pq test

[lovethecold]

Recently a large multinational construction firm requested that we do a soil investigation for a project. What we find odd, is that they "required" soil capacity to be determined using a pocket penetrometer. How common is this elsewhere? My experience in using the pq is that it is not terribly reliable. It may be ok for the field as a reference to a complete soil report, but not for providing recommendations.

What are your thoughts, particularly in relation to its use in giving recommendations elsewhere.

 

[dgillette]

Run. Run like hell. Then, from a safe distance, tell the contractor that the pocket pen is simply not able to provide allowable bearing pressure, regardless of whether it is governed by settlement or by bearing capacity (which is much less likely), but that you would be happy to submit a proposal using whatever methods you deem appropriate. (This assumes you need the work.)

If you propose on it as was requested, with bearing pressures determined using a pocket pen, it could come back and haunt you.

The only case I can think of where it would be of much value is if you needed a rough estimate of short-term bearing capacity for a fndn on clay (so the pocket pen can give you an estimate of undrained shear strength you could apply in spread-footing bearing capacity calculation with phi=0).

Best regards,
DRG

 

[msucog]

ha ha ha...that's funny. dgillette offered the best advice i could imagine other than asking who the heck wrote that spec.

 

[DRC1]

It really depends on what is trying to be done. Would I design a forty story building foundation on pecket pens? Probbaly not, but it does beat out SPT N values and I have seen a lot of big stuff on clay designed soley on SPT N values. If it is a preliminary concept study, or for moderate depth excavations it may be reasonable. For final design I agree there should be more. You may want to point out that for probably very little more money unconfined compression could be done to give a better number, although for somewhat more money, lab testing will give you information you can actually use with confidence. Explain to the owner there is a difference between having numbers and numbers you can rely on.

 

[fattdad]

In a field exploration program, I would consider a project requirement for pocket penetrometer data on cohesive soils an appropriate adjunct to SPT values. If the resulting data requires followup testing (i.e., CUTXC, UU, 1-d consol, etc.) then sobeit. I would not necessicarily (sp) "Run Like Hell". Then again, I do hate project requirements that don't make sense. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[lovethecold]

The project in question is for a lift station approximately 40 feet into fat clay. Apparently we have done work for these folks in the past and that historically they are pretty good about wanting work done properly. The comment my supervisor made was that geotechnical engineering has been going downhill over the years. To my knowledge, no other firm in our area performs uncofined compressive strenghs, only pq's to provide bearing capacity recommendations. And, apparently there is a geotechnical engineer south of us (outside our service area) who will do a complete soil report using only augered samples. Now, I know I have a lot to learn, but that to me is ridiculous.

Judging by some of the questions in other posts, and this information, I wonder how things are in other areas of the country, and if this is the general direction of the industry. This certainly isn't the case for all architects, contractors, and engineers, as some definately know what it is they are doing and needing in regards to testing.

 

[dgillette]

40 feet down, presumably requiring cut slopes in the clay? So its more like an exc stability question than a bearing-capacity or settlement question? For that purpose, it is POSSIBLE to use Su from pocket pen, but I'd MUCH rather have field VST or CPT, with lab vane or Torvane being a fallback (as well as a generous FS).

(Still, I regard the pocket pen as more of an index property to use in logging samples in the field, or at the ends of Shelby tubes when opening undisturbed samples, not something whose results I would use to design a cut slope of any consequence.)

It isn't necessarily ridiculous to do a soils report using only auger samples. All depends on the situation. If the guy knows the local soils well, that may be all he needs for the purpose at hand, if it's light residential, a cut slope in sand and gravel, or something like that, where knowing the material descriptions could be sufficient. For a heavily loaded structure or a steep cut in clay (where Su is critical), that's not likely to be adequate.

 

[fattdad]

For a 40 ft cut into fat clay, I'd agree the issues relate more to the conditons of construction. Bidding contractors should be provided with undrained shear strength and direct shear data so they have the data needed to design the excavation bracing or the construction slope.

