Settlement 4 cm in 2 months, time for 10 cm

Settlement is proportional to the degree of consolidation $U$, and time is proportional to $T_v$. For $U < 60\%$, $T_v = \frac{\pi}{4}U^2$, so $t \propto U^2$ (same layer, same $c_v$ and drainage).

Step 1: Degrees of consolidation

$$U_1 = \frac{4}{20} = 20\%, \qquad U_2 = \frac{10}{20} = 50\%$$

Both are below 60%, so the parabolic relation applies.

Step 2: Time ratio

$$\frac{t_2}{t_1} = \left(\frac{U_2}{U_1}\right)^2 = \left(\frac{50}{20}\right)^2 = 6.25$$ $$t_2 = 6.25 \times 2 = 12.5\ \text{months}$$

Answer: about 12.5 months from the start of loading. The double drainage condition does not change the ratio, because $H_{dr}$ is the same in both cases.