Factors affecting degree of consolidation

The degree of consolidation $U$ is the ratio of the consolidation settlement at time $t$ to the final primary settlement (or the fraction of excess pore pressure dissipated). For one-dimensional consolidation, $U$ depends only on the time factor:

$$T_v = \frac{c_v\, t}{H_{dr}^2}, \qquad c_v = \frac{k}{m_v \gamma_w}$$

So the factors that affect $U$ are those that change $T_v$:

1. Time $t$. $U$ increases as time passes. For $U<60\%$, $T_v=\frac{\pi}{4}U^2$.
2. Coefficient of consolidation $c_v$. A larger $c_v$ gives faster consolidation. It rises with permeability $k$ and falls with compressibility $m_v$. Sand has a very high $c_v$; clay has a low $c_v$.
3. Coefficient of permeability $k$. Fine, plastic clays with low $k$ consolidate slowly because water escapes slowly.
4. Coefficient of volume compressibility $m_v$. A more compressible soil must expel more water, so it takes longer.
5. Drainage path length $H_{dr}$. Time varies with $H_{dr}^2$. Doubling the layer thickness makes the time four times longer.
6. Drainage condition. Drainage at top and bottom (double drainage) gives $H_{dr}=H/2$, so it is four times faster than single drainage with $H_{dr}=H$.
7. Distribution of initial excess pore pressure. With one-way drainage, uniform, triangular or sinusoidal distributions give different $U$–$T_v$ curves, the difference being largest in the early stages. With two-way drainage, any linear distribution gives the same curve as the uniform case.
8. Presence of sand lenses, drains or seams. Vertical sand drains and thin sand layers shorten $H_{dr}$ and speed up consolidation.