Which limitation concerns k and mv?

They are assumed constant, but in reality fall as the soil consolidates

They are assumed to vary linearly

Secondary consolidation occurs

after excess pore pressure has become zero, at constant effective stress

at the same time as pore pressure rises

2078 Poush · 2+2 marks

Terzaghi limitations; primary vs secondary

Short answer

Terzaghi's theory assumes constant k, mv and cv, homogeneous saturated soil, Darcy's law, one-dimensional flow and strain, instantaneous loading, small strain, and ignores secondary compression. Primary consolidation is caused by pore water expulsion during excess pore pressure dissipation; secondary consolidation is creep after pore pressure reaches zero and is not covered by Terzaghi's theory.

Discuss the limitations of Terzaghi's theory of consolidation. State the difference between primary and secondary consolidation.

Limitations of Terzaghi's theory

It assumes constant permeability $k$ and coefficient of volume compressibility $m_v$ (so constant $c_v$ ); in reality $k$ and $m_v$ fall as the soil consolidates.
The soil is assumed to be homogeneous, saturated and to obey Darcy's law; real deposits are layered.
It is one-dimensional (flow and compression only vertical); in reality, lateral flow and strain occur near the edges of footings.
The load is assumed to be applied instantaneously and the total stress increment is constant with depth and time.
It ignores secondary compression (creep), which is large in organic soils and soft clays.
It assumes small strain and a linear $e$ – $\sigma'$ relation, and incompressible solids and water.
The theory fits the experimental curves well up to about 60% to 70% consolidation only.

Primary vs secondary consolidation

Point	Primary consolidation	Secondary consolidation
Cause	Expulsion of pore water as excess pore pressure dissipates	Plastic readjustment of grains/creep at constant effective stress
Time	During dissipation of excess pore pressure	After excess pore pressure is zero
Governing theory	Terzaghi's theory	Not covered by Terzaghi; time-log curve
Magnitude	Large part of total settlement (inorganic clays)	Small, but large in organic clays and peat
Depends on	$C_c$ , $e_0$ , $\Delta\sigma'$	$C_\alpha$ , time

✓ Answer checked by Claude Opus 5.5 (high effort)

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