Depth of Excavation

Greetings acres,

Being that I am in electrical construction my knowledge is limited based solely on occasion, we have to work with the excavator and concrete contractors to place conduit systems below grade through foundations at varying elevations.

Most foundation design specs and drawings I have seen are based on the soil exploration contractors report (boring logs) based on soil borings throughout the given construction site which reveal what any given soils load bearing will be based on the underlying subsurface structure.

I could be wrong but I personally doubt that there is any "one size fits all" formula to determine foundation depth or design. There are too many variables not only regarding soil but also varying building designs.

Geotechnical engineers in my neck of the woods usually recommend a minimum foundation depth of 1.5m because that is the depth of frost penetration here. If frost is not a consideration, the depth of excavation must be adequate to remove organic materials and disturbed soil.

A forty storey building often has underground parking in which case, the depth of foundation may be determined by the number of levels of parking. The depth of excavation for such buildings is also dependent on the type of soil encountered on the site. Prior to deciding on the type and depth of foundation, a soil investigation is carried out by geotechnical engineers. Sometimes alternative recommendations are made by the geotech recommending shallow footings, driven piles, compacted piles belled caissons or other foundation type, each with potentially different depths.

Rarely, if ever, are foundations intended to support a forty-story building designed from information exchanged on a blog-site.

Hey, there's a first time for everything....

To get the ball rolling, let's assume soil compressive strength is 100,000 tons per acre and overturning moment is 1,000,000 newton-feet (combined wind and seismic).

Over to you, CaptMoosie....

It all depends on the ground that you're standing on. Not to far from me the foundation rules in Manhattan are relatively simple, anchor the beams into the bedrock. In contrast nearly all of Long Island NY is several hundred feet of sand and clay above the bedrock.

It depends upon the soil testing report. Trained structural engineers design the required foundation based on the soil testing and type of load of the building. Books on strength of materials ,soil mechanics, Civil Engineering hand book will provide more details.

I would think, if you are indeed the civil site engineer, that you would realize the depth and size of a footing depends on several parameters, including, but not limited to, applied dead and live loads, allowable soil bearing capacity, and soil freeze depth for your area.

@ shawdog2,

"I would think, if you are indeed the civil site engineer".

Anyway Thanks.

Thanks also to you KJK/USA & Bruce, GA.

aCres

usaly you digg down to solid bedrock, but if it is to deep, then you can drive down columns for suport.

Hello! gudz777, I like your answer. I myself am an electrical engineer but my expertise became quite interdisciplinary and have been a designer for foundations of towers. The limited experience that I have on buildings, one excavates to original ground in cities. That is to say excavate through old fill. A shallow foundation would be if smaller width of foundation to depth ratio is more than say 2, you need piles. I am not considering frosted locations, which need to be dealt with special standards.