(c) Rating Cone Index. The CI value for a particular layer of fine-grained soil or remoldable sand
may be multiplied by the corresponding RI value to arrive at a value known as the RCI of the soil layer. For
example, referring to table 3-4, page 3-30, the CI corresponding to the critical layer for one pass of a 105-
millimeter Howitzer over fine-grained soil is 49. Referring to table 3-5, page 3-34, the RI for the same layer is
0.87. Therefore, the RCI for this particular layer is approximately 43 (49 x 0.87). Likewise, referring to table 3-
4, page 3-30, the CI corresponding to the critical layer for 50 passes of a 105-millimeter howitzer over fine-
grained soil is 56. Referring to table 3-5, page 8-34, the RI for the same layer is 0.85. Therefore, the RCI for this
particular layer is approximately 48 (56 x 0.85). These two values are entered into a composite chart, such as the
one shown in figure 3-18, as RCI1 and RCI50 respectively.
Because materials other than fine-grained soils and remoldable sands do not possess RI values, their RCI values
cannot be calculated.
When extensive field investigations for the determination of soil trafficability conditions are not practical, 81Qs
often use charts to quantitatively estimate RCI values for known soil types under various environmental
conditions (see table 3-6, page 3-38).
Although gravels containing appreciable amounts of fine material are not subjected to field remolding tests, they
may experience changes in shear strength due to the passage of traffic. Consequently, RCI values for these types
of soils have also been included in the charts based on estimations made from organic, textural, and plastic
properties exhibited under given moisture conditions.
(d) Vehicle Cone Index (VCI). The VCI is a value assigned to a given vehicle for a given number
of passes (1 or 50). Table 3-7, pages 3-40 through 3-46, contains the VCI values for numerous types of military
vehicles. Once the VCI values for a particular type of vehicle have been determined, they are entered into a
composite chart (see figure 3-18). For example, the VCI for one pass of a 105-millimeter howitzer is 21, and the
VCI for 50 passes of a 105-millimeter howitzer is 49. Therefore, these values are entered into the chart as VCI1
and VCI50 respectively.
(e) Comparison of Cone Index or Rating Cone Index to Vehicle Cone Index. The final step in
determining the trafficability of a soil on level terrain with respect to a specific type of vehicle is the comparison
of the CI value (in the case of coarse-grained materials containing little or no fines) or the RCI value (in the case
of fine-grained soils or remoldable sands) with the VCI value obtained for the corresponding critical layer. In
general, if the VCI for a critical layer exceeds the corresponding CI or RCI (whichever is appropriate), the soil is
not trafficable for the specified number of passes of that particular vehicle. If, on the other hand, the appropriate
CI or RCI value exceeds the VCI value for a particular critical layer, the soil is considered to be trafficable under
the prescribed circumstances. The trafficability of the soil under question is recorded on a composite chart (see
figure 3-18). For example, when the RCI of the critical layer for one pass of a 105-millimeter howitzer over the
fine-grained soil sample depicted on the chart in figure 3-18 (43) is compared to the VCI for one pass of that
vehicle (21), it is evident that the RCI is greater. Therefore, this particular soil is capable of supporting the
movement of one 105-millimeter howitzer. However, when the same comparison is made for the critical layer
representing 50 passes of the vehicle, it can be seen that the VCI (49) is greater than the RCI (48). Therefore, this
soil will not support the