evaluation of potential for foundation settlement

[eng003]

For a non-geotech, what are the general methods & procedures & tests used by geotechs for analyzing potential for foundation settlement of both a new site and in the case of an existing structure the potential for further settlement? I assume geotech evaluate several things i.e. type of soils, bearing capacity, compaction, moisture content, and of course nature and loading of the foundations.

I deal with primarily sandy soils, is there any simple field test that can be used as an approximate gauge for evaluating potential for settlement? Could you make some rough correlation with evaluating bearing capacity by using an inexpensive field test like a penetrometer? At the very least I think you could use it as a relative in field test to identify areas of concern and areas of no concern right? I always recommend a geotech follow up with a boring and report but I often have the need to render and initial recommendation whether it is necessary to follow up by a geotech. Thanks!

 

[Ron]

You want a 30 word dissertation on geotechnical engineering? The process is too complex for that. I would suggest that you take a supplemental course in geotechnical engineering.

Would you want a geotechnical engineer to ask for a quick and dirty method to design a structure?

 

[eng003]

Ron, Re-reading my original post I realize how general of a question I posed. Are you by any chance the Ron I may be thinking of in the JAX area? If so I am familiar with your work and greatly respect your opinion and any input you can offer. Let me pose the following scenario, a prospective home buyer is concerned about cracks in the exterior cmu wall of a home. He consults a local engineer during his due dillegence period to evaluate these cracks. Does he hire a geotechnical, structural, or civil engineer? I would say all three...

A strucutral engineeer to evaluate the severity of the cmu wall cracking and determine if it may have been caused from foundation settlement and to propose a way to patch it: i.e. with grout, mastic, a flexible sealant, or a epoxy.

A geotechnical engineeer to do a soil boring to determine nature of the underlying soils and their potential for future foundation settlement

A civil engineer to evaluate the drainage around the house likely contributing to saturation and consolidation of the soil manifesting in the settlement of the foundations.

Apart from getting all the local experts in all these fields involved the local engineer experienced in structural & civil engineering takes the job, although not a geotech he reviews a geotech report at least once a week and is generally familar with the field and their testing techniques.

After visting the site the engineer determines the cracking is likely the result of primary setttlement of the foundation (20 yr+ old home) likely to have stopped. Apart from telling his client this is what he thinks but to be sure he should have a geotechnical engineer do a boring, what could he offer in the way of basic testing of the existing soils to get a rudementrary guage of their nature?

On a side note the engineer generaly sees geotech reports state settlement is expected to be 1" or less all the time which appears to be some blanketed statement copy and pasted... I greatly appreciate any advice you thank you.

Lets also presume the engineer has already ordered a penetrometer from Grainger...

 

[Ron]

eng003...my apologies if I seemed a bit snarky! We get a lot of questions here that seem to be "tell me everything I want to know about this or that"! Your clarification puts everything in a different light.

Yes, guilty. I'm in the Jax area. Thank you for the kind words.

As for your issue, I agree with your approach, as difficult as that is sometimes for single family dwellings.

I offer a few points for your consideration....

Unless you are dealing with significant subsidence, "after the fact" geotechnical investigations have to be carefully approached for small projects. There's usually not enough money available to do an adequate forensic geotechnical study, so the typical approach is to put down a couple of borings and rule out the obvious. Can you do that yourself? Yes, provided you have a reasonable understanding of geotechnical techniques and properties, and you know enough to get a more experienced geotech involved if you get in over your head.

The same is true of the civil. Unless there is errant drainage being dumped onto the lot from adjacent property, there's not much influence the drainage will have on the structure, except for the possibility of undermining a footing or foundation element, or the contribution of excessive moisture to wood rot or other deterioration. Most general civils don't deal with either of those conditions, so it's left up to you to know a bit about drainage, groundwater influences (rising/falling groundwater and its effect on the structure), hydraulic gradients and flow, etc.

As much as we see specialization in engineering disciplines and subdisciplines, there is still (and in my occasionally humble opinion will always be) a need for engineers to be cognizant and seek training/knowledge in areas not of their specialty. A "Jack of All Trades" so to speak. Keep in mind; however, that when you cross over into another related discipline, you are held to the same standard as those who might specialize in that area. So if you practice structural engineering most of the time, yet you need to interface some geotechnical engineering done by you, then be aware that you need to be as good at it as the average geotech!