An undisturbed soil sample was taken from the middle of a clay layer (i.e., $1.5$ m below GL), as shown in figure. The water table was at the top of clay layer. Laboratory test results are as follows:
$\begin{array}{lcl} \text{Natural water content of clay} & : & 25 \% \\ \text{Preconsolidation pressure of clay} & : & 60 kPa \\ \text{Compression index of clay} & : & 0.50 \\ \text{Recompression index of clay} & : & 0.05 \\ \text{Specific gravity of clay} & : & 2.70 \\ \text{Bulk unit weight of sand} & : & 17 \: kN/m^3 \end{array}$
A compacted fill of $2.5$ m height with unit weight of $20 \: kN/m^3$ is placed at the ground level.
Assuming unit weight of water as $10 kN/m^3$, the ultimate consolidation settlement (expressed in mm) of the clay layer is _______