What does the compression index (Cc) represent in soil mechanics?

The compression index (Cc) is a crucial parameter in soil mechanics that specifically represents the logarithmic slope of the virgin compression curve. This curve illustrates the relationship between effective stress and void ratio during the compression of saturated cohesive soils, particularly clay.

In practice, the virgin compression curve shows how soil consolidates under increasing load, indicating how much the soil will compress as a result of additional vertical stress. The slope of this curve, which is represented by the compression index, allows engineers to quantify the amount of potential consolidation that will occur when changes in loading conditions happen. A steeper slope indicates a greater degree of compressibility, which is vital for predicting how a soil will respond under different loads.

The other options, while related to soil mechanics, do not accurately define what the compression index represents. For instance, the vertical displacement of soil during loading involves overall settlement calculations but does not specifically describe the nature of the compression index itself. The relationship between moisture content and density pertains to soil's physical properties rather than its compressibility characteristics. Lastly, while predicting soil settlement is important in geotechnical engineering, it is not the direct definition of Cc, which focuses on the slope of the compression curve rather than the broader settlement prediction process.