In one-dimensional consolidation testing (oedometer), the slope of the e–log p curve for a soil mass is used to determine which compressibility parameter?

Introduction / Context:
Compressibility parameters from the oedometer (consolidation) test are essential for predicting settlement of clay layers. The void ratio (e) versus logarithm of vertical effective stress (log p) plot captures how soil structure collapses under load and is the basis for estimating primary consolidation settlement in fine-grained soils.

Given Data / Assumptions:

• Test: one-dimensional consolidation (oedometer) under incrementally applied loads.
• Plot: e versus log p (p is vertical effective stress).
• We seek the parameter represented by the slope of the virgin compression line.

Concept / Approach:

The compression index Cc quantifies the virgin (normally consolidated) portion of the e–log p curve: Cc = (e1 − e2) / log10(p2/p1). It is used in settlement formulae to compute change in void ratio and corresponding strain for stress increases beyond the preconsolidation pressure. Other parameters relate to different aspects: Cv governs the rate (time) of consolidation; mv is the coefficient relating strain to stress in linear terms over small increments; k is hydraulic conductivity, not a direct slope on the e–log p plot.

Step-by-Step Solution:

Verification / Alternative check:

Recompression index Cr is the slope of the unloading–reloading (overconsolidated) portion and is smaller than Cc. This distinction confirms that the slope in the normally consolidated region reflects Cc.

Why Other Options Are Wrong:

• k: Derived from permeability tests, not the e–log p slope.
• Cv: Obtained from time–settlement curves using Terzaghi’s theory.
• mv: Incremental parameter; not the e–log p virgin-line slope.

Common Pitfalls:

• Confusing Cc and Cr or using natural logarithms without converting constants.
• Reading slopes over mixed regions (preconsolidation break not identified).

Final Answer:

compression index, Cc