Cohesive soil embankments: Iowa DOT design and construction specifications and resultant embankment quality

Abstract

Embankment Quality Research Phase I and II programs were initiated to identify embankment quality deficiencies under the current Iowa DOT specifications and to field test alternative embankment acceptance procedures and methods for cohesive soils that would result in improved embankment quality. Field research activities included observations of fill placement, in-place moisture and density testing, and dynamic cone penetrometer (DCP) index testing on several highway embankment projects throughout Iowa. In addition, several experiments involving rubber-tired compaction, changes in lift thickness, and aeration by disking were carried out. It was concluded that the current Iowa DOT specifications for sheepsfoot roller walkout is not, for all soils, a reliable indicator of degree of compaction, adequate stability, or suitable compaction moisture content. During fill placement, much of the fill material is typically very wet and compacted at high levels of saturation, which causes reductions in soil effective stress and subsequent instability. It was observed that earthwork construction processes including lift thickness and roller passes were not consistent on several embankment projects. Compacted lift thickness was measured to vary from 8 to 22 inches and compaction effort averaged 4 to 5 roller passes. Also, it was observed that reduction of clod size and aeration of wet soils by disking, which is currently a part of the Iowa DOT specifications, is rarely enforced in the field. Upon completion of one of the embankments, subsurface explorations involving Cone Penetration Tests (CPT), Standard Penetration Tests (SPT), and Shelby tube sampling operations were performed at selected locations to obtain information on actual finished embankment conditions and to develop an engineering evaluation for the project. The DCP was found to be a valuable field tool for quality control. Furthermore, by testing for soil stability, shortcomings from density tests (density gradients) are avoided. From these investigations, moisture control and soil design charts were developed to improve soil design specifications and field construction methods. Swell potential, susceptibility to frost heave, and performance under load are soil engineering properties related to pavement subgrade performance and were included in the author's newly developed Iowa Soil Design and Construction (SDC) chart and Iowa Moisture Content Construction (MCC) chart.