Page:America's Highways 1776–1976.djvu/331

 “cut-and-try” system. However, the variable results obtained by casual blending soon made it apparent that more than eyeball engineering was needed in dealing with soils and developing their potential as road material.

Most of the experiments up until 1920 were concerned with dust prevention and road surface preservation. Then, in 1920, the Bureau of Public Roads began investigations “to obtain accurate scientific information regarding the characteristics of soils which affect their bearing value.” At about the same time, several State highway departments established soil-testing units and each attacked the problem in its own way. However, through exchange of information at conferences and through American Society for Test- ing and Materials (ASTM), AASHO, and HRB, a soil classification system and standard soils tests were developed.

One significant milestone was the development, during the latter part of the 1920’s, of tests to measure certain key characteristics of the various soil types and their blends. These were the so-called soil constants which included the liquid limit, plastic limit, and plasticity index, the latter being calculated from the other two. These constants, which identify the moisture retention and flow characteristics of a particular soil, have become indispensable criteria for evaluating and predicting the performance of a given soil as a pavement foundation and for use as a control tool to facilitate the blending of inherently unsatisfactory soils to produce acceptable foundations.

Research in the late 1920’s also disclosed that other soil properties, in addition to plasticity, affect the performance of soils in road foundations. It became apparent that the wide range of soil types should be classified on the basis of measurable characteristics, such as fineness and chemical composition, to provide highway engineers a working soil language. Based on data obtained by testing thousands of soil samples and observation of behavior of soils under field conditions, the Public Roads’ soil classification system was developed and first published in 1929. The best soil for use in road foundations and earth structures was classified A-1. The series continued through seven more main groups, the poorest being A-8. This system, with appropriate revisions, later became the ASTM and AASHO standards.

This nuclear moisture-density gage was developed in the late 1950’s to instantly test subgrade compaction without the need to take samples and complete a laboratory analysis.

Continuing soils research included studies of resistance to frost heaving in cold climates (frost susceptibility) and consolidation or settlement characteristics of soils. It was determined that frost heave was greatest in silty soils and that the best remedy was to remove and replace the silt with coarser grained materials, such as gravel, to a depth at which frost action was not detrimental. Under the guidance of Dr. Charles Terzaghi of the Massachusetts Institute of Technology, serving as consultant to Public Roads, apparatus for measuring the consolidation characteristics of soils was also developed in the late 1920’s, and the test data were used to determine the rate and amount of settlement of highly compressible soils.

Occasional highway surface failures where the quality of the foundation soil was considered to be good indicated insufficient compaction during construction. To provide adequate compaction control, a test method for determining the moisture-density relations of soils for earth dams was adapted to highway earthwork in the mid-1930’s. Thereafter, compaction specifications for highway embankments, subgrades and soil-aggregate base courses required that the material have a proper amount of moisture when compacted so that a target density could be reached.

This is a circa 1935 electrical resistivity test along the George Washington Memorial Parkway in Virginia. An electric current is passed through steel rods to measure ground resistance. This geophysical method is used to locate subsurface rock formations.

Significant progress was made in soil stabilization during the 1930’s. Research showed that soils for use in pavement subgrades could be improved by the addition of portland cement, lime or bitumen, and that calcium and sodium chlorides were effective dust palliatives. During World War II, the Soils Laboratory collaborated with the Engineer Board, Fort Belvoir, Virginia, in the evaluation of chemicals for 325