The objective of soil sampling is to obtain soils of satisfactory size with minimum disturbance for observations and laboratory tests. Soil samples are usually obtained by attaching an open-ended, thin-walled tube—called a Shelby tube or, simply, a sampling tube—to drill rods and forcing it down into the soil. The tube is carefully withdrawn, hopefully with the soil inside it. Soil disturbances occur from several sources during sampling, such as friction between the soil and the sampling tube, the wall thickness of the sampling tube, the sharpness of the cutting edge, and the care and handling of the sample tube during transportation.
To minimize friction, the sampling tube should be pushed instead of driven into the ground. Sampling tubes that are in common use have been designed to minimize sampling disturbances. One measure of the effects of sampler wall thickness is the recovery ratio defi ned as L/z, where L is the length of the sample and z is the distance that the sampler was pushed. Higher wall thickness leads to a greater recovery ratio and greater sampling disturbance. One common type of soil sampler is the “Shelby tube,” which is a thin-walled, seamless steel tube of diameter 50 or 75 mm and length of 600–900 mm .
Another popular sampler is the “standard” sampler, also known as the split spoon sampler (split barrel sampler), which has an inside diameter of 35 mm and an outside diameter of 51 mm . The sampler has a split barrel that is held together using a screw-on driving shoe at the bottom end and a cap at the upper end. In some
a) A thin-walled tube and (b) A split barrel sampler
countries a steel liner is used inside the sampler, but in the United States it is standard practice not to use this liner. Consequently, the soil sample has a greater diameter. The results of SPT are different for lined and unlined samplers. The thicker wall of the standard sampler permits higher driving stresses than the Shelby tube, but does so at the expense of higher levels of soil disturbances. Split spoon samples are disturbed. They are used for visual examination and for classifi cation tests.