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e e e 3-13 In the process of investigation, the geologist should bear in mind that one part of a stream bank may be more subject to erosion than another due to differences in the hydraulic stresses that are exerted by the' flow. These stresses are greatest on the bed, on the outside of bends, and along the toe of the banks. Greater emphasis in logging, sampling and testing should be used in searching for and identifying by thick- ness and location any easily erodible stratigraphic units from midbank to the invert level of the proposed channel. A bank is only as resistant as its weakest segment, particularly if this segment is low in the bank. Determining the Sample Types to be Obtained Prior to the detailed investigations, the geologist and engineer have made a tentative decision as to whether data from undisturbed and disturbed samples are to be obtained and the best means for obtaining them. Each stratigraphic unit observed in an undisturbed condition is evaluated before the decision is made as to the type of sampling to be done. It is a good rule to obtain undisturbed samples when in doubt as to whether the unit is characteristically composed of discrete particles or of aggregates that would resist erosion as a mass. The effects of the environment on sediment, once it has accumulated, is discussed in the section on the geomorphology of alluvial deposits. The in-place behavior of materials that resist erosion as a cbherent mass cannot be determined by testing disturbed samples. Hence, undisturbed sample analysis is critically important if the in-place deposits differ in erodibility from that of the individual particles. The Unified Soil Classification System and similar systems do not identify erosion characteristics of an undisturbed coherent soil mass except that soils with a high plasticity index (P.I..) are usually more resistant to erosion than noncoherent soils. Some undisturbed soils with a low P.I. are about as resistant to erosion as those with high P.I., and under certain conditions, material with a high P.I. can be as erodible as one with a low P.I. such as soils that are highly dispersive. Another such condition exists when the soil mass is weakened by a fine network of fractures such as that which may be caused by expansive clays. The fractures mayor may not be filled with dissimilar material. The above would have to be determined by appropriate tests and observations. Careful selection of investigation tools and procedures for investi- gations are necessary to obtain samples and logs of the natural, undis- turbed in-place materials. Augers, excavation equipment and many laboratory procedures disturb the soils and provide data which result in erroneous interpretations of in-place properties.