Extrapolation of Geotech Study Results

[CANeng11]

I have a project to install a truck scale and foundation in Southern Alberta. We have a geotech study at the site that was completed for a different project, but the area that the study was done is approximately 150m away from where the new truck scale will be situated. Is it reasonable to use the results from this geotech study, or is this too far away to extrapolate the results?

 

[oldestguy]

Sometimes yes and sometimes no. If you are familiar with the geology of he area and that geology has generally the same condition from one place to the other, maybe yes. If it is known for abrupt changes from one area to the other, maybe not. Also, if the structure will impose a light bearing pressure or is quite rigid, the"yes" may be considered. However, your experience in the area should be well involved in any decision.

Reminds me of a question I frequently would get through the years. "How many borings needed?" My routine answer was "At least one". But here "maybe more".

 

[cvg]

It might be reasonable for a qualified geotechnical engineer to use that data to analyze your site. But if you are going to rely on just the other study, and you aren't a qualified geologist or geotechnical engineer, able to do the "extrapolation", then definitely no. as the engineer of record, you will take on all liability for not having a geotechnical report for your site.

 

[fattdad]

it's just a matter of risk. Consider using the data that's 400 ft away (more than a football field) and performing a few hand auger borings or backhoe-excavated test pits. Just to confirm that the adjacent data is reasonable for your new site.

In my practice, I'll stop a consultant if they rely on extrapolation. Then again, I work for a public agency and don't want to adopt the problems of my consultant.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Ron]

If you carefully read the geotech report you have, there is probably a prohibition or warning about extrapolating the results. There are two reasons most geotechs put such caveats and warnings in their reports. First is that they likely know the soil conditions at the site and surrounding areas better than others, and thus can make an assessment as to whether or not extrapolation can be effectively and safely done. The second is that of continuity of services. While that might be slightly self-serving, they still have the upper hand of information and its interpretation at that site.

Consult the original geotechnical engineer!

 

[bb29510]

I would say no, even if the material on top is the same, doesnt mean the material fifty feet down is. There could be a layer of muck, fat clay even a sand layer. water table could be different.

master ICC inspector, AWS CWI

 

[BigH]

With respect, bb29510, the material 50 ft down would/should have no effect on a truck scale. One always needs to know the zone of influence of the foundation.

I've compared soil reports (mine and one the company did 20 years earlier) in an area where we also had several other reports in the same geological setting. Given the same geological setting (such as the Peel Till Plain in Ontario) one can be pretty safe in having consistent designs - 150 ft apart is not all that far in such a setting - one would need to be cognizant, though, of old stream beds that might have been infilled - you might check out old agricultural maps as they can get pretty detailed. Another problem, even if you have investigated the site, is whether there are old utility lines or former house basements that have been infilled - the former should be covered in your report (if you are writing one). I used former reports of our company on a major paper mill site in Northern Ontario - no probs.

The big thing is, an none of us can help you out here as we aren't "local", how consistent is the geological setting in your area of the project. Only you can have a gut or reasoned feeling as to the uniformity in the area. (I did a location in southern Ontario that for a small development, had three different founding conditions - where only one was expected).

As an aside, years ago, I read some papers in an Alberta geo conference or workshop where a residential development ran into problems in that they encountered a dinosaur latrine!!! So, in Alberta, you might have to consider this!!!

 

[JedClampett]

BigH, by the way he said 150m, not feet. Don't you hate that?
Call the Geotechnical Engineer that did the existing report and ask. If you offer to pay him to write a sealed letter, he might be agreeable to that. If he/she says no, get more borings. This is very cheap insurance against a very big problem.
We work on water and wastewater treatment plants. These are large area facilities and are constantly being expanded and revised. Every area in the plant has been analyzed at one time or another. I use existing reports a lot, but I always feel a little dirty about doing it. I sometimes ask the PM if we can get a letter from the original Geotechnical Engineer verifying using the report for another project. It's usually about $500 or so. Of course borings plus a full report costs more.

 

[BigH]

Actually JedClampett - I work all the time in metres . . . I had to change when I got out of school - Canada was just changing from feet to metres. If I started a job in feet, I pretty well had to stay in feet - if in SI, then metres was okay - but to change mid-stream (like we had to in the early days of Syncrude) was not easy. Now, after so many years overseas, I pretty well think in SI . . . as for your Water plant - we pretty well peppered one in Ontario for years but always drilled a few extra holes (but with the knowledge of what was generally there) - in one respect it was good as the shale rock took a nose-dive within a 10 metres (oops, 30 ft) from another borehole.

 

[Ron]

I work in coastal plains soils where within 30 feet (oops, 10M..) ...we can get a siqnificant change in soil parameters, so we typically warn against extrapolation!