UC testing on Split Spoon Samples 1

[Scotto]

Problem: Conflicting opinions

Question: ASTM D2166 specifies unconfined compression testing conducted on "undisturbed", remolded or compacted samples. It states nothing about testing on disturbed samples that might be collected through standard split-spoon sampling. I've heard several opinions however that this is standard practice.I am interested in understanding more the pros/cons of conducting UC testing (on cohesive samples, CL, CH, CL-ML, etc.) on samples collected via standard split spoon

Desired outcome: A broader perspective on what is considered standard (and acceptable practice) for conducting UC tests on disturbed samples

Any thoughts? Thank for the input.

 

[22222222]

Hi Scotto.

Standard split spoon samples you probably mean samples taken from SPT test. If so, this kind of samples you can use only to test GP of soil ( grain size distribution, water content, unit weight etc,...), but not for UC test.I have never heard of this testing on split spoon samples and probably nobody would accept this way.Use mazier sample or thin wall tube sampling to get "proper" samples.I did recently quite a few on cohesive soil what we may call extremely weak rock(mudstone/stiff clay).In your case I think CIU test is more appropriate, but depends what is your looking for. For soft clay try very easy Vane test to get undrain cohesion.

 

[BigH]

One of the learned Doctors in our company used to run UC tests on recovered SPT samples. His rationale was that (1) he knew it was disturbed, (2) the soil was generally of low sensitivity. Thus, he had a "lower bound" value that he could "play" with. Sure, it is better to do tests on undisturbed samples, but for stiff to very stiff clays and high end firm clays, it is, unless you are set up, sometimes more difficult to push the thin walled tubes into the ground.

Use with caution but some lower bound values might be of very good help at times.

Best regards

 

[Ron]

Scotto,
I doubt that even the lower bound premise is valid for UC testing of split spoon samples. In some instances, compaction occurs during the push up the tube, so the density of the split spoon sample could be greater than an undisturbed sample. In another scenario, the overburden pressure is removed thus the clay can "rebound", thus reducing the density. The difficulty is that you don't know with much certainty which case you have, thus making the data suspect. In short, you wouldn't know if you were at the "lower bound" or not.

 

[BigH]

Ron:

Point taken. However, if low sensitivity high end firm to very stiff as I suggested initially, the sample has OCR. I don't see how compaction of such soil would take place especially since compaction requires removal of water (incompressible)in very short term - certainly won't happen in short duration of taking the sample. With respect to release of overburden pressure, this occurs with undisturbed samples too unless you have devised system (for only highly critical projects) to maintain overburden pressures upon tube extraction from ground. If you are talking about expansion if sample is put without confinement into bag or jar - you can always insert sample into a small dia. pvc pipe and wax around it, eh? Of course, this is all out window for soft clays and sensitive clays.

Still, if one understands the various possibilities, I think that some use, in many situations, can be made of such results. (To advise further, I haven't done this in my practice over the years - yet - but I know those who have.)

Good comments Ron - I follow the various threads throughout. Stay in touch.

Best regards

 

[Focht3]

This is an excellent thread. Kudos to all.

I hate running UC tests on SPT samples. At one point in my career I even refused to use the results from such "flawed" tests. But I have had occasion to use them where other sampling techniques were not available or were unsuccessful in obtaining suitable samples. And I got high strengths and stiffnesses for a shaley clay (CH): S[sub]u[/sub] values in excess of 20 ksf (480 kPa) and initial slope moduli of over 500 ksf (24 MPa.) Way too hard to push with a Shelby tube!

I guess that I'm closer to BigH's position on this issue: use high quality sampling if at all possible, but test the SPT samples if nothing else is available.

 

[Scotto]

To all who have posted in response to my original thread thank you. This is the kind of dialogue that I'd hope to generate and what in my opinion needs to happen now and then.

Thanks again for the good discussion.

 

[pigdog]

Hello Scotto et al
If I understand, one condition that would justify the use of SPT samples is the firmness of ground. Has anyone used a Pitcher sampler? It is a sort of soil coring sampler. I found a picture and explanation here:

http://www.stearnsdrilling.com/Pitcher Sampler.htm

I have not used this sampler, because the assumption I have run into is that if sampling a soil requires coring, it is strong enough. (Not counting dessicated soils etc.)
roger

 

[erich]

Hey Pigdog,
I was recently on a job near Washington DC and we used a Pitcher sampler in very hard clays. I haven't seen the results of the tests yet, but the technique was solid. The only problem I saw with using it is that you would have to plan on it's use from the start of the boring. The cutting headis greater than standard H sized drilling equipment so 5 inch or larger casings would have to be used.
Erich

 

[dirtman]

Performing unconfined compression tests on remolded samples ignores an important point for some soils - the sensitivity issue. The definition of sensitivity is the ratio of the undisturbed qc strength to the disturbed qc strength. Sensitive clays lose a great deal of their strength when remolded, and without having both values, one would not be able to evaluate this possibility.

