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e EM 1110-2-1913 31 Mar 78 general sample borings are selected based on Phase 1 results. Undis- turbed sample borings are located where data on soil shear strength are most needed. The best procedure is to group the foundation profiles developed on the basis of geological studies and exploration into reaches of similar conditions and then locate undisturbed sample borings so as to define soil properties in critical reaches. . b. Depth. Depth of borings along the alignment should be at least equal to the height of levee but not less than 10 ft. Boring depths should always be deep enough to provide data for stability anal- yses of the levee and foundation. This is especially important when the levee is located near the riverbank where borings must provide data for stability analyses involving both levee foundation and riverbank. Where pervious or soft materials are encountered, borings should extend through the permeable material to impervious material or through the soft material to firm material. Borings at structure locations should extend well below invert or foundation elevations and below the zone of significant influence created by the load. The borings must be deep enough to permit analysis of approach and exit channel stability and of underseepage conditions at the structure. In borrow areas, the depth of exploration should extend several feet below the practicable or allow- able borrow depth or to the groundwater table. If borrow is to be ob- tained from below the groundwater table by dredging or other means, borings should be at least 10 ft below the bottom of the proposed excavation. i . 2-10. Geophysical Exploration. a. Use of geophysical methods of subsurface exploration is ex- pected to increase as a part of foundation exploration for levees because of the long, relatively narrow areas to be explored and the in- creasing cost of borings. Table 2-4 summarizes those geophysical methods most appropriate to levee exploration. These methods are a fairly inexpensive means of exploration and are very useful for inter- polating between borings which, for reasons'of economy, are spaced at fairly wide intervals. Geophysical data must be interpreted in conjunc- tion with borings and by qualified experienced personnel or misleading information is almost certain to result. Because there have been significant improvements in geophysical instrumentation and interpreta- tion techniques in recent years, more consideration should be given to their use. '" b. divided surface Currently available geophysical methods can be broadly sub- into two classes: those accomplished entirely from the ground and those which are accomplished from subsurface borings. 2-6 e