Drive Core Sampling

Hydrologists usually rely on a mixture of drill cutting samples bailed from the hole to determine the grain-size distribution of an aquifer material from which to select the proper screen-size-slot openings for constructing an observation well or production well. However, the cable-tool drill provides an easy method for determining the true grain-size distribution by means of drive-core sample analyses. Drive cores can easily be taken in dry materials or saturated materials with the cable-tool rig. After cuttings have been bailed out, fasten a drive-core barrel with inner liners and core retainer to the tool joint just below the drill jars (fig. 30). Lower the sampler to the bottom of the hole; make a mark on the drill line 20 in. above the casing or at some other reference point; and, by alternately lifting and dropping the jars, drive the sampler to the desired depth. After the sample has been driven, try to pull it out with the rig hoist; if it is too tightly anchored, use the jars in the driving mode for bumping it loose. Note:- When driving the sampler with the jars, do not raise the jars to the end of the slip-joint travel of the jars, or a direct upward blow will be imparted on the sampler, damaging the integrity of the sample. Using these jars for driving is contrary to their normal use; but, if the method is done carefully, no damage to the jars will result. This method provides a sample of the aquifer material that truly represents the grain-size distribution, not a mixture of materials drilled.

This same method can be used to take a relatively undisturbed core of cohesive soils, by using a Shelby-tube sampler. Use the lightest jars available and careful driving techniques to lessen the chance of disturbing the sample or even crumpling the thin-walled Shelby tube. In addition to collecting cuttings and core samples, the cable-tool method of drilling offers an excellent means of collecting water samples from the hole. This is especially true when drilling in unconsolidated materials, where the hole has been cased. Compacted materials behind the casing and on top of the casing drive shoe almost always prevent any water from moving down the hole; after cuttings have been removed and further bailing is performed to partly clear the water, water samples can be collected for analysis.

The cable-tool method can also be used to collect continuous, uncontaminated drive-core samples of unconsolidated material. This method of sampling could be used in areas where hollow-stem auger drilling cannot be contracted. This method was used by the Washington State District of the Water Resources Division of the U.S. Geological Survey for a study involving a TNT (trinitrotoluene) waste contamination. It was accomplished in the following manner: 1. A 4-in. drive-core barrel containing acetone-cleaned inner liners was driven 18 in. for the first sample, using drill jars as the hammer. 2. A section of 4-in. casing with a casing drive shoe was driven to the bottom of the sampled depth, and the 4-in. drive-core barrel was run inside the casing and driven to the original 18 in. to clean out the cuttings brought into the casing by the shaving action of the casing drive shoe. 3. After the cuttings were removed, the sampler containing new inner liners was again cleaned with acetone and another 18-in. sample was driven and removed. This sampling, driving of casing, cleanout of cuttings, sampler cleaning, and sampling again were done in several holes. As a comparison and evaluation of the

Sampler head assembly-

-Casing

Drill -jars

Sampler -connector sub

-Drive shoe

Solid or split-tube 3 sample barrel- >

Solid or split-tube 3 sample barrel- >

Sampler inner liners

Sampler drive shoe

Sampler inner liners

Sampler drive shoe

Figure 30.—Cable-tool drive-sampling apparatus.

method, a Water Resources Division auger-drilling rig used hollow-stem augers and drive-sampling equipment to sample one hole at the same location. Analytical results of the two methods compared favorably, but the hollow-stem-auger-drilling and drive-sampling method was several times faster than the cable-tool-drilling and drive-sampling method.

The method of cable-tool drilling and drive sampling was provided to show that there is another way of the sampling in this type of environment, if hollow-stem-auger-drilling and drive-sampling equipment is not available. Although this sampling was performed in unsaturated materials, it might also work for sampling below the water table if the Church method of driving and sealing behind the casing were used. Basically, the Church method utilizes a casing-drive larger in diameter than the diameter of the drive casing used to make an oversized hole. A drilling mud is pumped around the drive casing at the same time that it is being driven. The purpose of the drilling mud envelope is to lubricate the outside diameter of the drive casing while also maintaining a pressure seal around the casing and preventing upward artesian flow of water between the outside of the drive casing and the borehole annulus.

Procedures of Sampling in Air-Rotary Drilling

Procedures for sampling an air-rotary-drilled hole are essentially the same as those used for obtaining samples from a hole drilled by the hydraulic-rotary method. Drilled cuttings are collected from the return airstream using a sieve, colander, or shovel. Again, as with sampling the return cuttings from holes drilled by any of the other methods discussed, careful judgment must be exercised in togging the cuttings. Factors to be considered by the person taking the sample include: uphole- air velocity and lag time of returned-cuttings sample; sample mixing and balling as the cuttings are moved uphole in the return airstream (might contain mist, foam, polymers); periods of tost circulation and no cuttings return; and the competency and expertise of the driller to help assess the validity of the cuttings and depths from which they were drilled. In-situ sampling of materials using core barrels (air-rotary or drive-core type) can be taken anywhere in the hole if the driller is equipped with the necessary tools for doing so. If this type of sampling is done, standard- rotary air-coring techniques using dry air, foam, or mist would be applicable.

SAMPLING TOOLS AND THEIR APPLICATION

A sample may be defined as a representative unit or part of the formation penetrated in the borehole, that is obtained for purposes of analyses and description. Samples taken can be either disturbed (grab samples, drill cuttings) or undisturbed (samples obtained under in-situ conditions), depending on the techniques with which they are obtained from the borehole. In general, the less sample disturbance required, the more costly the method for obtaining the sample.

Sampling of soil and rock involves many engineering techniques and a variety of specialized downhole tools. These tools are generally referred to as samplers or core barrels; many of them are discussed in the following section.

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