Hollow Stem Auger Sampling in Saturated Materials

Core sampling through hollow-stem augers in saturated materials encounters problems that require different techniques than those described for the unsaturated zone. The greatest problem is piping. When the center-assembly plug and pilot bit are withdrawn, there is a tendency for the viscous slurry of cuttings, still contained in the bottom of the hole, to be pumped upward into the hollow-stem augers. Withdrawal of the center-plug assembly is analogous to a piston pump that pulls this viscous slurry into the augers. Before presenting coring methods through hollow-stem augers in the saturated zone, some methods to prevent piping from occurring and some remedial measures if it does occur are described in the following paragraphs.

When a required sampling depth has been reached below the water table, check to see if the center-assembly plug and the inner wall of the auger are sand locked together. If a sand lock or seal exists and the center-assembly plug can be pulled, a vacuum will be created at the bottom of the plug assembly that will pull the slurry or viscous sands in behind it. To verify the sand lock and to break it, remove the capscrew from the adaptor cap that fastens it to the top auger flight while leaving in place the center-rod bolt that couples the rod-to-cap adaptor. Now, exert a slow upward pull on the inner rod and move it up an inch to two, while observing the auger flights to see if they stay in place. If the augers start to move, a secure lock between the two can be broken loose by alternately pushing and lifting on the center-rod assembly. When the center rod is free, raise it just enough above the top auger flight to enable removal of the rod-to-cap adaptor. After removal of the adaptor, push the center rod back down until the pilot bit is at its original drilling depth; lower the hollow-stem auger adaptor cap back down to its original position; reinsert the threaded capscrew that couples it to the auger. Now proceed with the technique that will, in most cases, seal off the bottom of the hole and prevent slurried cuttings from moving up into the inside of the augers. Apply heavy down pressure to the hollow-stem and move the clutch just enough to thrust the auger head 3 or 4 in. into the previously uncut formation (the center rod and pilot bit will slide up without rotation because the rod-to-cap adaptor previously had been removed). As the auger is forced this short distance into the formation (using a minimum of rotation), a firm seal usually will be produced against the bottom of the auger head and the first auger whorl, which will prevent the slurried materials from breaking through. The pilot bit and center-assembly plug is withdrawn at a very slow rate to induce no pumping action. After the center-rod assembly has been removed sound the bottom of the hole to verify that the sealing technique was successful and that no large thickness of cuttings is in the bottom of the hole. There will be at least 3-4 in. of loose material in the hole as a result of from the pilot bit being pushed up in the sealing operation. If this thickness of material cannot be tolerated and handled in the manner previously described (pouring out or discarding the upper-contaminated portion of the sample), then a 3-in. clean-out barrel can be run in and the contaminated material can be removed prior to sampling.

Situations can occur where sealing off the hole (as described previously) is not successful. Ear example, when auger drilling into a viscous sand (which may be under artesian head and will flow into the hole during withdrawal of the assembly), then more involved remedial measures are needed, such as: 1. While auger drilling, keep the inside of the hollow stem filled with clean, prepared viscous drilling fluid (viscosity about 50-s). This fluid can be added as each auger is coupled by pouring from a standby barrel of mixed mud. During the auger-drilling operations, mud below the auger head is seldom lost because the natural tendency of drill materials is to move upward and close off the small annular space between the center-assembly plug and the wall of the hollow stem. This added fluid will not be lost through the joints of the auger, when using a screw-coupled auger or O rings between the joints of spine-coupled augers. Maybe less than one full column of drill fluid is required to prevent piping; prior drilling experience in the study area will usually dictate the fluid head needed in the hollow-stem auger to prevent this problem. 2. Another method of introducing viscous fluid into hollow-stem augers is to drill a V4-in. diameter outlet hole in the bottom section of the center rod (as close as possible to the top of the center-assembly plug), and pump fluid through the top in the center rod as needed. This technique is particularly helpful if a considerable amount of piping of the material is anticipated. Using this method, completely fill the hollow stem with whatever weight or viscosity of fluid is needed, while, at the same time, continuously pumping additional fluid into them to maintain full augers as the center rod and center-assembly plug is removed. Use caution in the speed at which the center-rod assembly is withdrawn. Withdrawing the rod at a rate that is too fast for the fluid to move down through the restricted (y4-in.) annular area between the inside wall of the hollow stem and the center-assembly plug will cause a vacuum at the bottom of the auger head, and loosely consolidated material will be pumped up the hole. These methods usually will keep any large quantity of contaminated material from entering the inside of the hollow-stem augei; and representative or undisturbed samples can be collected using any of the previously described methods.

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