Soil Borings 1

This was from a document that a geotechnical engineer once gave me.


No. of Borings depends on:

1. Size of the structure
2. Configuration of the structure (area vs. perimeter)
3. Variation in load characteristics from one part of the building to another.
4. Budget constraints
5. Experience with other structures in the area.
6. Availability of other borings in the area or on the site.
7. Requirements of local building codes.
8. Requirements of governmental agencies.

The BOCA code once required one boring for each 2500 sf of building.

The FHA and HUD agencies used to (don't know about today) require one boring for each 60 feet of perimeter for buildings more than three stories, and one boring for each 100 feet of perimeter for those less than three stories (basements count as a story).

One recommended guideline was:

Buildings less than 5000 sf: Min. 2 borings
5000 to 20,000 sf: 2 to 4 borings
20,000 to 40,000 sf: 4 to 6 borings
40,000 to 60,000 sf: 1 boring for each 10,000 sf plus 1 boring for each 800 feet of perimeter
over 60,000 sf: 1 boring for each 15,000 sf plus 1 boring for each 1,000 ft of perimeter

Add 1 boring for each special installation such as chimney, elevator pit, concentrated machine load, extra deep basement, high sign, etc. unless it occurs where a typical boring occurs.

Also, try to keep them within 100 feet of each other. 150 feet max. We tend not to do borings for buildings with greater spacing than that. If the soils in the area you are in is highly variable, than the spacing should be closer. Consult with a local geotechnical engineer, he should be familar with the area and know what the best course of action would be. There are certainly cases where you can get by with less, can't think of any atm, unless you are wanting to assume a fair amount of risk.

Another thing to know to, is what are the expected loadings and/or height of the building. These will impact how deep the borings need to go.

Also keep in mind, if the borings are far apart and one boring shows a deep area that would need to be excavated do to poor soils, there may be more excavating being recommended than might otherwise be needed, closer borings could narrow in the bad area. Or you could do more borings later.

I go about 200 ft and NEVER extrapolate (interpolation is a necessary evil, however). The shape of the building needs to be enveloped by the boring layout. Now is the question of how deep. . . ?

f-d

¡papá gordo ain’t no madre flaca!

fattdad (and other of you geotechs out there):

guidelines on depth would be interesting. Have any?

Here's the typical guide:

for pier footings - 2 times the anticipated width.

for strip footings - 4 times the anticipated width.

look at the grading plan as if there is a 10 ft cut and the bearing grade is 2 ft below that you need to drill 12 ft before you get any foundation design data (you will have 12 ft of data for your earthwork evaluations though).

Understand the geology: If there is a soft clay layer at 2 times your foundation bearing depth, you really need to fully penetrate the soft clay before you terminate the boring.

There is somewhat of a chicken and egg problem here. You need to have some perspective on the column loads and general sense of bearing pressure. So, if 2,000 psf is a safe bet and you have 200 kip column loads, your footing needs 100 sf (10 ft square). That relates to a 20 ft boring below the anticipated bearing grade. If you start by anticipating 4,000 psf, you'd end up with a shallower "design" boring depth, which may prove too shallow.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

we use 2.5x for columns and 4x for strip...below bearing level as fattdad points out. also, if there's significant fills, boring are taken deep to identify/evaluate the compressible zone. also, for seismic site class, some borings are taken to refusal if the client doesn't want to pay for shear wave velocity assessment (in my area, shear wave velocity assessment is almost always much more cost effective).

if i suspect old fill, the borings are tight...if i suspect residuum, then spread them out. i typically use 50x50 and 100x100 as starting points (depending on the size of the building--wouldn't likely do 50x50 on a million sf building). start with identifying the max anticipated loads, history of site, topo maps, etc and go from there. consult a geotech that is familiar with local conditions.

There are several rules of thumbs that others have given - some required by agencies and the like. I think that knowledge of the general area and geology as fattdad indicates is extremely important in choosing the borehole programme. Also, one needs to know preliminary details of the structure. Normally, I would seldom stop a boring shallower than 20 to 25 ft. even if I know the area pretty well (and assuming spread foundations). And, it would be a great idea, if the area is "new" to your experience database - or if the area is without history (little in the way of previous knowledge) to take at least one borehole to 50 ft. Sometimes you can do a few deeper holes and then use shallower holes in between to confirm the upper influenced strata.
If one was going to have to use piled foundations - especially if friction/adhesion piles, then you would go at least 2 to 3 x the likely dimension of the pile cap (i.e., the perimeter of the pile group) below the pile tip (see Tomlinson and others for zone of influence of settlement for pile groups). This would be modified of course should you "hit" rock or glacial till/drift, etc.
Sampling, in my world, would be at 2.5 ft intervals to 15 ft then at 5 ft intervals to 50 then space at 10 ft (as general rule). The 2.5 ft intervals is to ensure that you have sufficient samples in the footing zone (or, for thoses strutures with basements) to make a reasonable assessment of the soil properties (sufficient N values - or torvane values in tips of undisturbed samples). Oh, and dig a few shovel pits to ascertain, with some precision, the thickness of topsoil (important from a construction/stripping point of view).
In India, I ran a combination of 4 inch thin-walled tubes at the metre interval, then dropped a standard split spoon down the thin-walled hole. This way I covered 900 mm of the 1000m intervals. Because we were very interested in the upper 6 m or so, we did this for 8 m before increasing the sample spacing.
Getting back to the geology. I was on a site that had 30 to 35 ft of very dense sand and gravel. Good stuff, eh? A previous job (by others) in the same area had, oh, a large area fill of 10 to 15 ft involved with the structure - on one side. Mmmm At 35 ft, 100 ft of soft compressible normally consolidated "fat" clay (for you fattdad, mon ami). Can you guess what happened after the previous structure was built (say 3 to 4 years later)??
So, know your geology, know your development/structure, understand the influence that the likely foundations will have, and taylor your programme accordingly. Guidelines are only that - experience rules. Also, always have a geotechnical techician or engineer (junior to intermediate) log the boreholes - do not rely on the drillers.

Let us not forget the Leaning Tower of Pisa!

(good post BigH - thanks for the proper spelling of fat clay.)

f-d

¡papá gordo ain’t no madre flaca!

Wha? You thought I'd spell it phat? - it would never be fatt! (p.s., never ever used this as a written term in my life until now! - you've had a bit of influence on me!)