Geotechnical Report Interpretation

[tclat]

Hi,

I have an geotechnical report that I'm having difficulty interpreting. The Geotechnical Engineer gives a table titled "Allowable Design Parameters". In this table he has a column "Bearing Pressure = 44,179 psf". Further on in the report there is a straight line graph labeled "Allowable bearing vs Footing Size". The allowable bearing capacity varies from 4000psf for a 2' wide footing up to 10700psf for a 10' wide footing. The material is limestone. Can anyone shed some light? I have asked the geotechnical engineer if the table is meant to be ultimate parameters but he has confirmed it is allowable. Other parameters in the table are

"skin friction = 2813psf"
"passive pressure = 5900psf
"Sliding friction coefficient = 0.35

I generally get reports which give a bearing capacity irregardless of footing size. I will likely use the graph for design since it gives lower values than the table.

Thanks

 

[BigH]

Sorry tclat - I laugh out loud (my wife asking me what? what?) when I actually see numbers reported as 2813 . . . I ask myself "Who is zooming whom?"

I am wondering why you have a bearing pressure of 44,179 psf (44.2ksf) and then the graph is from 4 ksf to 10.7 ksf. Something doesn't jive. You also indicate the material is limestone (rock???) if this is so, these seem like very low bearing pressures. Something is amiss, in my view.

When dealing with clays, we typically gave a graph of footing size (x axis) vs allowable bearing capacity (based on SF = 3) and then would have a superimposed curve on it of allowable bearing pressure (for 25 mm settlement) (y axis). We then, at times when required, would do something similar for footing interactions. For many jobs this isn't absolutely necessary because one isn't going to do very small adjustments which is why a "single" number is many times given.

 

[Ron]

Agree with BigH...it doesn't make sense. It reminds me of one of my favorite little sayings....In analysis, we measure with a micrometer. In design, we mark it with a crayon. In construction, we cut it with an axe.

I'm happy if I hit the actual bearing capacity within a few hundred psf. As for skin friction and other calculable parameters, we have to consider that there are many loosely held variables and assumptions present in each of our proclamations. When we don't understand that, we are often greatly disappointed!

 

[oldestguy]

Is this from a recent grad? Sounds like there is need for his boss to be involved. Maybe hand the logs to another firm for a practical answer.

 

[tclat]

Hi all,

Thanks for the comments. I agree with the comments about the precision of the numbers. What I don't understand is how do you start with an allowable bearing capacity of 44,179psf and end up with a allowable bearing pressure, for a 2 foot wide footing, of a tenth of this overall figure. What is the 44ksf meant to represent? I thought it was ultimate at first. What about the other "allowable" design parameters? At this stage I don't mind designing for the for the lower allowable pressures even if I think it is conservative. Not sure if I should trust the other figures.

The building footings at being cast on top of the limestone rock. I would like to key atleast the ends of the strip footings into the rock to help with sliding for my shearwalls. I asked the geotech to give me the shear strength of the rock and his recommendations in terms of the depth of the key and over what area I should apply the shear strength to resolve my sliding issues. He has given me skin friction and passive pressure. For keys in rock I would have thought that the shear strength of the rock would be more relevant than passive pressures. Any advice?

 

[miningman]

Cant comment as to the specifics here, but if inexperienced personnel are suspected, be carefull. In my experience the top 30 feet or so of EVERY rock exposure I've ever worked with is, to use a technical mining term, " rotten rock" If your techie gives you numbers derived from say a 100 foot diamond drill hole , you might end up with some very optimistic values.

 

[oldestguy]

No doubt some formula is being used incorrectly for the bearing values. For rock support, isn't there a local building code for "allowable" values. That may be all you need.

 

[hokie66]

Given those numbers, I wouldn't even try to interpret the report, as the author obviously has just plucked the numbers out of some program without understanding anything he has written.

 

[fattdad]

this is bad for all the reasons cited above.

f-d

¡papá gordo ain’t no madre flaca!

 

[darthsoilsguy2]

weird. i almost wonder if the units are suppose to be pounds and not psf... but that still doesn't make sense.

The statement "I generally get reports which give a bearing capacity irregardless of footing size" merits a little followup tip. The size of the footing the structural uses does matter and may be prohibited in the geotech report. In a typical geotech report the GE will state their assumptions for column loads to develop the bearing pressure. A common source of confusion between geotech and structural is that in a lot of conditions the design is governed by settlement. as the footing gets bigger, the influence zone for that settlement increases and the allowable bearing capacity can decrease. If the Structural Egr has column loads that will exceed this stated design assumption, then the theoretical footing the geotech developed a bearing pressure for is not adequate for the design. The answer is not just simply increasing the footing size using the original design bearing pressure... structural should make the geotech aware of this column load that exceeds design assumptions.... in case the allowable bearing pressure might be subject to decrease. Also, the geotech will likely call out a minimum footing size due to punching failure, regardless of how small the load.