Hand Probe

[kumaran856]

Is using a hand probe a good method to determine foundation bearing capacity? For MSE wall foundation, what would you recommend to be used to check/calculate bearing capacity?

 

[fattdad]

There may be a vocabulary problem here. MSE walls do not typically have a "foundation" per se. As such, there is typically no "bearing capacity" to check or calculate. Rather, MSE walls must be designed for internal, external and global stability. On the matter of global stability this is a slope stability problem, which in turn will trump any "bearing capacity" calculation that you may come up with.

Not trying to be difficult, however. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

mse wall designers around some parts like to specify a "bearing capacity" requirement. it includes the area under the facing block and entire reinforced zone. in my opinion, i think it's their way of trying to imply even more liability on someone else. i would love to forward you a set of plans and specs for these things being built around my part of the country to see what you think about the language. because of the apparent trend, a lot of testing firms are moving to doing the designs themselves...at least if they're going to get stuck with the liability, they will get compensated for their efforts and exposure.

probing essentially checks the surficial consistency. in my opinion, the probe rod is probably the best tool we can carry with us. we also perform hand auger borings with dcp during construction and soil test borings during the exploration phase. the wall designer should specify the testing criteria so that the geotech "understands" what is expected.

on another note, the wall designers (at least some) attempt to throw the stability analysis back on the geotech. i still maintain "what the heck are they designing if they use a cookbook approach selling the same design year after year and don't actually do any engineering to design the wall for the site soils instead of simply providing a wall design with the expectation that the owner or owner's rep is responsible for making the onsite soils satisfy the design". simply put, the wall designer should be responsible for contracting the geotech for these things so that the owner is not stuck taking the liability. then, if the designer's geotech wants to do all the design work with no fees, that's between the designer and geotech.

ok, i've worked in my soapbox speech so i'll not beat it to death.

 

[msquared48]

An old method I have used on the sly (without anyone else watching) to check subgrade capacity or degree of compaction on my own is to put 10 to 15# of pressure on a 1/2" diameter rebar section I carry with me in the field. If it penetrates 1/4" or less into the stratum, then I am satisfied. If otherwise, then I look harder.

Kinda seat of the pants engineering, but it works for me.

Mike McCann
MMC Engineering

 

[msucog]

msquared48, quite often simple is better (not always but often). but if it goes to court, it's harder to defend so it should be backed up with some amount of test data. and my probe rod has worn down to a point...but it's calibrated to me (although it is getting rather short these days...so it's hard to pull off the casual conversation while leaning on the probe rod without looking a little foolish). i do keep a nice new shiney one in my truck too as backup.

 

[BigH]

msucog - have you ever referred to BS-8006 titled "Code of Practice - Strengthened/Reinforced soils and other fills." The version I have is from 1995 (BS = British Standards). On page 56, they clearly indicate that "For design, a bearing pressure qr based upon a Meyerhof distribution may be assumed . . . The imposed bearing pressure qr shoiuld b ecompared to the ultimate bearing capacity of the foundation soils . . ." While globally, the reinforced wall would act as a gravity structure with the slope stability aspect needing to be checked (and fall outside the reinforced section), they imply that the "reinforced mass" must also meet the bearing capacity requirements. In practice - not likely to be critical compared to the overall global slope stability (deep seated basal mode). Just a point. Not sure if things have changed in the particular Code of Practice since the publication of my copy.

 

[oldestguy]

What do you need for "bearing capacity" for the wall itself? My guess would be in most cases 2,000 psf would be fine. Then look in your local building codes or one that may apply. Chances are it will not show that for 2,000 psf that you need much of any stiffness or density of earth. As I recall,one old code calls for 2,000 psf on mud. So try to relate the code general presumptive bearing capacity to your job site and the loadings from only the wall units. Of course going back into the hill, that earth load is something else again. However, it may be permitted to settle, as it is built up. This may also take the wall down a little with it. this is not a fancy structure and no one will see or be concerned with some settlement.

