Question $2$

$($a$)$ A site is underlain by a $10m$ thick stratum of clay which has an average undrained shear strength of $60$kPa. Below $10m,$ there is a stiffer clay stratum with undrained shear strength of $150$kPa. Both clays have saturated densities of $2\mathrm{.}0M\frac{g}{m^3}.$ It is proposed to use $0\mathrm{.}6m$ diameter and $10m$ long bored piles on the site.

$($i$)$ Determine the ultimate load carrying capacity of a single pile, using Figure $2$ and an adhesion factor of $0\mathrm{.}4.$

$[8$ marks$]$

$($ii$)$ Determine the ultimate load carrying capacity of a square group of $16$ piles spaced at $1\mathrm{.}2m$ centre to centre. Assume a block failure mode.

$[8$ marks$]$

$($iii$)$ Determine the efficiency of the pile group.

$[4$ marks$]$

Use Figure $2$