Tractive Effort 
The force developed in the soil to propel a vehicle is called tractive 
effort. This force is made possible by the shearing strength of the soil (Bekker, 
1956, p. 255). 
In Equation 8, it was shown that the soil shearing strength, s, is equal 
to the soil cohesion, c, plus the normal pressure of the shear plane times the 
tangent of the angle of internal friction, @: 
s = c + ptangd (8) 
The shear occurring at the ground contact area between the soil and 
a vehicle, at the point close to a stall, is a large-scale replica of the shearing 
process produced in the laboratory by means of the shear-box under the 
conditions expressed by the above equation. 
Figure 39 shows this analogy and implies that the horizontal tractive 
force, H, and vehicle weight, W, acting upon the ground may be related by 
the same equation as above, 
H = Ac + Wtang (12) 
where A = the shear area, which is approximately equal to the track ground 
contact area. 
In a purely frictional soil such as dry sand, c = O and the tractive effort 
becomes 
H = Wtan@ (13) 
In this case the tractive effort is proportional to vehicle weight, W. 
In a purely cohesive soil such as soft, saturated clay, however, where 
¢ = 0, we have seen that s = c (Equation 9) and only the soil cohesion contri- 
butes to the shear strength. In this case, the tractive effort becomes 
H = Ac (14) 
Here the vehicle weight does not enter the formula and the tractive effort 
basically depends only on the ground contact area, A. Thus, the criterion 
for track design for use on the saturated plastic soils found on the ocean 
bottom, is track size: the larger the ground contact area, the better the 
traction. 
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