The following is in response to an increase in the number of inquires regarding sample retention. Most is based on my 50 years of drilling and sampling work as a participant either drilling, helping, consulting or designing equipment for sampling. To the best of my knowledge none of it has been printed or presented at seminars.

H. E. "Hank'' Davis 6/1999

 

PROBLEMS and some SOLUTIONS in GEOTECHNICAL SAMPLING

FOREWORD


Geotechnical sampling is accomplished with different samplers in different parts of the United States. East of a north-south line approximating the eastern border of Colorado, the Split Barrel Standard Penetration Sampler (ASTM D-1586) is commonly used. West of that line, the Ring Lined Barrel Sampler (ASTM D-3550) is preferred and used by most geotechnical firms. Most western firms have D-1586 Samplers but rarely use them unless the client insists on their use. Thin-Wall Tubes (often improperly called "Shelby Tubes")1 are rarely used in the West. Samplers used both in the east and west are usually furnished with a ball check valve, and in this writers opinion, should also be furnished with a ''sling-shot'' in order to remove the ball as far from the job as possible thus eliminating a common sampling problem. More later!

The samplers mentioned above are generally used for routine geotechnical work. Piston Samplers, Rotary Samplers (Denison, Pitcher, Denver, etc.) Cryogenic Samplers and others used for special work are not specifically included in this discussion2 

DETAILS & DIMENSIONS

The ASTM D-1586 Split Barrel Sampler has very specific dimensions. The outside diameter is 2.0"and the inside diameter is 1.5". The Drive Shoe (cutting shoe) is 1.38" bore and the Split Halves can be 18 to 30-inches long but normally are 18 inches long. The ASTM D-1587 Thin Walled Tube Sampler, used in the East, usually has a 2.0" outside diameter with a specified wall thickness of 18-gage (0.049''). It is rare, however, to find a 2" thin-walled sampler with anything but a 16-gage (0.0625'') wall.

The ASTM D-3550 Sampler used in the West (often erroneously referred to as a modified California Sampler)3 is actually a Split Barrel Sampler. But it is a split-barrel sampler adapted to be loaded with sample rings or liners, which is the primary reason for the ASTM D-3550 designation as opposed to the ASTM D1586 designation. To be used with rings or liners the inside diameter of the sampler must be capable of containing the rings or liners without binding, without excessive end play and without excessive diameter play. This requires special manufacturing techniques. The surest of these techniques, honing, just about doubles the cost of the finished sampler. Various other techniques used by manufacturers to avoid the high cost of honing, tend to be manufacturer specific. Thread configuration connecting the various parts of the sampler vary depending on the manufacturer. Consequently, the user is advised to select the best for his use and then stay with that manufacturer.

Unfortunately the manufacturer of the most commonly used D-3550 sampler, Sprague and Henwood (S&H), closed its doors in the early 1990's. Many drill manufacturers bought the S&H sampler and resold it as their own. Most geotechnical firms in the West have more than one D-3550 sampler and even more spare drive shoes. Since the closing of S&H, many geotechnical firms have been cannibalizing and re-machining as necessary from parts on hand to keep at least one sampler workable. Cannibalizing & re-machining can only work so long. Then what?

The worst solution is generally the local machinist. He may be a superb machinist with excellent equipment, but very rarely has the alloy steels and especially the heat treating and hardening capability needed. He will rarely have the parts Ňoff the shelfÓ. A good D-1586 or D-3550 sampler will utilize two different alloys, two different heat treats and at least one hardening process.

The D-3550 sampler is made in a variety of diameters, all very close to each other, the most common of which (standardized by the Diamond Core Drillers Association (DCDMA)) and complying with American Society for Testing and Materials (ASTM) is nominally 3'' O.D. by 2.50" I.D. with a drive (cutting) shoe bore of 2.38" and capable of carrying 12 or 18-inches of rings or liners, often with a 6 inch long non-split section of barrel above the split barrel section. This 6 inch long section, if present, may be 2.50" inside diameter allowing for use of a 6 inch long liner or it may be 2.38" inside diameter which precludes the use of a liner. In either case, the 6-inch long section is a "spoil barrel" and is intended to receive loose soil (spoil) which may have been on the floor of the bore hole prior to driving of the sampler. We recommend use of a spoil barrel which will accept a 6'' long liner. This provides "extra" material, which, collected and transported in a sealed liner, can be used for moisture determination, Atterburg limits or gradation testing, thus preserving the ring or liner samples for consolidation or shear testing. Liners or Rings for this ''standard'' sampler are nominally 2.50 '' O.D. by 2.38" I.D. Rings are normally 1‑inch tall (long) and liners 4 or 6-inches long.

DEFINITION - A Ring is equal to or shorter than its inside diameter. A Liner is longer than its inside diameter.

The Dames & Moore Sampler is either 3.25" or 3.0" O.D. being heavy duty or regular duty, respectively. Both sizes of the Dames & Moore Sampler have a nominal I.D. of 2.5" and a drive (cutting) shoe bore of 2-7/16" for use with 2-1/2" O.D. by 2-7/16" I.D. rings or liners. In my opinion, either version of the Dames & Moore Sampler complies with the ASTM D-3550 Standard. Both the Real Modified California Sampler and Dames & Moore Sampler have been in use since well before World War II although ASTM did not write the D-3550 designation until 1976.

WARNING - It is almost impossible to see or casually measure the difference between a 2-7/16" and a 2-3/8'' I.D. ring or liner without using a vernier or dial micrometer; and using the wrong I.D. ring or liner will assure problems.

PROBLEM

Sample is driven or pushed but sample is not in the sampler when brought to the surface.

