The excessive settlements and failures of many embankments constructed of shale materials has led to major investigations concerning the properties and behavior of shales. Durable shales, which can withstand the weathering processes, will perform satisfactorily when placed as rock fill. Nondurable shales however, must be thoroughly broken down during compaction and placed as soil fill. Shales which are mechanically hard but nondurable present special problems relative to construction techniques.

The policy of thoroughly breaking down shales during embankment construction emphasizes the importance of understanding shale degradation and its relationship to the compaction variables. Appropriate definition of the degradation functions would be most directly achieved through field compaction tests. Yet the expense of field tests and the limitations of present knowledge on shale degradation would reduce the effectiveness of any major field testing program. A standard compaction- degradation test could be used to generate the degradation functions in the laboratory using an assortment of compaction variables. Ultimately, the results and experiences from the laboratory studies could be coupled with field compaction observations. With sufficient data from both laboratory and field studies, the quantitative prediction of shale degradation during field compaction could be made from the results of laboratory testing only.

For this study, an impact type compaction test using an effort level of 790 kN-m/m2 (16,500 ft-lb/ft 3) was selected as a"standard" compaction-degradation test (Hale, 1979). This test was used for a laboratory evaluation of the effects of:

  • initial shale gradation,

  • maximum aggregate size, and

  • compaction water content, on shale degradation and compacted density.

Numerical Representation of Gradations

During the compaction process, fracturing, abrasion, and moisture effects break down individual shale pieces. The result is a compacted material with a gradation different from that of the uncompacted material. Therefore, a measure of the gradation change serves as an indicator of the amount of degradation that has occurred.

The Index of Crushing (IC) is a gradation index based on the summation of the weighted fractions of several size groups. Aughenbaugh et al. (1963) described the use of the IC as a measure of aggregate degradation during compaction. The percentage of the sample by weight within a size range is multiplied by a factor equal to the mean equivalent mesh size of that range. The summation of the values from each size group represents one gradation. The actual IC value is computed as the difference between the numerical representation of the initial and final gradations and expressed as a percentage of the value from the initial gradation. Since the gradation values in the IC represent the mean or average aggregate size of the initial and final gradations, the IC is a measure of change in the mean aggregate size. Previous successful use of the IC by Bailey (1976), the ability to use the IC for samples with different initial gradations, and the concept of mean aggregate size led to the use of the IC as the primary measure of degradation for this study.

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