UNDISTURBED soil samples are those that are cut, removed, and packed with the least possible disturbance. They are samples in which the natural structures, void ratio, and moisture content are preserved as carefully as possible. Samples of this type are used for determining the density (unit weight) of soil in the laboratory and investigating the strength of undisturbed soils in the laboratory by the CBR or unconfined compression tests. These samples may be shipped to more completely equipped laboratories for shear, consolidation, or other strength tests. Types of undisturbed samples are chunk samples, cut by hand with a shovel and knife, and cylinder samples, obtained by use of a cylindrical sampler or the CBR mold equipped with a sampling cutter. Expedient methods of obtaining cylinder samples are also used.

The method of sampling chosen depends upon the equipment available, the tests required, and the type of soil. All undisturbed samples must be handled with care. Cohesionless soil samples must be kept in the container until ready for testing, and the container should be handled without jarring or vibration. Some soils are too hard or contain too many stones to permit sampling with the cylindrical samplers and can be sampled only by cutting out chunks by hand. Taking of un-disturbed samples frequently requires a great deal of ingenuity in adapting the sampling devices to job conditions and in devising schemes for their use. Whatever method is used, the sample must be taken and packed in the container for shipment without allowing its structure to change. Protection against change in moisture content during sampling and shipment is also required.

The simplest type of undisturbed sample is the chunk sample. It should be noted, however, that these can be obtained only from soils that will not deform, break, or crumble while being removed.

Figure 15-9.-Taking a chunk sample from a level surface.

Figure 15-9 shows the process of taking a chunk sample from a level surface, such as a subgrade or the bottom of a test pit. After smoothing the ground surface and mark-ing the outline of the chunk, the first step is to excavate a trench around the chunk. Then deepen the excavation and trim the sides of the chunk with a knife. Finally, using a knife, trowel, or hacksaw blade, cut off the chunk at the bottom and carefully remove it from the hole.

To take a chunk. from the vertical face of a test pit or trench, as shown in figure 15-10, smooth the surface of the face and mark the chunk outline. Then excavate the soil from the top, sides, and back of the chunk. After shaping the chunk with a knife, cut it off and carefully remove it.

After removing the chunk sample from the hole, you need to seal it. One method is to apply three coats of melted paraffin, as shown in figure 15-11. Each coat is al-lowed to cool and become firm before the next coat is applied. This gives adequate protection for strong samples that will be used within a few days. When the samples are weak or may not be used within a few days, wrap them with cheesecloth or other soft cloth and seal them with paraf-fin (fig. 15-12). If cloth is not available, you can reinforce the sample with several loops of friction tape or twine. Then apply three coats of paraffin. Take extra precaution in these operations so that the sample is not damaged. Another method is to dip the entire sample in melted paraffin after the first brush coat is applied and the sample is wrapped (fig. 15-13). This requires a larger container and more paraffin. However; this method pro-vides a more uniform coating that, by repeated dippings, can be built up to 1/8 in. or more in thickness.

When samples are to shipped, as from a construction battalions remote detail site to the battalions main body site, additional protection is required. This can be accomplished by applying many coats of paraffin or by placing the chunk in a small

Figure 15-10.-Taking a chunk sample from a vertical face.

Figure 15-12.-Wrapping a weak chunk sample before final sealing.

Figure 15-14.-Packing a chunk sample for transportation or shipment to laboratory.

box and packing it, as shown in figure 15-14.