Project Geotechnical Report

[SteelPE]

I would post this in the Geotechnical forum., but that forum isn't as active as this one.....

I have a project that I am designing a foundation for. The project is a single story industrial/manufacturing building that will be constructed using a metal building. I received the geotechnical report this morning and was happy that they recommended a shallow type foundation system with an allowable bearing pressure of 3ksf..... Then I took a look at the borings they did under the building footprint. I know very little about reading these tables but I do know that having low numbers in the "blows per 6"" category is not a good sign..... and having no numbers is even worse.

The boring log indicates some very loose sands 10'-20' below the building (mostly 1-2 blows per 6" and one instance of the probe going sinking into the ground under its own weight).

I can't recall ever seeing similar numbers without some sort of soil remediation recommended by the geotechnical engineer. Am I right to question this report?

 

[JAE]

My first recourse is to call the geotechnical engineer and discuss.

 

[phamENG]

I would certainly call and discuss, but I'm guessing they're just not concerned with long term consolidations of those soils. Here in swamp country where we have all clay, that would be a big problem. I don't think granular soils have the same issues, especially at that depth, but it's been a while since I took my geotech classes...

 

[JStephen]

I'd call and ask. They may have overlooked something. It may be far enough down that it's a non-issue.
I can visualize hitting little pockets down there vs hitting an entire weak layer, and that might make a difference. You'd think if it was an entire weak layer, the surface soils would all just settle by that amount.
I have seen soils reports where they had loose material and recommended vibratory compaction, preloading with fill, etc to remedy it.

 

[CarlB]

That soil type (silty sand), very low blow counts, below the water table, present very high chance of liquefication during a seismic event, leading to settlement, sand boils, etc.
Unless site is in a very low seismic region.

 

[SteelPE]

Thank you for your responses. In 25+ years this is the first report that I have questioned. I have a call into the architect to set up a meeting between all parties. I don't mind talking with the geotech directly, I just want to make sure other people are aware of the questions should problems ever arise.

 

[dik]

and frost heave if an issue...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Rabbit12]

I'm going a bit off memory here, but I think the stress distribution into a soil is a bulb kind of shape. If your column loads are low enough the actual stress imparted into that soft zone could be relatively small.

 

[Tomfh]

If definite query it. We are fixing a 25 year old warehouse at present, which was likewise built on soft ground (based on tests we just did), and which has settled 100mm along one edge.

 

[bones206]

See my recent post with a metal building that settled 18 inches over 40 years and is still sinking...

But that case was a building founded on 100 feet of soft clay. With sand, I think most of the settlement would be immediate from initial dead load. Still prudent to check with the geotech to understand the basis of the recommendations.

Regarding liquefaction, I've had some recent projects in MA where the geotech has flagged the soil as susceptible to liquefaction and included design requirements for dealing with that seismic scenario. So it is definitely something to pay attention to, even in New England.