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The answer is 11.6m, 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

`6.0m`

thick and consists of sands and gravels with bulk unit weight of

`22k(N)/(m^(3))`

and angle of friction of

`\phi _(crit )^(')=35\deg `

. The bottom layer is composed of clay with bulk unit weight of

`18k(N)/(m^(3))`

and undrained shear strength of

`60kPa`

. Determine the necessary minimum pile length immediately following construction assuming that the water table is at the depth of

`2.0m`

from the ground surface involved. The average adhesion factor may be taken as 0.4 for the clay layer. Assume that the horizontal effective stress at any depth is equal to

`(1-sin\phi _(crit)^('))`

times the vertical effective stress at the same depth, and that the angle of friction between the concrete and the sand is equal to

`0.6\phi _(crit )^(')`

. 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 2.0 , while the end bearing safety factor may be taken as 2.5 .