I don't see much of a bearing capacity issue. I do see a lateral earth pressure issue, which should include a piezometer in the fat clay, just to see if you can get hydrostatic conditions from partings in the clay. I also see an issue on backfill materials. Just how will the excavated fat clay perform during backfill operations (i.e., will the contract requirements specify 90 to 95 percent compaction when the excavated soils are slow to dry and saturated)?

The field program needs to consider piezometers, Shelby tube sampling, direct shear, UUTXC and likey a 1-d consolidation test (just to look at the extent of preconsolidation). That said, I'd still require the field exploration to include pocket penetrometer tests on all SPT test samples (and Shelby tubes also). Just for reference, the test program that I'd suggest would likley be less than $2,000.00 in many of our local markets. Where our profession is going downhill is engineers are having a hard time marketing the necessary data (and shade tree geotechnical firms are willing to under price what's needed).

f-d

¡papá gordo ain’t no madre flaca!

 

[lovethecold]

dgillette and fattdad, it makes perfect sense what you are saying. I guess the guys who I work for scoff at the idea of doing pq's for anything other than field reference to a prepared report, and I don't necessarily disagree. But, as you indicated, perhaps this is one job where qu's are not really required, there certainly won't be an issue with bearing capacity. I am not working on the project, but do know that numerous tubes where collected, and a complete battery of tests are being run. I don't think a piezometer was installed though, but water table here tends to be just a few feet below grade and oversaturated anyway. The CH tends to have numerous laminations of silt, and would show a hydrostatic pressure within a few days, or a week at the most.

Question, is the UUTX an abreviation for the triaxel test? Thanks.

 

[fattdad]

UUTXC (unconsolidated, undrained triaxial compression test). You'd typically take a Shelby tube sample from the ground, place it into the triaxial test chamber and replicate the confining stress from the sample depth without allowing drainage (undrained). Then shear it.

If there is a battery of testing on "undisturbed" samples and these data were used in the overall engineering analyses, then I'd also want to use a pocket penetrometer during construction to document (qualitativelly) that the soils in the field relate to the soils from the design field exploration.

Sounds like an interesting project.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

UUTXC - hadn't seen this abbreviation before.

 

[Stillerfan]

Hey fattdad, what do you mean by "shadetree geotechnical firm"? I assume derogatory, however, I don't like to assume anything.

 

[howardoark]

I do pocket pens on everything and have seen values six times lab shear strengths. So, I'd kind of want to use a factor of safety of 16 (or 8 times whatever you would normally use). If the extra building foundation cost ends up being 100 times more than the owner saved on the geotechnical investigation, I don't see that as my fault.

I pointed out to an architect the other day that the reason his building cost so much wasn't because the structural engineer had been conservative (for some reason they blamed the poor structural guy) it was because the geotechnical engineer had given them allowable bearing capacity of 2500 psf for clean sands with SPT blow counts of 80. But I think they went back to that same geotechnical firm for their next building because he was $5,000 cheaper than me.

On UUTXC, my practice has been to consolidate the samples (drained) to slightly more than the in-situ stress and then shear them undrained. I figure the strength gain from the slight extra consolidation won't be significant and the consolidation should remove some of the effects of sample disturbance. I'd be happy to be talked out of that though.

 

[moe333]

I would call that a CUTXC since you are consolidating it. Consolidating beyond the insitu stress will give unconservative strengths. The vertical consolidation may not be that significant, but since you are probably consolidating isotropically, your horizontal consolidation will be much higher than the insitu was.

 

[fattdad]

Yes, I meant "shadetree geotechnical firm" in a derogatory sense. There are alot of companies that will price low for design work just to get the construction testing.

CUTXC is a different test, which is first consolidated. UUTXC takes the Shelby tube sample and replicates the in-situ stress with out consolidation, then shears without drainage. You'll get the undrained shear strength at that confining stress.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

Whatever happened to the nomenclature CD, CIU, CAU (each with and without "bars" over the top), UU, etc?? (See Lamb and Whitman).

 

[fattdad]

Yeah, I'm O.K. with those. I just added TXC at the end of the UU out of some sort of habit, I guess. . .

f-d

¡papá gordo ain’t no madre flaca!