PROBABLE CAUSES:

Check valve is not working or not present in Sampler Head. Check valves are intended to expel the buildup of fluid pressure in the sampler above the incoming sample by permitting fluid (please note that both air & water are fluids) to flow upward through the port and valve and out into the borehole through vents, or upward into the drill rods. As withdrawal begins, the valve is supposed to close, preventing backflow of fluid and allowing a vacuum to develop over the sample. As withdrawal continues, the vacuum increases and friction between the sample and the sampler drive shoe combined with the vacuum should hold the sample in the sampler. Most samplers utilize a ball check valve because it is cheap, compact, and durable. Ball valves work very well in squeaky clean environments and at very high pressure. Unfortunately, neither condition is normally present when sampling soils. The ball cannot seat because of grit or dirt on the seat. Vacuum cannot be developed because the valve has not sealed. Numerous attempts have been made by various manufacturers to coat the ball or seat or to otherwise improve the seal. I know of none that work reliably.

A cure for loss of samples in areas where water (but not saturation) is a problem can range from simple to complex depending on the severity of the problem.

First, try using a perforated ring, as a retainer, in place of the bottom ring, closest to the drive shoe. A perforated ring is a sample ring having 8 or more evenly spaced 3/16" or 1/4" diameter holes drilled through its wall.4 The holes in the sample ring wall provide a place for the sample to expand into and "grip'' the wall of the sampler. Typically, after the sample drive is made but prior to withdrawal of the sample, the driller whacks the drill rod a time or two with a hand hammer, which causes a slight collapse of the soil into the holes in the perforated retainer ring, and particle friction does the rest. Content of sample rings above the bottom ring should not be severely affected.

Second, try using a 2 or 3 mi1 thick polyethylene bag five to six inches long which is a fairly snug fit around the outside diameter of the sampler. Slit the bag near its bottom with a sharp knife to create 2 each 1/2"± long slits. Pull the bag up around the sampler about 2- 3 inches and lightly tape the top of the bag to the outside of the sampler with masking tape. (Sort of like pulling on a sock part way). Tuck the bottom part of the bag up into the sampler. Proceed with your normal sampling procedure. As the sample is driven the bag is cut off inside the sampler by the drive shoe cutting edge or is torn free from the tape. Either way the bag becomes a short shroud over the top of the incoming sample thus assuring a seal for a vacuum hold, which, with the perforated ring, will handle most tough jobs. The slits become the valve, which closes when the bag collapses on top of the sample upon sampler withdrawal. The bag is sufficiently pliable that any grit becomes encapsulated. When done properly, the shroud will be in the top two or three rings. The bottom ring may be unusable because of the slump of the material into the retainer holes but the middle rings should be reasonably suitable for testing and infinitely better than no sample at all! We always recommend 18 inches of split section samplers to provide sufficient rings.

Third, tape the sampler splits closed full length on the outside of the sampler. The splits can be a source of air leakage, which may defeat the vacuum hold we're trying to develop. Masking tape is usually okay, but stainless steel foil tape (available from most large hardware stores) is better.

DEFINITION - (re sampler terminology) -VENT is a passage for expelling fluid and is always open; PORT is a passage for expelling fluid and may be open or closed. VALVE is a device for closing a port. Vent(s) commonly, but not always, open out the side of the sampler head. A port is normally a passage through the axial centerline of the sampler head. Valve(s)are normally located on the top of the port, when present. [added 4/2005 HeD]

Fourth, modify the sampler head port (the axial bore through the sampler head which is normally counter-bored and machined at its upper end to provide a seat for the ball) by counter-boring and threading from the bottom to receive an adaptor and nipple. External thread on the adaptor to fit into the now bottom threaded port. Upper end of the nipple to be barbed to be a stretch fit into a childŐs birthday balloon which has been cut off an inch or two above the opening. All parts can be found at hardware and variety stores but some machining of the sampler head will be required. Properly made this system will bring water out of a hole just as a bailer would. In use fluid is expelled through the cut off balloon as the sample is pushed or driven but the balloon collapses upon sampler withdrawal encapsulating grit and sealing. [added 4/2005 HeD]

Sample retention when using Thin Wall tubes can usually be accomplished by drilling 8 or 10 each 1/4" diameter holes through the tube wall in the bottom 2 or 3 inches of the tube then wrapping the outside of the tube, over the holes, with stainless steel foil tape. Works the same way as the ring retainers described above. [added 6/2005 HeD]

1Shelby is a registered trademark of U.S. Steel. Current Thin-Wall tubes are made of ERW-DOM (Electric Resistance Welded-Drawn Over Mandrel) steel tubing which is much stronger, cheaper, and made to tighter tolerances than older Shelby. The term Thin Wall Tube should be used and the term Shelby should be discouraged.

2Some aspects of samplers described in this writing can be applied to more exotic samplers but it must be done with care and solid knowledge.

3The real Modified California Sampler is a very precisely made expensive Retractable Piston Type having non-split coupled sections and carrying brass liners. It employs several exotic metals and various heat treat and hardening methods. It sells for about 25 times the cost of the 3" O.D. Modified Split Barrel Sampler and must be furnished with special extruder equipment. It is used almost exclusively by the California Department of Transportation who developed it in the 1930's. Obviously, using the term Modified California if you mean Modified Split Barrel is not cool.

4Perforated Sample Retainer is made by drilling 8 or 12 evenly spaced 3/16" or 1/4" diameter holes drilled through the walls of the ring. It is used in place of the bottom sample ring (closest to the drive shoe). Sample passing through the ring as the sample is driven is normally not excessively disturbed. When the drive is completed and withdrawal begins, the sample opposite the perforated ring tends to "slump" slightly into the perforations which provides some "grip" for the sample. Rings above the retainer usually provide reliable test specimens.

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