin-walled tube hole. This way, while, yes, the SPT might be argued to yield "bad" results, I did end up with 6 thin-walled tubes and 6 blow counts with a chance to look immediately as per any laminations within the soil on opening up the split spoon.

Oh, and be careful of desiccated crusts.

 

[ONENGINEER]

BigH, many thank for your enlightening post. The two silos are said at this stage to be on separate rafts. But with only 10 ft away, it seems the foundation soil behaves as if it is one raft 90 ft long by 40 ft wide as it may be implied from the kissing towers you pointed out.

I wonder if depth to 20% stress which is ~1.5 B (Bousinesque)is taken for borehole depth that would suffice. Otherwise the client seems to have another engineer on board who has recommended a lower depth.

 

[fattdad]

I'd calculate the elastic response and drill to the depth where the delta sigma V is within 10 percent of the overburden pressure.

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

Use the idea of a single slab and you will end up with a picture like the one shown, unless that slab is heavily reinforced..

 

[ONENGINEER]

Oldestguy, sorry I cannot see the picture you sen me. Errand browsers prohibit me from seeing the picture. But I understand you recommend a single slab (?). Thank you

 

[Okiryu]

ONENGINEER, Look at Chapter 4, page 57 of this link:

https://www.wbdg.org/ccb/DOD/UFC/ARCHIVES/ufc_3_220_03fa.pdf

This document is used in DoD projects. Although is cancelled there should be another one which supersedes this with similar information.