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3-14 Selection of Equipment for Logging and Sampling e The frequently limited depth required in field investigations for channel improvements and the. advantages afforded by in-place examination of the alluvial profile indicates that at least a framework of pit excavations is desirable. Safety precautions must be rigidly maintained. The back- hoe is the preferred equipment for such excavations. Once in-place profiles can be examined and sampled above water tables, the pits can be supplemented by drill holes below water tables. Limitations of pits include the possibilities that the depth of sampling required exceeds the capabilities of available equipment, that a high water table would restrict or prevent investigations below that level, and that access is sometimes limited for physical reasons or property owner objections. The advantages of drilling equipment are no depth or ground-water limitations, in-place testing with less site disturbance, and greater safety. For guidance in maintaining safe working conditions, refer to the safety manual for geologic investigations. Light weight drilling equipment mounted on track vehicles may provide a means of collecting disturbed and undis- turbed samples from areas that are not accessible to heavy drill rigs. Sampling equipment is described in NEH, Section 8, Chapter 2. In instances where drilling equipment is used, push tube sampling of relatively soft fine-grained soils or Denison core barrel sampling of hard fine-grained soils are recommended. Special equipment has been devised for undisturbed sampling of soils in investigations for channel improvements where pits or stream banks are the sampling sites. Figure 3-2 is a drawing of the e sampling tool which is designed for placement on a shelf dug in the side or bottom of a pit or bank. It is built to obtain more than one sample from the same setup. The thin-walled brass tubes used should be at least two inches and preferably larger in diameter by five or six inches in length. This size is usually sufficient for shear strength tests. Duplicates from the same stratum must be obtained in the event one is damaged in shipping or handling. Following tests in an undisturbed condition, the same material is used for classification, or an additional sample of the disturbed soil can be obtained. The sample or samples are judged to be undisturbed if the soil column on the inside of the tube and on the outside are both level with the upper edge. A tendency for compaction to occur in a tough, dry soil because of the frictional resistance can be reduced by pre-wetting the inside of the tube with water or a small amount of a light lubricant. On completion of sampling, the tubes are sealed. It is essential that the undisturbed samples do not become desiccated. The cutting of cubes or other block shapes for laboratory testing is another means of obtaining undisturbed samples. It may, in fact, be necessary to obtain such samples if other equipment compacts or otherw:j.se e