is measured with a cone penetrometer containing an aluminum shaft and a 30-degree cone that has an area of 0.5
square inches (see figure 3-11, page 3-28). During operation, the analyst places both hands on the handle of the
instrument, with one hand on top of the other (see figure 3-12).
Next, with the shaft of the instrument in a vertical position, a slow, steady, downward force is applied. At the
instant the cone base is flush with the ground, a second analyst reads the numeric value indicated on a radial dial.
This value, which may range from 0 to 300, is then recorded for a depth of 0 inches on a chart similar to the one
shown in table 3-4 page 3-30.
As downward movement of the cone penetrometer continues, the dial readings are recorded at successive 6-inch
intervals to a depth of at least 6 inches below the critical layer of the design vehicle. It was previously determined
(Lesson 3.E.2.b.(1)(c) page 3-27) that the critical layer for 50 passes of a 105-millimeter howitzer over fine-
grained soil is 6 inches to 12 inches; therefore, under these circumstances, the dial readings must be recorded to a
depth of 18 inches (see table 3-4, page 3-30). Following the completion of each test, the cone penetrometer is
extracted by either the handle or the shaft. One the results of the required number of tests have been recorded, the
individual values obtained for each test at a particular depth are
Figure 3-12. Determination of cone index