Building From the Top Down

Building from the Top Down A Story of the Development of a Great System of Oiled Highways in Arizona

By JULIAN W. POWERS Engineer of Materials, Arizona State Highway Department.

ROAD BUILT FROM the top down that is the story of the development of Arizona's lowcost type highways, which have become standard for oilsurface construction in nearly every quarter of the globe. It is, perhaps, not a good engineering practice, but experience through a period of several years shows the statement to be a fact.

It is a story of failures. Only through analysis and diagnosis of failure have we been able to overcome weak points and approach a degree of perfection. But the work is not finished. We continue to analyze and we continue to diagnose, and by that process we hope, eventually, to overcome the weak spots and produce a road surface which will serve us as well as the more expensive types which we cannot adopt because of prohibitive cost.

The Arizona oil-surface highway was born of necessity. Long distances to be traversed through sparsely settled sections of mountains and plains, coupled with extremely limited revenues for construction, caused the Arizona State Highway Department engineers, a few years ago, to begin casting about for a road surfacing material which would withstand the ravages of wind and rain and heavy traffic, yet keep within cost brackets which we could afford.

We began to experiment with oil, and today Arizona has 1100 miles of oil pavement.

We began at the top with the oil layer (oil or cutback asphalt). Flaws began immediately to appear. So we went deeper, and by a process of elimination we are gradually developing a sound and durable pavement.

Research by the Materials Department has not yet progressed to such a point that no difficulty is encountered in building an absolutely stable and satisfactory surface layer. However, it has made much progress. It has shown, for example, that after certain lengths of time, accumulations of moisture often the layer immediately below the surface, causing displacement or breaking up of what otherwise might have been a satisfactory oil cake. It was found that most of these subgrade layers could be effectively stabilized with the addition of an intermediate layer between the subgrade and the oil.