One-way vs two-way drainage time

Time for a given degree of consolidation comes from the time factor:

$$t = \frac{T_v\, H_{dr}^2}{c_v}$$

For the same $U$ (same $T_v$) and the same $c_v$, time is proportional to $H_{dr}^2$.

• One-way (single) drainage: water leaves through one face only, so $H_{dr}=H$.
• Two-way (double) drainage: water leaves through top and bottom, so $H_{dr}=H/2$.

$$\frac{t_{two}}{t_{one}} = \left(\frac{H/2}{H}\right)^2 = \frac{1}{4}$$

Result: two-way drainage needs only one quarter of the time required for one-way drainage. Equivalently, one-way drainage takes four times longer. For example, a clay layer that needs 8 years to reach 90% consolidation with one-way drainage reaches the same degree in 2 years with two-way drainage.