A more important point to me is the need to use the most appropriate field equipment to obtain data of interest. If the saturated unconfined compressive strength of a clay soil is of interest, you should be performing field vane shear tests, not SPT tests. Results of the field vane shear test can be used directly to estimate the undrained shear strength of clays (after correcting for plasticity)

 

[Focht3]

dirtman:

You are correct - to a point.

Remember that a field vane has a relatively low upper limit on S[sub]u[/sub]. (I don't remember what that limit is at present.) My discussion (and BigH's) centered on samples too hard for a field vane or Shelby tube - or DMT, CPT, SBP, etc. A Pitcher sampler would usually work, but is not locally available in large sections of the country. And importing one may be prohibitively expensive with respect to the project's real needs and budget.

Sensitivity is normally not an issue for these soils - UDW is often > 120 pcf and S[sub]u[/sub] > 10 ksf. You can't turn a field vane in these soils - partly because you can't insert it, at least not at depth. When I have had occasion to use SPT samples (which I can count on one hand and have digits remaining), even a remolded strength was more than needed. The final design hinged on other factors such as constructability or overall problem geometry.

Think of this another way: let's assume that the soil I previously referenced with S[sub]u[/sub] of over 20 ksf was at it's remolded strength and had a sensitivity of 2. Using the remolded strength, the lateral capacity calculations for a monopole communications tower near a major highway indicate that the 3 ft diameter base of the pole only required four feet of embedment. Good judgement - and previous experience - indicates that the poles' minimum embedment ought to be no less than about 15 to 20 feet. What practical benefit is achieved by sampling this site with a Pitcher barrel and measuring an S[sub]u[/sub] of 40 ksf?

I heartily endorse the main thrust of your post: to use the most appropriate equipment whenever possible. But I also think it's important to recognize that some projects cannot support this approach - and may not require it.

 

[BigH]

As Focht3 and I have pointed out - sensitivity is important and experienced engineers will recognize this. I agree wholeheartedly with Focht3 about not going overboard - don't use a yacht when a rowboat is all that is necessary!

Mention has been made of the pitcher sampler. In 25 years, dealing in very stiff to hard clays and clayey tills, I have never used one. Our firm back in early 60s had the rights to the foil sampler for very soft clays - anyone remember those??

Available equipment is important. In US and Canada - it is usually not a problem for obtaining non-everyday equip. But, presently I am here in eastern India - 'ell, they use motorized tripods for ALL investigations!!! I don't know of a single auger rig here in the Calcutta area - although there may be. Motorized tripods!! How many have used them?? Today, in fact, I took the construction site's split spoon to the founding level of a new (added) retaining wall necessary as we did not have enough room for full shoulder. How did we put down the "boring"? I took another weightlifter and he and I hoisted the 65kg hammer up the 30inches and dropped the darn thing on a jar rod that some poor labourer had to try to hold steady by holding the spoon/rod beneath - all this with an attached steel chain clanging about. Why? - we couldn't get a drill in there and we had/have worries about the retwall's footing on the edge (within 1.25m) of a canal slope (see thread on footings on slopes) - and the contractor was starting to pour his mud mat. I got blows per 6 inches of 1, 1, 1, 1, 1, 2, 3, 3, 4 from base of retwall footing. Sounds like we will need to go deeper to the firm clay and replace soft clay old fill with new granular fill. Still, I waxed up the "logs" of the firm clay - may do an unconfined since it is my only option other than correlations with Atterbergs - but even that, I have to send some 4 hours away to a lab that could do it. You see, all I need is 80kPa or so bearing - so if I prove firm clay (disturbed) has 40kPa, it, with judgment seems reasonable. We all need to understand - as we might hate it - that, there are times and situations where winging it is sometimes necessary.

This is a great thread - keep up the comments . . . and best regards.

 

[UdayRB]

All said and done one must use common sense and engineering judgement in determining if a particular testing procedure is providing useful data. In some soil types UC test can provide usable data to make soil strength judgements or at least correlate the SPT blows with the unconfined compressive strength. In some formations UC test on split tube samples would be too unreliable and should not be used.

 

[davecooper]

I have heard that the smaller the diameter the higher the strength will be. I have not researched this as it is not that important to me. I wouldn't suggest using anything under a 2.5 inch diameter for strength testing or 3 inch for consolidation testing unless a piston sampler is used. I have also heard that sampling can increase the density of a partially saturated sample. This of course could not happen with a fully saturated sample since water is incompressable and the speed of sampling would not give it enough time to drain. The wall thickness of the sampler would probably have a big influence whether the sample is disturbed or densified. All hearsay and guessing though.

Best Regards,

Dirtdoc1