Methods of Shelby tube sample extrusion 1

[Bigft64]

Does anybody have any tips/tricks/advice when it comes to extruding 3" OD Shelby tube samples? The firm I work for is in an area with very little soft, cohesive soils and where there are some, they tend to be stiff. Therefore, we almost exclusively use SPT and California drive type sampling. We are preparing to work in an area out of our normal area that has large amounts of soft clays and so we plan on taking numerous Shelby tube samples. However, we don't have a quick-and-easy setup for extruding the samples and nobody has a lot of experience with them either. I was just wondering if people had any tips, methods, or special tools that wouldn't necessarily be found in ASTM that could help us out. Thanks.

 

[BigH]

Our Toronto lab used to extrude them horizontlly. We had a horizontal half pipe (aluminum). The tube would sit in one half of it. We then had a hydraulic piston to push the top of the sample extruding it out of the tube and the sample would be collected in the extension of the horizontal half pipe. Sorry I have no pictures of it. Our firm dealt with many soft clay sites - we even had the exclusive use of the Swedish foil sampler which is quite a set-up.

 

[oldestguy]

Big H shows a nice design.

However, I have found that vertical extruding with hydraulic piston also works. If you cut off the tube in short sections so that you don't make that sample travel a long distance thru the full length tube, you may not be distorting it as much as when trying to salvage the tubes for more use later. I'd not extrude the whole tube sample.

Short of using hydraulic pusher, for soft soils, you can use the short cut off Shelby tube sections and manually shove the samples out with a piston that fits loosely inside it. Get a wooden dowel, metal shaft, or simlar cylinder of just under the proper diameter fit and set it vertical. Put the sample and tube on top of it and manualy push down. Your sample then sits on top of that "pusher".

 

[MRM]

Cut the tube into a short section. Run a wire through the sample, directly adjacent to the tube wall. Connect the wire to a wire saw and tension. Make a cut around the perimeter of the sample. This will further reduce disturbance of the sample, especially with softer clays, when pushing it out.

 

[jdonville]

Bigft64,

To expand on MRM's post, here's how I have heard it described (my firm extrudes horizontally - not ideal, but I'm still working on changing some minds):

1) Cut ST sample across long axis into lengths of interest using a band saw. Take appropriate care to protect eyes, hands, etc.

2) Carefully thread a hypodermic needle (just the fine needle part) along the inner edge of the tube parallel to the longitudinal axis of the soil sample.

3) Carefully thread a thin guitar or similar strong, fine metal wire through the needle. Once through, carefully remove the needle and set aside. Use the wire to break the surface adhesion between the soil sample and the tube section around the entire circumference of the soil sample.

4) Gently extrude the sample. Trim and/or test immediately.

Some research suggests that the middle 1/3rd of the ST sample has the least disturbance, so choose your cuts appropriately.

Jeff

 

[GeoPaveTraffic]

The firm I worked for in the mid west used a setup very similar to BigH. We had a hydraulic piston with a travel of about 30", used a steel bearing plate between the sample and the piston, and push from the bottom of the tube. This setup worked very well for all clays (soft to hard) and well for silts. Soft silts were a problem, but then again soft silts are always a problem.

Another comment, don't wait any longer than you must before extruding the samples. The longer you wait the poorer the condition and the harded to get the samples out. Also, instead of using a band saw to cut the tubes, a larger pipe cutter works very well.

 

[jdonville]

GPT,

The reason why I suggested the use of a band saw is that you want to cut across the tube and the soil sample. I can't imagine that this would be practical with only a pipe cutter.

Jeff

 

[msucog]

we use a ban saw to cut across the entire tube to shorten the length into a few shorter sections. the shorter sections are long enough that the middle part of that section is still "undisturbed". this also allows us to evaluate different sections of the tube since subsurface conditions are rather erratic in my part of the country. for example, it helps us confirm that the 4 bpf material we were trying to sample ended up in the tube.

 

[GeoPaveTraffic]

jdonville,

Your right. I was trying to remember, it has been quite awhile. However, we never cut the tube and the sample, and never had much trouble getting good samples out of the tube. The only trouble we had was when the tubes sat around the lab for a a many weeks or months.

We did use a pipe cutter when the end of the tube was damaged. In those cases we would cut off the damaged portion with a pipe cutter; then push the sample as normal.

 

[oldestguy]

A question.

Has anyone taken the time and effort to see the difference in distubance when the sample is extruded thru the full tube length, versus using short sections?

I recall taking extruded samples, slicing them on the axis, letting them dry some and looking at the degree of distortion along the sides. It is interesting to see how far in this takes place.

I prefer then to use the part closest to the bottom due to its having had less done to it than those parts farther in.

