Using a Statewide Soil Map in Lieu of a Site Specific Soil Test 4

[Ron247]

I keep running into People/Client's with the idea of using an existing Statewide Soil Map for either design or forensics at a specific site rather than paying for a site specific soil test. I have tried to explain to them that these maps are somewhat accurate over a very large area such as a 100 square miles but not that accurate over a smaller area such as 10 acres within the 100 square miles. The map cannot depict parameters such as thickness of questionable soils, depth of questionable soil from the surface, depth of groundwater and numerous other details I want to know that affect design or my rationale for why some issue is present on an existing structure.

I would like to know everyone's thoughts on this and would really like to know if there are an published papers on the subject. I have the Soil Survey Manual from USDA but it does not address this.

 

[WinelandV]

Ron,

It a job past, I helped perform a foundation design estimate for a power transmission line utilizing the USDA soil survey maps. I was new in my job, and thankfully had a very experienced geotechnical engineer guiding me. The foundations were going to be 3' - 5'Ø concrete shafts, and I needed to estimate (guess, ahem) if a specific pole location was going to be standard soil drilling, or if it would likely need some rock drilling. Using the soil code, and the estimated slope letter, we were able to come up with a decent guess for the hundred-some odd amount of foundations. This was all very very preliminary, so that the estimators could provide a bid quote. I'm 95% sure that had we got the job, we would have got a boring log at each pole.

Basically, there is good info to be had, but, as you pointed out, you can't know specifically what's going on below (iirc) 1 foot or so. I'd say the maps can get you started in a direction, but I wouldn't depend on them for final design.

Please note that is a "v" (as in Violin) not a "y".

 

[TigerGuy]

The USDA maps are only good for the surface 5 feet or so. What do these people think will happen when they need a 9-foot cut just to level the pad?

I am biased, but I always tell people that you don't want to try and save a few bucks in the site investigation and design. Very few understand the benefits of a detailed and high-quality geotechnical investigation and report. They only see that they can save $2,500 on their million dollar project.

 

[geotechguy1]

One reason is that nothing ever goes wrong, and civil and geotechnical engineers are extremely conservative to a fault, so it's difficult to justify the value of investigations. I've been involved in projects where millions are spent on investigations and fancy lab testing and in-situ testing and at the end with all the data the engineer goes 'Hmm, well we have this data showing end bearing of 25,000 kPa and skin friction of 200 kPa. But I am not comfortable with that so lets stick with 500 kPa end bearing and 50 kPa skin friction because that is the parameters I always use in these areas'.

 

[MTNClimber]

We have the highest liability and the smallest piece of the pie and now they want to get rid of us. So who takes on the liability? It won’t be the owner.

 

[phamENG]

I worked for a sign company for a while designing the poles, foundations, and connections. We did installations all over the country, and nobody was going to pay for a geotech report for a new monument sign in front of McDonalds. So I would use web soil survey maps to figure out the most likely kind of soil in the area and then used presumptive values. I left notes for the installers to call me if they encountered soft clay where I thought there would be dense sand.

Anything more substantial, though, and I wouldn't trust it for actual design.