Ch. 7] THE SUBGRADE AND BASE COURSES 143 



into a period of wet years and the position of the ground-water table 

 rises. Obviously the moisture content of the subgrade and base 

 courses may be drastically altered and, in consequence, the associated 

 bearing values. 



Middlebrooks and Bertram (1942), in a study of soil tests rela- 

 tive to design of runway pavements, arrive at the tentative conclu- 

 sion that, for testing the subgrade, the time-tested California bearing- 

 ratio test is most suitable for flexible pavements and that, for de- 

 termining the modulus of soil reaction and the use of this value in 

 Westergaard's center-loading formula, the field-bearing-test method is 

 the most satisfactory for rigid pavements. They also make a very 

 pertinent statement to the effect that no accurate methods for evalu- 

 ating the true bearing value in soils affected by frost action are 

 known. They also state: "Highway experience indicates that, in areas 

 which are subject to frost action the base course of non-frost heaving 

 material under a pavement used for highway loads, should extend 

 to a depth of at least 50 per cent of the average frost penetration, 

 in order to provide suitable subgrade reinforcement." The most 

 important effect of frost action is in the reduction of the bearing 

 values of the soils. 



In a critique of the above study, Campen and Smith (1942) make 

 a point of the fact that, in laboratory testing of saturated, compacted 

 soil samples, it should be borne in mind that soils beneath pavements 

 are chiefly affected by moisture that reaches them from below and not 

 through the essentially waterproof road surfacing. 



The effect of steady traffic is essentially that of vibration and con- 

 stant shocks which result in a densification of the underlying soils. 

 This is less in clays, where intergranular slippage is less because of its 

 cohesive bond, than in more granular materials. In quicksands, vibra- 

 tion or shock destroys the natural sand structure or bond in the sur- 

 face layers and causes an upward movement of the contained pore 

 water until the surface grains may actually be suspended, with a 

 consequent loss in the bearing value of the sand. It is believed that 

 a similar condition is approximated under modern high-speed high- 

 ways. If there is a time lag after the passage of a heavy vehicle dur- 

 ing which the subgrade can relax and regain the moisture "squeezed" 

 to the surface, no permanent damage results. However, if there is not 

 sufficient time for the soil to recover this moisture from the surficial 

 layers, it may travel in the direction of traffic flow until it passes 

 laterally from under the pavement or reaches a joint. If it is assumed 

 that some of the finer particles in the base courses and subgrade 

 move along with this moisture and are lost through "pumping" at the 