Triming the perimeter surfaces may off-set that disturbance action. Do you do that??

Imagine what goes on first sliding the sample into the tube at the site and then reversing that to extrude them. One reason for using that foil sampler in sensitive clays. It sure makes a difference in what that sample receives for disturbance. The one I observed was made by Sprague and Henwood and demonstrated at the St.Lawrence Seaway 1955.

 

[BigH]

OldestGuy - were you involved with or aware of Foundation Company of Canada - contractors? Fenco (Foundation Engineering Company) and Geocon were set up as separate companies instead of divisions of Foundation Company in the early-mid 1950s. They did an extensive amount of work on the St. Lawrence.

 

[oldestguy]

Nope. This was a demonstration of the gear. My boss (a professor) was a consultant for an earth mover who went broke due to low bid for moving dense glacial till. This was incidental to our work there. Hey, that clay sure is interesting stuff. Put a sample in your hand, wiggle the fingers and it is soup.

In 1956 I did interview with a Toronto based geo firm, but took another job. Can't recall the firm's name, but one principal was Mr.Cheney, a co-grad student at Cornell. If still alive, he'd be in his early 80's.

 

[mdhshanwil]

A nice overview of recommended practice is contained in Chuck Ladd and Don DeGroot's 2003 Arthur Casagrande Lecture: "Recommended Practice for Soft Ground Site Characterization", Proc 12th Panamerican Conf on Soil Mech and Geotech Engr, MIT, Cambridge, June 22-25.

Recommendations are provided for all types and phases of soft ground characterization, including drilling/sampling, in-situ testing, laboratory testing, and interpretation of results. In fact, some of the recommendations others have provided above should refer to this paper.

HIGHLY recommended reading.

 

[DPirate]

I know that a lot of those who read this will cringe! I currently work for an organization whose SOP is to extrude Shelby tubes in the field. However, we are gradually changing this to only allow extrusion in the laboratory. The method of extrusion used is a hydraulic ram mounted horizontally on the vehicle with a slightly undersized push head that fits in the tube. A 3"ID half-moon trough is used by the person extruding to catch the sample.
The tube is secured using a clamp, and the sample travels in the same direction sampled. The trough is held under the tube and moved along with the sample as it leaves the tube so there is minimal dragging of the sample. If there are any cuttings at the top of the sample they are discarded. The length of recovery is then measured, and then a decision made as to what 12" portion is representative of the recovery, or, if there is more than one distinct layer, where the sample should be split. We try to get the weakest portion visible.
The drilling mud is then lightly scraped off of the sample using a putty knife(cringe), and the sample wrapped in a layer of plastic wrap; a layer of foil; then sealed in a plastic bag. The sample is then placed in a styrofoam transport box which supports the sample and delivered to the testing lab within 2 days.

BTW if it's Louisiana you are going to, try the crawfish!

 

[BigH]

Where did you(they) get this SOP? I've never heard of this before and it "violates" all the bibles of soil exploration (Hvorslev, Winterkorn and Fang, etc.)

 

[moe333]

Jdonville (and Mdhshawnwil),

I assume your recommended procedures are based on the Ladd paper that Mdhshawnwil refers to. I have been reading this paper and it is extremely enlightening. I see this method recommended for consol samples.

My question is where can you find a hypodermic needle 7-12 inches in length, since this is the length you would need for triax. testing when you discard the upper and lower several inches as suggested in the paper. I would like to purchase some of the hypodermic needles if you know a supplier.

And, if you have used this method, does the portion of the sample where the hypodermic needle is pushed through show much disturbance? If so, do you trim the diameter of the sample?

 

[BigH]

In re-reading jdonville's post, from personal experience he is correct in that the middle of the thin-walled tube shows least disturbance. We carried out a number of miniature vanes at various locations within the tube and there was definitely a "stronger" soil in the tube centre.

 

[DPirate]

BigH:

Sorry for the delay in response...

This SOP has been in place for decades here. All geotech firms I have observed and the state DOT field crews have extruded in the field. This practice is not strictly forbidden in the ASTM specs. From what I've determined, the logic behind the practice stems from the fact that the soils encountered here are either predominately clays and we design for friction piles; or we encounter an adequate sand layer and design for end-bearing piles. The strength loss by this SOP allows our designs to be a little more conservative in either case.

A related issue is the number of tubed samples that are obtained from an average boring. For a standard 120-foot boring at 5-foot centers, it is not uncommon to sample only clays. For a typical 3 boring week, the ability to properly transport 70+ samplers becomes an issue.

Once again, this practice is being phased out in my organization, but overcoming the institutional inertia is a slow process.