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Undisturbed samples

Undisturbed samples

Undisturbed samples


Is any good way to take undisturbed samples for cohesionless type soils? For example, SM soils for triaxial or unconfined compression tests.

Thanks for your help.

RE: Undisturbed samples

rusty shelby tubes!

RE: Undisturbed samples

SPT N-values are around 10...not sure if I can push a shelby tube thru that soil...will try...

RE: Undisturbed samples

I've heard of freezing the ground and then obtaining the material by coring . . . I am not sure you would be able to do any unconfined compression testing - unless it was done in the triaxial cell and then it prove to be difficult to maintain the shape while handling and while the cell is being filled. Have you thought of doing direct shear for strength parameter?

RE: Undisturbed samples

Yes, I also heard about freeze sampling but it is not common here. For shear strength tests we normally do unconfined compression test or triaxial. I have to check with the lab if they can do DSS. But even for DSS, I will need undisturbed samples, so the issue is how to obtain the samples...

RE: Undisturbed samples

if the sandy material is heterogeneous without striations, layerings, partings, etc., you would only need to know the in situ unit weight and then you could reconstitute the sample in the direct shear to achieve that unit weight and "gradation". if the sample has partings and that, then it would be problematic as the plane of shear could be along a parting/layer in situ.

RE: Undisturbed samples

You can run a field shear strength test if you have the gear. Start with an undisturbed mound. I've used a 12"x 12" open steel box 3" high, with a roughened plate on top inside, filling in all gaps with compacted uniform sand. Plaster of Paris has been used also. Load on top with a proving ring and jack against a resistance (such as a vehicle), push sideways with a hand powered hydraulic cylinder, calibrated. Commercial auto body repair rigs can be used. One set up may do it for three runs, but otherwise use three set-ups. In excavations a back-hoe with bucket resting on a strut nearby works well for the vertical load.

This also works for weak stuff, such as waste bark or flyash, but much lighter loadings and load measuring equipment.

RE: Undisturbed samples

OG, thanks for the advice. Do you have a photo or sketch so I can visualize that better? Thanks again.

RE: Undisturbed samples

OG here.

Sorry, but the equipment is gone, some even scrapped as I moved from plenty of room country lot with shop to a city lot and standard house. One item of importance is with real strong ground, you need a pretty hefty normal load. So I made a proving ring from the outer race of a very large ball bearing that came from the junk box of a large construction equipment shop. It was roughly 8" OD with the "band" being about 2.5" x 3/4". A dial gage is fitted inside. The thing was calibrated in a compression testing machine. The loading was applied vertically through a screw jack, but a hydraulic jack cam be used. The box was made from steel angles with galvanized 3" legs, salvaged from a scrapped windmill. Also the horizontal strain was measured with dial gases suitable for 2" throw. You can buy kits for the horizontal loading hydraulic cylinder in either 10 or 20 ton capacity. Item 15616 at Northern Tool (Porta Power). Add a pressure gage and calibrate it in the testing machine also. In an excavation that pushes against a steel plate or plank against the nearby earth. You gather from this that I was a guy that made a lot of his own test gear. At 87 now, it's tough not to even have a welder here for simple fixes.

If you try to jack against a vehicle on rubber tires, it takes a lot of jack "throw" to get the vehicle up off the tires. I learned the hard way once with two scrapers back to back as the vertical reaction. The back-hoe bucket resting on a support (so it does not move) works great. Hopefully the operator does not fall asleep while you make use of his machine.

RE: Undisturbed samples

Okyru, in sands there is really not much choice.
Unless you are able to arrange a field contraption like OG's, or to freeze the soil (a very expensive and Tricky practice, as thawing it properly is also critical), the only way is as per BigH's suggestions, to reconstitute the sample. As far as I know, the comparisons between reconstituted sample and undisturbed samples (with frozen sands I believe) are pretty encouraging.


RE: Undisturbed samples

Thanks for your replies. I am still trying to figure out how OG's testing apparatus looks like...

RE: Undisturbed samples

Ok, here goes with a rough sketch. The back-hoe bucket vertical reaction is not shown, nor a post needed to support it and I forgot to show a pressure gauge on the horizontal hydraulic pump that powers the horizontal piston. Also, I'd do the measurement of horizontal movement with a dial gauge mounted back by the horizontal cylinder, in case of excessive horizontal strain needed and to save the gauge from damage. We only need some of the backhoe weight for this test, so the post supports it. My proving ring was much heavier construction than shown. You could put a pressure gate on a hydraulic jack for the vertical load load instead of the proving ring,I suppose. This set up does not allow for perfect application of the vertical load, but for as rough a test as it is, I doubt that in is important.

RE: Undisturbed samples

Thanks OG for taking the time on this. It is much more clear now. I feel that in the past engineering was more fun since engineers needed to create their own apparatus and tools. Nowadays with internet and computers, we are forgetting the basics and get more "robotized"...

RE: Undisturbed samples

Oldesy Guy--
I like your story--- We must be about the same age--I've been in this field since 1976. Improvised a lot of stuff. It was fun---now, not so much---time to move on.

RE: Undisturbed samples

Mike: I'm a little older, 87. However,have always been right in there since age 5 I guess. I recall one case in court as an expert witness dealing with a failed gas main causing a building fire. My attorney strayed a little and asked me to comment on a weld on the pipe, When opposition screamed that that was out of my field of expertise (soils engineering), I was told to show them my hands. It was pretty obvious that they were not those of someone who sat in the office all day. Enuf was said.

RE: Undisturbed samples

OG, wow 87!! I am in my 30's and hope I can still remember things at that age!! Thanks for sharing your experiences with us!!

RE: Undisturbed samples

Hey guys: You never stop learning and keeping one foot there (working on this stuff) is better than having one foot in the grave I think. Besides its fun.

RE: Undisturbed samples

You seem to have maintained a good attitude---better than i can say of myself. I am increasingly frustrated by young ( fresh out of college guys and gals) who want to impress the boss that they can do it faster, and cheaper, yet don't know anything about the testing we do, or why it is important. One recent example--I was doing roofing inspections on a shopping center addition. The roofers were working from column line F to L, but could go no further because the structural inspector had not inspected past line L, and there was, in fact, no decking beyond line M. The young guy knew he got a BONUS when the roof was installed to line P, and was demanding that the roofers meet his deadline. I asked if he wanted them to roll out the roofing and then slide the decking under it. He didn't understand....
Its time for me to at leaset semi retire, if I can find someone who will let me work just 2 or 3 days a week. the workload where I am is such that I need to be here everyday.

RE: Undisturbed samples

Mike: it may be a little late for you, but in my case I semi retired as part owner from a full time job and opened may own office at age 55. In the process a new drilling and testing firm started and I became their mentor. They have since expanded well and have young engineers that are learning. As long as they will handle the insurance, I continue to mentor them, even though my own work is very minimal. They seem to value the reviews, especially when I catch a possible drastic mistake before it goes out. Somehow those years of experience still can be relied on occasionally and the thanks are nice to hear.

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