Behavior of organic soils: identify the most characteristic statement Soils containing significant organic matter typically exhibit which of the following behaviors in the field?

Introduction / Context:
Organic soils (peat, organic silts/clays) pose challenges in foundations and embankments. Recognising their hallmark traits is critical for deciding on preloading, replacement, or ground improvement strategies.

Given Data / Assumptions:

• High organic content relative to mineral fraction.
• Field moisture near natural conditions (often high).
• No special stabilization added.

Concept / Approach:

Organic matter forms a compressible, fibrous matrix with high void ratio and water content, resulting in “spongy” feel, low dry unit weight, and large primary and secondary settlements under load. Volume change follows moisture reduction (drainage) rather than increase; they do not “swell as moisture decreases.” Conversely, adding water does not cause shrinkage; drying leads to shrinkage and oxidation, further degrading structure.

Step-by-Step Solution:

Verification / Alternative check:

Empirical correlations show very low undrained strength and high compressibility parameters for organic deposits; construction codes often require removal or preloading.

Why Other Options Are Wrong:

(b) and (c) reverse moisture–volume behavior; (d) ignores a well-known property; (e) is false without stabilization or significant desiccation/aging.

Common Pitfalls:

Confusing organic clays with expansive mineral clays; assuming drying alone makes an adequate foundation stratum.

Final Answer:

They are of spongy, compressible nature with low unit weight.