A (3 m x 8 m) footing is designed for a multistory building; the footing is constructed at a depth of 3m below the ground surface; the foundation is subjected to a stress of 110 kN/m Bedrock layer is located at 35 m below G.S.; saturated and dry unit weight of the soil is 21 kN/m3 and 17 kN/m3 respectively. Find The borehole spacing and The number of boreholes for a square area of 144 m and then The depth of the borehole required for the site investigation.