Borehole depth 3

[Sujitha Menon]

how do we determine the depth of borehole under a tower building with 4 basement and 15 stories

 

[human909]

Ask an more knowledgeable engineer. Some basic understanding of the expected ground conditions is very helpful.

 

[RabitPete]

Sounds like something a geotech would determine based on their knowledge of the local soils and a target bearing capacity you are expecting.

 

[BAretired]

Lower a tape measure into the borehole with a weight attached to the end. You will know it has reached bottom when you no longer feel the pull of the weight on the tape.

BA

 

[WARose]

I think a lot of it is experience. Where I live, they typically go to bedrock every time anyway because they run into it at 30' down.

But if you need something more concrete, one geotech text I have suggests this:

Z_b=10(S)^0.7 (for light steel or "narrow" concrete buildings)

&

Z_b=20(S)^0.7 (for heavy steel or "wide" concrete buildings)

where:

Z_b=depth to bore to [in ft]
S=the number of stories of the building

Another standard I saw once said (something like) boring to a depth where the increase of effective pressure (Δp) is equal to about 10% of the average effective bearing pressure of the structure. (In other words, to a point where the effect of the applied stress from the new structure is minimal.)

 

[blihpandgeorge]

need a geotech who understands the general ground conditions, we typically give a geotech an indicative column load and they will advise on likely BH depth.

 

[Enable]

While BART's answer is clearly the winner lol I suspect most introductory geotech text should have this. See the following table from Geotechnical Engineering by Donald Coduto and Co (though this is for shallow footings only)

 

[HTURKAK]

Sujitha Menon ,

Double posting is not allowed .. pls look to the forum rules. Apparently , the excavation depth will be 15 m . Literally the overburden pressure at that level should be around 270 kN/ m2 or 270 kpa. If the basement floors have similar area with upper floors, the service load of the bldg (19 floors + 2 m raft ) will be 240 kN/ m2 ..

If the podium and basement storeys have larger plan area ( most probably ..) the effective pressure will be negatif..I hope the GWL is below the raft so you will not have floating risk.

Apparently excavation depth is more than 15 m , so the minimum depth of bore holes should be 30 m .

 

[GeoEnvGuy]

You ask your local geotechnical engineering firm. If you pick a point on a map in a local geotechs area they can tell you what the expected subsurface materials will be, the likely foundation recommendation and what to expect in the investigation report.

The depth answer relates to the available bearing capacity and foundation widths for footings or strip foundations. In bad soil conditions the depth you are going to be looking for is to get piles as end bearing or friction fitting.