This Table gives the standard penetration numbers determined from a sandy soil deposit in the field. Depth (m) 3.0, 4.5, 6.0, 7.5, 9.0, 10.5, 12. Unit weight of soil Kn/M^3 16.7, 16.7, 16.7, 18.6, 18.6, 18.6, 18.6. N60 7, 9, 11, 16, 18, 20, 22. Determine the variation of the peak soil friction angle. Estimate and average value of soil friction angle for the design of a shallow foundation. Note For depth greater than 6m, the unit weight of soil is 18.6 kN/m^3