QUESTION # $4(30$ MARKS$)$

A$.$ A foundation $1\mathrm{.}5m1m$ is located at a depth of $1m$ in a clay soil. A softer clay layer is located below at a depth of $1m$ measured from the bottom of the foundation. The characteristics of the two layers are as follows:

$(15$ marks$)$

Top layer

Undrained shear strength $=120K\frac{N}{m^2}$

Unit Weight $=16K\frac{N}{m^3}$

Bottom layer

Undrained shear strength $=60K\frac{N}{m^2}$

Unit Weight $=15\mathrm{.}6K\frac{N}{m^3}$

Determine the gross allowable load for the foundation with FS of $2\mathrm{.}5.$

B$.$ A $6-$m diameter concrete tower silo $21m$ high was constructed over soft clay on a ring foundation. Figure $5$ shows the variation of the undrained shear strength $(cu)$ obtained from field vane shear tests at the site. The groundwater table was located at about $0\mathrm{.}7m$ below the ground surface. Just after it was filled to capacity for the first time with corn the concrete tower silo suddenly overturned due to bearing capacity failure. The failure surface extended to about $7\mathrm{.}5m$ below the ground surface. The following are the average parameters for the soil in the failure zone and the foundation:

Load per unit area on the foundation when failure occurred $=155K\frac{N}{m^2}$

Average plasticity index $($PI$)$ of clay $=36$

Average undrained shear strength $($cu$)$ from $0\mathrm{.}6$ to $7m$ depth obtained from field vane shear tests $=25K\frac{N}{m^2}$

Depth of foundation $=1\mathrm{.}8m$

Evaluate the above and suggest a reason why failure occur. Show calculates to support your argument.

$(10$ marks$)$

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C$.$ A square pad foundation is to be constructed in a sandy soil with the following characteristics;

Unit Weight $=19K\frac{N}{m}3$

Friction Angle $=$

$300C=0$

If the base of the column is restricted to $1\mathrm{.}8m1\mathrm{.}8m$ and one$-$way eccentricity, $e=0\mathrm{.}16m$ determine the ultimate load the foundation can carry.

$(15$ marks$)$