Current Sampling Equipment?

[dirtgirlhawaii]

Hi all, experienced and new geotechnical engineers...

How does your firm obtain relatively undisturbed soil samples? My current employer seems to have massively outdated equipment in the lab and in the field. I am curious as to what is the most widely used equipment for field sampling at this time. We use a modified California ring sampler, but the particular brass or Teflon rings we use are no longer available for purchase and have not been available for quite some time. I have only been doing this for a little over a year now and need to know if working here is essentially a time warp into 1960.

Mahalo!

 

[GeoPaveTraffic]

First, modified California ring samples cannot be considered the least bit undistrubed. I have seen a lot of folks in CA who call them undistrubed but this is bull.

There are methods of getting relatively undistrubed samples. The one we use most often is a Shelby tube. Typically a 3-inch diameter by 24-inch long sample. Two inch and 5-inch diameter tubes are also generally available.

The only other option I've seen specified is a Pitcher sampler, though I've never used one.

Mike Lambert

 

[BigH]

To add to GeoPaveTraffic's comments - disturbance using the thin-walled steel tubes can be minimized by the method of insertion. Using a piston sampler would be the best way, then hydraulic push, or manual push. In India, they used to gentle tap in the sampler (for me a no-no).

Other methods to obtain relatively undisturbed samples might be by the use of the Swedish foil sampler - but I doubt this has been used more than a few times outside of Scandanavia . . . you might find something about it in a book. Our company in Canada had the rights to this sampler - but it had been used years before I had joined (1975).

If you are in test pits, then the use of block samples would be appropriate. See USACE, Hsorlev for how to take the sample.

 

[aeoliantexan]

I believe you are asking about routine sampling (every 5 feet or so) to get samples for description, classification tests, and approximate strength tests. From what I see, the practice has advanced very little and actually regressed since Hvorslev's day. I grew up in a company with California roots, and we used the 2" California sampler routinely. It was OK for very stiff clays, and sample handling was convenient. It tended to compress unsaturated soils including loess and fill. I was able to improve on that by designing a tip with a sharp, long-tapered bevel and an interior bevel that made the tip cut a sample a little smaller than the ID of the tubes. If the sample could be pushed out of the tube with the thumb, it probably hadn't compressed much going in. We had our samplers built to spec by a local machine shop. You can probably have tubes made the same way. The main trick is to throw the tips away when they get dulled or chipped. For cohesive soils, I believe California sampler is still much better than a split spoon. It is seldom suitable for consolidation testing.

In much of the south, the clays are stiff to hard, and the main question is how much will the soil swell. The standard sampler is a thick-walled 3" tube that looks like a Shelby tube, but the wall is thicker and the tip is not swedged to cut an undersized sample. Some drillers are still using the tubes they started with 20 years ago. When they get nicked and bent on the tip, they saw them off. The more picky ones grind the tip sharp, but some just use the blunt cut end! The recovery is poor, but improved by pushing a 16-inch long tube 24 inches.

You probably won't have to try very hard to improve on the local practice. Read Hvorslev and recognize the principles. Are you concerned about reducing the undrained strength by remolding the sample too much, or increasing the apparent strength by compressing the sample due to high perimeter friction? buck the tide and try to improve your profession.

For more important strength and consolidation testing, Shelby tubes are still the standard. Piston samplers or foil samplers may be justified for very soft or sensitive soils. Larger-diameter Shelby tubes generally get less-disturbed samples. Don't over-push the sampler, and reduce the push to 12 inches or so for touchy soils.

Don't knock the 60s. Academics, in-situ testing,and computers have improved since then, but the local practice has deteriorated since the geotechnical report became a priced commodity.

 

[Ron]

Agree with most of the other comments. Shelby tubes, hydraulically pushed, the larger the diameter, the better for routine UD samples.