The answer is $11\mathrm{.}6$m$,$ please show all workings:

A $600mm$ square concrete pile is to be built to support a working compression load of

$300kN$ or a working tension load of $200kN$ at a site with two layers of soil. The top

layer is $6\mathrm{.}0m$ thick and consists of sands and gravels with bulk unit weight of $22k\frac{N}{m^3}$

and angle of friction of ${}_{\text{c}\text{r}\text{i}\text{t}}^{\text{'}}=35.$ The bottom layer is composed of clay with bulk

unit weight of $18k\frac{N}{m^3}$ and undrained shear strength of $60$kPa.

Determine the necessary minimum pile length immediately following construction

assuming that the water table is at the depth of $2\mathrm{.}0m$ from the ground surface involved.

The average adhesion factor may be taken as $0\mathrm{.}4$ for the clay layer. Assume that the

horizontal effective stress at any depth is equal to $(1-sin{}_{\text{c}\text{r}\text{i}\text{t}}^{\text{'}})$ times the vertical

effective stress at the same depth, and that the angle of friction between the concrete

and the sand is equal to $0\mathrm{.}6{}_{\text{c}\text{r}\text{i}\text{t}}^{\text{'}}.$ It may be assumed that the end bearing capacity

factor is equal to $9$ for the present conditions. The shaft resistance safety factor may be

taken as $2\mathrm{.}0,$ while the end bearing safety factor may be taken as $2\mathrm{.}5.$