Standard Practice- Subsurface Exploration 3

[Mazzman]

I received a copy of a subsurface exploration report prepared a licensed engineer with geotechnical experience. The project is a 8000-square-foot, wood-frame commercial building, and the work was conducted for a local developer.

The report states that the field sampling was done using a Giddings ATV rig. Apparently, this is a small pull-behind trailer with a power auger mounted on it (see attached photo from Giddings website). I'm told the engineer runs solid stem augers and conducts the field work himself.

No Standard Penetration Tests are conducted; however, the report of geotechnical exploration reports “Estimated N-Value” on the boring logs.

The logs in the report include “Cohesive Strength (psf)” values. The engineer told me he runs the augers down, and then lifts the augers exposing some soil within the auger flights. He then uses a hand-held penetrometer on the soil exposed on the augers to measure strength. He says the value he reports is equivalent to undrained shear strength.

I asked how he gets N-values if he does not run the SPT. He said the “Estimated N-Value” reported on the boring logs is based on some correlation he has between hand-held penetrometer value and Standard Penetration Test value.

The logs include USCS classification, but there is no indication in the report that soil classification tests, or any tests, for that matter, are conducted.

There are statements in the report that do not seem to be supported by data:

The report states “Penetrometer tests were performed on representative soil samples to evaluate soil shear strength, compressibility and consistency in-situ”. Based on the description of the field testing, it does not appear the sampling was conducted in-situ. Also, hand held penetrometers are not usually considered useful to predict compressibility.

The report says “This report…has been prepared in accordance with generally accepted geotechnical engineering practices.”

My question to you: Are the practices I've described here consistent with local geotechnical engineering practice for a similar project in your area?.

 

[Patgeotech]

As a geotechnical engineer who thrives in practical application, I often come across 'below par' geotechnical investigations that are often done by one-man bands. Their estimates are always cheap and never include all the infomation (geologically) that it should. Needless to say we often follow up doing the remediation work when it goes wrong. I do not know what the other geotech professionals views are, but I have found that these so-called geotechnically experienced people (as mentioned by Mazzman) have no geotechnical qualification/training and underestimate the unpredictable nature one can find in the ground.

It is worrying when one sees the comments/opinions made by structural engineers and compares them against geotechnical engineers. I think that the chains of communication between structural and geotechnical engineers are important and one can never get enough information from the structural guys and their requirements. The cost of getting a proper geotechnical investigation is a mere fraction compared to the costs of should it go wrong. Something that most clients do not consider - the cheaper geotech always wins but sometimes it is not always the best advice one gets!

As a competent and professional person, I would certainly get another geotech opinion - it is not the question of what may happen to this relatively light structure, but what worries me is the situation on his next job where he does the geotechnical investigation for a multi-storey building?

You mentioned that he has geotech experience - what level may I ask? Using a mere pocket penetrometer to classify augered (disturbed) soils I find worrying. The thought comes to mind "one gets what one pays for".