Increasing in Bearing Capacity with Depth and Width (Cookies Cutter Geotech Report Recommendations)

[Geotech_Pavement]

Hello All

I am trying to become more proficient in writing geotechnical reports and providing recommendations but sometimes get frustrated by things that just don't make logical sense in the industry.

I am from Southern California and all reports I have seen (from other companies) have a sentence about increase of allowable soils bearing capacity with depth and/or width. I am not sure how they calculate these percents or number increases. It seems made up to me (prove me wrong). Is this normal verbage in all geotechnical reports in other parts of the USA ? Increase in 20% per ft of depth and 10% per ft of width seems the easiest for me to place in every report lol. I will list a few examples from other reports below:

Example Company 1: "May be increased by 100 psf for every footing width and 400 psf for every footing depth to a maximum allowable bearing pressure of 2,500 psf."

Example Company 2: "An allowable soil bearing capacity of 1,500 pounds per square foot may be utilized for design of isolated 24-inch-square footings founded at a minimum depth of 12 inches below the lowest adjacent final grade for pad footings that are not a part of the slab system and are used for support of such features as roof overhang, second-story decks, patio covers, etc. This value may be increased by 20 percent for each additional foot of depth and by 10 percent for each additional foot of width, to a maximum value of 2,500 pounds per square foot."

Example Company 3: " The bearing values may be increased by 10 percent for each additional foot of embedment and/or width, to a maximum value of 2500 psf."

Example Company 4" "The allowable bearing pressure may be increased by 500 psf for every additional foot of width and by 800 for every additional foot of embedment depth up to a maximum of 4,000 psf."

I appreciate your input.

 

[geotechguy1]

The real reason is that a large number of geotechnical engineers, especially at smaller firms, are essentially useful idiots. What you'll probably find is that presenting the numbers those way is convenient for the structural engineers design and also transfers liability away from the structural engineer, and structural engineers / engineering firms simply refer to or recommend using the geotechs that help them this way.

Unfortunately that's the sad state of the industry in alot of places.

 

[DanielSgeo]

If you look at Terzaghis bearing cap formula it’s pretty self evident that bearing cap is a function of width and depth of footing…

 

[MTNClimber]

Is this normal verbage in all geotechnical reports in other parts of the USA ?

Not in the areas I have practiced (southern New England and Colorado).

We provide a net allowable bearing pressure for footings. We provide minimum footing dimensions and depth. The net allowable bearing pressure for footings less than three feet wide is reduced proportionately.

 

[Greenalleycat]

Here in NZ most geotechnical reports are written with reference to a paper by Stockwell in 1971 (lol literally the most up to date info we have...)
This has simplified formula for changing bearing pressures depending on footing width and depth

Generally, depth always increases the capacity
However, the width only increases the capacity of isolated footings (pads) in sand
Otherwise, wider footings in sand w adjacent slabs, silts, or clays decrease the capacity