Soil Sampling

Soil samples are usually taken at regular intervals during drilling to be analyzed for chemical composition and tested for physical properties such as particle size distribution, textural classification, and hydraulic conductivity. The samples are generally taken from the bottom of the borehole and at the necessary depth when the sampling device is driven with the aid of a 140-pound hammer. The hammer is connected to the sampling device by drill rods. The number of hammer blows, usually counted for each 6-inch increment of the total drive, indicates the compaction and density of the formation being penetrated.

The most commonly used soil sampling devices are the split-spoon sampler and the Shelby tube. The split-spoon sampler is a 12- or 18-inch long hollow cylinder consisting of two equal semicylindrical halves held together at each end with threaded couplings. The sampler is lowered to the bottom of the borehole and driven with a hammer to the necessary depth. When the sampler is brought to the surface, it is disassembled, or split, to remove the soil sample. Split-spoon sampling provides representative soil samples for physical or chemical testing. The samples, however, are disturbed; therefore, the results of the analysis should be used with caution. When the same sampler is used to collect different samples, the engineer should decontaminate it after each sampling event to prevent cross-contamination.

The Shelby tube sampler is a thin-walled tube made of steel, aluminum, brass, or stainless steel. The cutting edge of the tube is sharpened, and the upper end is attached to a coupling head by cap screws. The sampler must meet certain criteria, such as a clearance ratio of 0.5 to 1.5 and end area ratio of 10, to ensure the least disturbance to the sample (U.S. EPA 1993). The sample collection procedure is similar to split-spoon sampling except that the tube is pushed into the soil by the weight of the drill rig rather than driven. When the sampler is brought to the surface, the sample is sealed and preserved for laboratory analysis. Shelby tube sampling is used for soil analyses that require undisturbed soil samples, such as hydraulic conductivity testing.

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