Page:America's Highways 1776–1976.djvu/124

 and Ohio. One was failure of the base due to overloading of a frost-softened subgrade, such as Hubbard described, resulting in sudden and complete failure. The other was internal failure of macadam bases due to pressures high enough to crush the stone particles. Harrison found failure by internal crushing in macadam bases as thick as 18 inches near Chicago. In Wayne County, Michigan, he found that thin concrete pavements had broken up so badly that they had to be resurfaced, but that those which were made 7 or 8 inches thick had withstood heavy trucking with little damage.

Concrete mixer patching a bad spot in the roadbase (asphalt top has been removed).

Albert T. Goldbeck and F. H. Jackson of the OPRRE’s Division of Tests found that brick roads on concrete bases in Ohio, Illinois, and Indiana had stood up well except where the underlying earth foundation had become softened due to poor soil or poor drainage.

All investigators agreed that pavements on sandy, well-drained soils had given much better service than those on clay soils, especially at places where drainage was poor because of inadequate ditches.

The most significant observation of all was by Harrison who noted that hundreds of pavement failures were obviously due to moisture softening clay soils, yet the shoulders had adequate slope and drainage was excellent. Some failures were on low fills where there was no possibility that surface water could have penetrated to the subgrade. Harrison concluded that these failures were caused by “non-gravitational water,” that is, water held in the soil by capillary attraction. Not only would such soils not drain by gravity; they would actually take up more water from below by capillary action due to their fine pore structure. He also declared that any theory of design (such as McAdam’s) that held that soils under a pavement could be kept dry by an impervious covering was fallacious, and therefore only soils that do not lose their strength when wet, such as gravels and sands, should be used there.

The widespread destruction of the roads by trucking inspired angry demands for limits on the weight of vehicles, and for crushing taxes on trucks to make them “pay their fair share” of the maintenance bill. However, the more thoughtful leaders, such as Delaware’s influential chief highway engineer Charles Upham, warned that the motor truck was not just a wartime phenomenon, but would be around for years after the war was over. “. . . the motor truck,” he said, “which has been developed during abnormal times has shown that it has solved an economic problem, and this solution assures us that. . . the heavy truck will be utilized for transporting freight and express within expanding limits. Therefore,. . . we must build and maintain in such a way that our roads will withstand, as permanently as possible, the demands of the future heavy truck traffic.”

Upham’s viewpoint was shared by most of the State highway department heads, but none of them had a clear idea of how truck traffic might develop or how to deal with it. However, they agreed completely that legal limits must be placed on the weight of the vehicle and its load, and that without such limits there would be no way to protect the rural roads from destruction. There was a general consensus that the 5-ton capacity truck was the largest that should be allowed on the rural roads, although about 15 percent of the trucks then in existence were of greater 118