What is the cause of immediate settlement?

Elastic distortion of soil at constant volume

Plastic readjustment at constant effective stress

What governs primary consolidation settlement?

Dissipation of excess pore pressure as water leaves saturated clay

Secondary compression is most significant in which soils and when?

Organic soils, soft clays and peat, after primary consolidation

2072 Chaitra · 3 marks

Immediate, primary and secondary settlement

Short answer

Immediate settlement is elastic distortion at constant volume, primary consolidation is volume reduction from pore water expulsion in saturated clays, and secondary compression is slow plastic readjustment. They are computed respectively by elastic theory, consolidation theory, and creep coefficient.

What is the difference among immediate settlement, primary consolidation settlement and secondary compression settlement?

Total settlement is $S = S_i + S_c + S_s$ .

Point	Immediate (elastic)	Primary consolidation	Secondary compression
Cause	Elastic distortion of soil at constant volume	Expulsion of pore water as excess pore pressure dissipates	Plastic readjustment of soil particles at constant effective stress
Time	Occurs during or just after loading	Slow, months to years	Continues after primary consolidation, very long term
Soil	All soils; mainly sands, dry silts and unsaturated clay	Saturated clays	Organic soils, soft clays, peat
Volume change	No (undrained)	Yes, water leaves	Small, continuing creep
Computation	$S_i = \dfrac{qB(1-\mu^2)}{E}I_f$	$S_c = \dfrac{C_cH}{1+e_0}\log\dfrac{\sigma'_0+\Delta\sigma}{\sigma'_0}$	$S_s = \dfrac{C_\alpha H}{1+e_p}\log\dfrac{t_2}{t_1}$

✓ Answer checked by Claude Sonnet 5.5

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