$4.$A three storied building is constructed on a silty sandstone $($as shown in attachment$).$ An undisturbed soil sample has been collected from $50$ m depth where the lower limit of the building foundation is located. The direct shear test data of the collected soil sample is given in the attachment. Determine the shear strength parameter $($c and $)$ of the soil sample. $($Use the graph paper for data plotting$).$

$5.$Determine the safety factor of the structure if the soil sample has a consolidation normal stress of about $1300$ kN$/$m$2.$ Based on your findings comment on the ground condition.