5.2 Soil Strength
The soil strength module was developed for the Soil Moisture Strength Prediction Model
Version II (SMSP II). The original documentation is found in Sullivan et al. (1997).
Section 7.2.1 is a paraphrase of pages 3233 and 7576 of the aforementioned report.
Historically, cone index (CI) as measured with a standard WES (USA Corps of Engineers
Waterways Experimental Station) cone penetrometer, has been used to predict the
strength of a soil in establishing vehicleterrain interaction relationships. The cone index
is a measure of the resistance to penetration developed by the cone as it is pushed into the
soil. It equals the vertical force applied to the sleeve divided by its surface area. The WES
cone has a 30 apex angle with a 0.5-in.2 base area that is connected to a circular shaft of
The rating cone index (RCI), on the other hand, has been used to represent the proportion
of the original soil strength retained after vehicles have passed over the area. The rating
cone index is the cone index multiplied by the remolding index (RI) of the soil where the
RI equals the ratio of the after-to-before CI values of a soil subjected to a remolding test.
The first step of the remolding test is to obtain an undisturbed soil sample using a
trafficability sampler. This sample is then placed in a cylinder where the cone
penetrometer tests are performed. Measurements are taken at 1-in. intervals. After placing
the soil in the cylinder, the sample is subjected to 100 blows with a 2.5-lb hammer, 12-in.
fall per blow. A second cone penetrometer test is performed and the resulting RI is
defined as the ratio of the average CI value after the 100 blows to that of the CI value
before the blows.
Most soils have a minimum strength that will permit a vehicle to complete a specified
number of passes over it. This value is referred to as the vehicle cone index (VCI).
Insufficient soil strength may cause wheeled and tracked vehicles to become immobilized
if the in-situ CI or RCI is less than the VCI required for a specified number of vehicle
Soil strength depends not only on the soil type, but also on the soil moisture. To help
quantify the relation between soil strength and moisture, over 1000 field and laboratory
tests were performed on samples collected from the United States and elsewhere. Except
for gravels and peat, the resulting curve fits for the different USCS soils are
CI = exp[c1 - c2 ln(MC )]
RCI = exp[c1 - c2 ln(MC )]
where c1 and c2 are given in Table 7.2.1 below and MC (%) is the soil moisture content
by weight, MC = θ ( ρ w / ρs ) ⋅100% , and ρs is the soil density. The cone index and rating
cone index for gravels is 300, the maximum soil strength allowed. Peat is assumed to
have no strength.