$0\mathrm{.}3(25$ marks$)$

A trapezoidal masonry retaining wall shown below: $1m$ wide at top, $3m$ wide at bottom and $4m$ high. The face of wall towards backfill is vertical and the backfill inclined at angle $(=20)$ with horizontal. Unit weight of soil is $20k\frac{N}{m^3}$ and that of masonry is $24k\frac{N}{m^3}.$ Take ${}^{\text{'}}=30$ and coefficient of friction at base as $0\mathrm{.}45.$ Determine factor of safety against sliding and overturning. No underground water and no passive pressure are being considered.