and thickness of the critical layer vary with the type and weight of the vehicle, the type of soil, and the number of
In addition, the nature of the soil profile may also influence the position of the critical layer. Within a fine-
grained soil having a normal soil profile, the critical layer for one pass of a light (46,000-pound), self-propelled,
full-tracked, 10-millimeter, M37 howitzer is 0 to 6 inches. The critical layer for 50 passes of the same vehicle is 6
inches to 12 inches. The critical layer must be identified before proceeding with trafficability determinations.
(d) Sampling Density. The total number of individual sites that must be tested in order to accurately
describe the soil trafficability conditions of a given region may be determined by using the soil trafficability test
site template provided in figure 3-10, page 3-28.
The number of test sites required for a questionable trafficability area is listed within the template oval that most
closely approximates the size of that area as depicted at final graphic scale. Care should be taken to ensure that
samples represent typical soil conditions and that selected sites serve to define soil boundaries.
(2) Field/Laboratory Procedures. An in-depth quantitative field analysis of soil trafficability involves
the use of a soil trafficability test set (see figure 3-11, page 3-28). Where field analysis is not possible,
trafficability determinations may be made based on remotely collected information.
(a) Cone Index (CI). The first step in the field analysis of soil trafficability is the determination of
the CI of the soil. The CI, which is an indication of the soil's shear strength,
Table 3-3. Variation of critical layer with respect to vehicle type and weight, soil type, and number of