Where to learn how to interpret boring logs.

[KingPyrrhus]

I have been doing drilling observation for a few years and I have been trying to figure out how to interpret the boring logs I create and make recommendations for Geotechnical Reports.

Right now I am trying to figure out how to determine the allowable bearing capacity for shallow foundations. I got a boring log and converted the N values to N60 values. Then I was going to use Terzaghi equations for ultimate bearing capacity. The problem is I can't figure out what to use for densities or friction values off the boring logs. (My Foundation engineering book gets Relative Density from Kulhaway and Mayne, and Friction Angle from a DeMello guy) The soil is a SM with Rock Fragments. First, the relative density doesn't give me a density in pcf, only as a percent. Do I just interpolate? And can I use the relative density for a SM soil, or does it have to be a clean sand? What do I do if I have a clay or silt? I think I can get the friction angle, I think I just need densities to get overburden stresses.

I'm trying to self teach this to myself. I just don't know where to start. I have been on Google all day. I found some things from the US Navy, but didn't find it helpful. Does anyone know somewhere I should start? Like books, specs, manuals, whatever?

 

[Ron]

I don't mean to be flippant; however, you start with an engineering degree with a heavy emphasis in geotechnical engineering.

 

[KingPyrrhus]

I have an Engineering degree in which I took both Intro to Geotech and Foundation Engineering, and my E.I.T., and I have the P.E. coming up soon. But I'm stuck right now as an Engineering Technician. And my office doesn't have time to train me, so I am trying to learn on my own. I'm trying to find out where to find what to do with the data from the boring logs.

Can anyone point me in the right direction?

 

[motorbiketocrank]

I applaud your desire to learn. First, start by carefully reading and understanding a good basic foundation text. I don't know what you're currently reading but I'd recommend those by Coduto, Das (and there are others). Taking a good university course to help while doing this would go a long way.

The textbook learning above is only the start. Frankly, you're not going to go far without being trained by and working with an experienced geotechnical engineer. Sounds like that's a problem in your current job so I'd recommend either pushing the training issue within your current job or looking for another. The textbook learning will help you to learn how to "solve" a problem with known parameters. But the "art" of geotechnical engineering is knowing what parameters to use in the first place (ex. when you have 10 borings all showing somewhat different conditions, what parameters do you use).

To more directly answer your question: In foundation design you usually consider the material as either a cohesionless or cohesive soil. A cohesionless soil (relatively clean sand/gravel) will be governed by friction (phi angle and associated Nq and Ngamma in the bearing capacity equation which you get from correlations with the N60 values). A cohesive soil will be analyzed using cohesion (undrained shear strength) in the bearing capacity equation. The cohesion would usually be acquired by pocket penetrometer readings, unconfined compression tests, etc. And the bearing capacity is only half of the story.....you then need to analyze for settlement (usually using correlations with modulus in cohesionless soils and consolidation in cohesive soils). And don't forget that allowable bearing capacity is not just based on the soil conditions......it also depends on the magnitude of the loads (i.e. the allowable bearing capacity wouldn't necessarily be the same for a building with 100 kip column loads and for one with 500 kip column loads).

 

[Ron]

Unfortunately you are employed by a firm that values other things over advancement, training and mentoring of young engineers. This is unfortunate and puts you at a distinct disadvantage.

You need a mentor...someone who can sit with you and go over the boring logs and explain their meaning and nuances. Maybe you can entice one of the senior level engineers in your firm to do so. I've always considered this a priority and have done it numerous times with younger engineers. It is an obligation that Senior level engineers must fulfill (in my not so humble opinion). If your senior level engineers are not willing to do so...shame on them. If your company does not support that...shame on the company. Move on and find a company that will provide some guidance and mentorship.

Good luck.

 

[Hanks864]

All you are asking are things I learned during my bachelor degree in Civil Eng. I don't know what to say to not sound offensive. I am a geotechnical engineer now but this all is some pretty basic stuff you learn in school.

Of course you should learn a lot from the senior geotech engineers at your office, and you should be able to ask them any question that comes to your mind, this is the time to ask and reinforce knowledge. I can relate to this somehow since I started working at a geotechnical firm full of very competitive colleagues who didn't like to share and sit to discuss anything. Fortunately I always had my boss, the manager of the company who directed me through difficult projects until I gained enough experience.

 

[FixedEarth]

"Soils and Foundations" by Lieu and Evett & "Geotechnical Engineering" by Coduto are great places to start. Also look for a 3 to 5 day type refresher courses by University Extensions.

A general rule of thumb is to use 200 psf x N as your allowable bearing capacity when you are not close to a descending slope. So if you have N = 12, allowable bearing capacity is 2,400 psf. This is just to check your calculations. Most often, allowable bearing capacity is not just ultimate qu divided by factor of safety equal to 3. Settlement, location of nearby footings, future excavations and a host of other loadings need to be considered.

Try to find a senior engineer in your City who will mentor you. Go to the local ASCE Geotechnical section monthly meetings. Good luck.

If you happen to be in the San Diego area next month, come to my class:

https://www.halfmoonseminars.com/continuing_education_seminar.php?seminar_location_id=2564