What does the recompression index (Cr) measure?

The recompression index (Cr) measures the logarithmic slope of the recompression segment of a soil's compression curve. This index is vital in understanding how saturated soils behave when they are subjected to changes in effective stress after being preloaded or after recovering from a previous loading condition.

In practical terms, Cr provides insights into the compressibility of fine-grained soils during the recompression phase, where the soil regains its strength and volume due to the reapplication of load after unloading. It reflects how much the soil structure reaffirms itself following a reduction in stress, helping engineers predict settlement and other characteristics of soil behavior under varying pressures.

The other choices do not accurately describe Cr's function: the slope of the virgin compression curve pertains to a different aspect of soil behavior before any unloading occurs, while the maximum allowable load relates to load-bearing capacity rather than compression characteristics. Furthermore, the failure point under shearing forces pertains to a different parameter entirely, typically analyzed through effective stress and shear strength principles.