A layer of normally consolidated, saturated silty clay of $1$ m thickness is subjected to one dimensional consolidation under a pressure increment of $20$ kPa. The properties of the soil are: specific gravity $=2.7$, natural moisture content $=45 \%$, compression index $=0.45$ and the recompression index $=0.05$. The initial average effective stress within the layer is $100$ kPa. Assuming Terzaghiâ€™s theory to be applicable, the primary consolidation settlement (rounded off to the nearest mm) is

- $2$ mm
- $9$ mm
- $14$ mm
- $16$ mm