Then, using the rod probe technique, related to what you have experienced when you run field density tests and look for some degree of percentage of compaction on production of compacted fill, chances are you will find that probe is not too bad for a general idea of relative density or percentage compaction, etc.

On the thought as to where you may stand in court using this as an "engineer's tool", tied in with your years of experience, it would be difficult for someone to argue that I don't know what I am talking about using this method, when you look at the hundreds of cases where I've used it with no failures to make quick evaluations of soil conditions, followed by more sophisticated testing to back it up. It is a step above an umbrella that once was common.

 

[BigH]

My Big Red colleague does hit the point that settlement shouldn't normally be a problem - as I have indicated in other threads, I have been involved with RE Walls up to 11 m thick that settled over 1000 mm (over 40 inches) during construction without problems.
Still, we had a few failures of a 9m high wall sitting on Su = 20kPa clay some 6m thick. Rightly, this embankment fill (made of the RE Wall) might technically be called a slope stability problem/failure (using Bishop or other methods of analysis) clearly the 10m x 20 kN/m3 = 200kPa > 6 x 20kPa (ultimate bearing - approx) - and the wall did a tumbling in rotation - like the bearing capacity problem in Lambe and Whitman (Chapter 32 on Shallow Foundations with Undrained Conditions (Fig 32.1).

 

[msucog]

bigh, i have not put my eyes on that particular reference but will look it up. bearing capacity seldom controls much of anything here and i don't really see how it's applicable to anything other than the facing block. but, as a matter of checking the entire foundation zone, i don't necessarily argue otherwise because i do intend to treat the entire area the same (i'm not going to leave a big muddy spot under the reinforced zone). however, i do see wall designers cutting the FS down to 2 for fine grained soils except they want someone else to sign off on it. we typically recommend a higher FS for fine grained soils. settlement and slope stability are always the driving factors here. once you get off in to the argument of settlement for a 40' tall wall, total and differential will typically be high (12-18"+) even for "good" foundation soils with a shallow to moderately thick soil profile which often has very high consistency soils and/or shallow soil profiles nearby to make the differential even worse. you figure the settlement of a mass fill area, you have a fair amount at the facing, then much more as you head in to the reinforced zone...and it's even worse due to highly erratic consistencies and soil profile thicknesses. i've seen wall designers throw up the red flag due to the possibility of that much internal differential. i have no doubt the walls can withstand large settlements since i've seen it before without "failure" (although the facing did crack and bulge rather ugly). however, i've seen the things splayed out very ugly all over the place too.

don't construe my somewhat negative comments about mse walls or their designers as opposing the use of the things. i think they are an excellent choice in some situations. however, some (not all) of the designers here (which apparently is not the "norm" elsewhere) choose to take a contractual approach that transfers their liability to everyone else involved...but owners and even some geotechs unknowingly accept it because they don't see the potential implications. the more and more geotechs i've seen that get bit by lawsuits for these things, the more and more they reject the ridiculous language and assumptions put out there by the designers.

 

[escrowe]

The advantage to the 'rebar probe' method is the sheer number of 'tests' you can run in a short time. If your inspection is more sophisticated, but limited to a few discrete points in the foundation subgrade, you are more likely to miss the mass of peat that the contarctor overlooked near the east end...

My company started out providing geotechnical parameters for MSE walls, then evolved to providing plans, then just said to heck with it and started building the darned things.

I once saw an MSE wall failure attributed to tree growth at the top of the wall. We estimated a surcharge of about 75 tons of vegetative matter.

 

[Mickney]

To respond to the original question, the bearing capacity should be determined for the structure (building, wall, etc.) through a geotechnical investigation. This investigation will include soil test borings, CPT borings, laboratory testing and engineering analyses specific to the structure and geologic area.

Personally, I would not utilize a probe rod only to determine bearing capacity. I have used the probe rod in the field during site grading and construction, with experience and supplemental subsurface testing (hand auger borings, test pits, additional soil test borings, etc.) to complement the original geotechnical investigation.

Personally, most MSE wall plans I have seen do stipulate a bearing capacity. Now, when checking the bearing capacity using the traditional formulas, I use the reinforced length as the foundation width.