Boring location and amount of borings 3

[eej268]

A quick question. I have a project for a foundation engineering class and must choose the location and amount of borings for a 16 story structure that measures approx. 320' in length and 160' in width. I was wondering how many borings are necessary and the general location of said borings? Should they be at the corners, along the walls, in the middle, etc?

Thanks

 

[Bobber1]

Before I answer this, how many credits will I receive?

 

[ARS97]

Is money an option? If not, heck, get one at every column location. Seems like it's too open-ended of a question though.....

 

[fattdad]

Generally-accepted engineering practice would include borings at a spacing of about 200 ft. If the ground conditions are highly variable, maybe closer, if they are very uniform, maybe further.

I'd say for a 320x160 footprint, I'd go with the four corners and the center (i.e., 5 total). You could also go with six borings each along the perimeter and 160 ft apart.

Now how deep? For a 16 story building and to get to where the change in effective stress is less than 10 percent of the existing overburden pressure, you'd likely have to go pretty deep - maybe like 60 or 80 ft, unless you happen to know there is some intact granite within 30 ft or so. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

May want to take one boring to 100' for seismic site class determination (unless there is shallow rock).

 

[BigH]

Before starting, do your homework on the "liklihood" of buried stream channels, etc. that might show up on old mappings of the site but may have been infilled for some reason or another. If this has occurred, you may very well need to up the number of borings.

 

[oldestguy]

I'd tell the prof that any answer you give will be only one person's opinion and practice. Heck, I sometimes answer that when a client asks, I say "at least one". That sort of puts them back in their seat, with less of the resistance that sometimes come from spending money.

If there is no budget and if the client is a repeat client, he usually will go along with any thing you say.

Then, after reviewing the number chosen, (five seems ok), reserve the possibility to go back and do more, or to go deeper, to search out apparent variations.

As to how deep, you might use a simple rule that borings go to the depth where added stresses are below 10 percent of existing stress. This added stress includes any grade raising inside as well as outside. Neglect unloading of a basement as a starter.

 

[grabens]

Others have provided sound advice. I would say a boring spacing of 100 on-center or 1 per 10,000 s.f. would be reasonable for a project like this. As for depth, the column loads would probably control this. A good rule of thumb would be 100 kips for every story of the buildig. A 16-story building would generally have column loads on the order of 1600 kips. Assuming a bearing capacity of 3 ksf, the area of the column footing would be a 23 ft by 23 ft footing. Using the 2B rule for stress distribution, the boring depths would need to be at least 50 ft. A building of this size with these loads is probably going on a deep foundation. Depending upon the anticipated geology (i.e. shallow or deep rock, end bearing vs skin friction piles), borings on the order of 75 feet to 100 feet may be reasonable.

As others have said, the question is too open ended. In the real world you would ask for some additional information.

 

[pelelo]

fatdad, would you please explain how you came up with the 60 to 80 feet depth?.

I agree with grabbens approach about stimating the boring depth, the only problem is not all the time you will have a 3 ksf bearing capacity, of course, this depends on the experience of the geotech engineer in the area.

IF you have less than than 3 ksf, you might need more information.

According to what I have seen, the deepest boring is located on the elevators as the heaviest loads are located in this area. Then, you can distribute (depending on the area of the site) the rest of the borings on the columns.

These borings don;t need to have the same depth, even though the deeper the more info you have. Also remember, you might be fighting for a project, your competitors sometimes might come up with shorter borings, so sometimes conservatism doesn't help.

 

[FixedEarth]

Depth of Boring for good soils can be determined by: Boring Depth = (10) x (# Story)^0.7 + Df. Where Df = depth of foundation.
In our example, 16 story hight, we get (10 x (7)ft + 4ft = 74 ft. You would probably make this the depth of the deep borings and take several intermediate depth (30 to 40)ft borings.
As for location, I am partial to S or Z shaped layout.

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